Update //third_party/protobuf to version 3.

third_party/protobuf/csharp/src/Google.Protobuf/JsonFormatter.cs

Index: third_party/protobuf/csharp/src/Google.Protobuf/JsonFormatter.cs
diff --git a/third_party/protobuf/csharp/src/Google.Protobuf/JsonFormatter.cs b/third_party/protobuf/csharp/src/Google.Protobuf/JsonFormatter.cs
new file mode 100644
index 0000000000000000000000000000000000000000..7b99f31465323ebe70ff76e1b5b1c29cc4159b4e
--- /dev/null
+++ b/third_party/protobuf/csharp/src/Google.Protobuf/JsonFormatter.cs
@@ -0,0 +1,906 @@
+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2015 Google Inc.  All rights reserved.
+// https://developers.google.com/protocol-buffers/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#endregion
+
+using System;
+using System.Collections;
+using System.Globalization;
+using System.Text;
+using Google.Protobuf.Reflection;
+using Google.Protobuf.WellKnownTypes;
+using System.Linq;
+
+namespace Google.Protobuf
+{
+    /// <summary>
+    /// Reflection-based converter from messages to JSON.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Instances of this class are thread-safe, with no mutable state.
+    /// </para>
+    /// <para>
+    /// This is a simple start to get JSON formatting working. As it's reflection-based,
+    /// it's not as quick as baking calls into generated messages - but is a simpler implementation.
+    /// (This code is generally not heavily optimized.)
+    /// </para>
+    /// </remarks>
+    public sealed class JsonFormatter
+    {
+        internal const string AnyTypeUrlField = "@type";
+        internal const string AnyDiagnosticValueField = "@value";
+        internal const string AnyWellKnownTypeValueField = "value";
+        private const string TypeUrlPrefix = "type.googleapis.com";
+        private const string NameValueSeparator = ": ";
+        private const string PropertySeparator = ", ";
+
+        /// <summary>
+        /// Returns a formatter using the default settings.
+        /// </summary>
+        public static JsonFormatter Default { get; } = new JsonFormatter(Settings.Default);
+
+        // A JSON formatter which *only* exists 
+        private static readonly JsonFormatter diagnosticFormatter = new JsonFormatter(Settings.Default);
+
+        /// <summary>
+        /// The JSON representation of the first 160 characters of Unicode.
+        /// Empty strings are replaced by the static constructor.
+        /// </summary>
+        private static readonly string[] CommonRepresentations = {
+            // C0 (ASCII and derivatives) control characters
+            "\\u0000", "\\u0001", "\\u0002", "\\u0003",  // 0x00
+          "\\u0004", "\\u0005", "\\u0006", "\\u0007",
+          "\\b",     "\\t",     "\\n",     "\\u000b",
+          "\\f",     "\\r",     "\\u000e", "\\u000f",
+          "\\u0010", "\\u0011", "\\u0012", "\\u0013",  // 0x10
+          "\\u0014", "\\u0015", "\\u0016", "\\u0017",
+          "\\u0018", "\\u0019", "\\u001a", "\\u001b",
+          "\\u001c", "\\u001d", "\\u001e", "\\u001f",
+            // Escaping of " and \ are required by www.json.org string definition.
+            // Escaping of < and > are required for HTML security.
+            "", "", "\\\"", "", "",        "", "",        "",  // 0x20
+          "", "", "",     "", "",        "", "",        "",
+          "", "", "",     "", "",        "", "",        "",  // 0x30
+          "", "", "",     "", "\\u003c", "", "\\u003e", "",
+          "", "", "",     "", "",        "", "",        "",  // 0x40
+          "", "", "",     "", "",        "", "",        "",
+          "", "", "",     "", "",        "", "",        "",  // 0x50
+          "", "", "",     "", "\\\\",    "", "",        "",
+          "", "", "",     "", "",        "", "",        "",  // 0x60
+          "", "", "",     "", "",        "", "",        "",
+          "", "", "",     "", "",        "", "",        "",  // 0x70
+          "", "", "",     "", "",        "", "",        "\\u007f",
+            // C1 (ISO 8859 and Unicode) extended control characters
+            "\\u0080", "\\u0081", "\\u0082", "\\u0083",  // 0x80
+          "\\u0084", "\\u0085", "\\u0086", "\\u0087",
+          "\\u0088", "\\u0089", "\\u008a", "\\u008b",
+          "\\u008c", "\\u008d", "\\u008e", "\\u008f",
+          "\\u0090", "\\u0091", "\\u0092", "\\u0093",  // 0x90
+          "\\u0094", "\\u0095", "\\u0096", "\\u0097",
+          "\\u0098", "\\u0099", "\\u009a", "\\u009b",
+          "\\u009c", "\\u009d", "\\u009e", "\\u009f"
+        };
+
+        static JsonFormatter()
+        {
+            for (int i = 0; i < CommonRepresentations.Length; i++)
+            {
+                if (CommonRepresentations[i] == "")
+                {
+                    CommonRepresentations[i] = ((char) i).ToString();
+                }
+            }
+        }
+
+        private readonly Settings settings;
+
+        /// <summary>
+        /// Creates a new formatted with the given settings.
+        /// </summary>
+        /// <param name="settings">The settings.</param>
+        public JsonFormatter(Settings settings)
+        {
+            this.settings = settings;
+        }
+
+        /// <summary>
+        /// Formats the specified message as JSON.
+        /// </summary>
+        /// <param name="message">The message to format.</param>
+        /// <returns>The formatted message.</returns>
+        public string Format(IMessage message)
+        {
+            Preconditions.CheckNotNull(message, nameof(message));
+            StringBuilder builder = new StringBuilder();
+            if (message.Descriptor.IsWellKnownType)
+            {
+                WriteWellKnownTypeValue(builder, message.Descriptor, message, false);
+            }
+            else
+            {
+                WriteMessage(builder, message);
+            }
+            return builder.ToString();
+        }
+
+        /// <summary>
+        /// Converts a message to JSON for diagnostic purposes with no extra context.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// This differs from calling <see cref="Format(IMessage)"/> on the default JSON
+        /// formatter in its handling of <see cref="Any"/>. As no type registry is available
+        /// in <see cref="object.ToString"/> calls, the normal way of resolving the type of
+        /// an <c>Any</c> message cannot be applied. Instead, a JSON property named <c>@value</c>
+        /// is included with the base64 data from the <see cref="Any.Value"/> property of the message.
+        /// </para>
+        /// <para>The value returned by this method is only designed to be used for diagnostic
+        /// purposes. It may not be parsable by <see cref="JsonParser"/>, and may not be parsable
+        /// by other Protocol Buffer implementations.</para>
+        /// </remarks>
+        /// <param name="message">The message to format for diagnostic purposes.</param>
+        /// <returns>The diagnostic-only JSON representation of the message</returns>
+        public static string ToDiagnosticString(IMessage message)
+        {
+            Preconditions.CheckNotNull(message, nameof(message));
+            return diagnosticFormatter.Format(message);
+        }
+
+        private void WriteMessage(StringBuilder builder, IMessage message)
+        {
+            if (message == null)
+            {
+                WriteNull(builder);
+                return;
+            }
+            builder.Append("{ ");
+            bool writtenFields = WriteMessageFields(builder, message, false);
+            builder.Append(writtenFields ? " }" : "}");
+        }
+
+        private bool WriteMessageFields(StringBuilder builder, IMessage message, bool assumeFirstFieldWritten)
+        {
+            var fields = message.Descriptor.Fields;
+            bool first = !assumeFirstFieldWritten;
+            // First non-oneof fields
+            foreach (var field in fields.InFieldNumberOrder())
+            {
+                var accessor = field.Accessor;
+                if (field.ContainingOneof != null && field.ContainingOneof.Accessor.GetCaseFieldDescriptor(message) != field)
+                {
+                    continue;
+                }
+                // Omit default values unless we're asked to format them, or they're oneofs (where the default
+                // value is still formatted regardless, because that's how we preserve the oneof case).
+                object value = accessor.GetValue(message);
+                if (field.ContainingOneof == null && !settings.FormatDefaultValues && IsDefaultValue(accessor, value))
+                {
+                    continue;
+                }
+                // Omit awkward (single) values such as unknown enum values
+                if (!field.IsRepeated && !field.IsMap && !CanWriteSingleValue(value))
+                {
+                    continue;
+                }
+
+                // Okay, all tests complete: let's write the field value...
+                if (!first)
+                {
+                    builder.Append(PropertySeparator);
+                }
+                WriteString(builder, ToCamelCase(accessor.Descriptor.Name));
+                builder.Append(NameValueSeparator);
+                WriteValue(builder, value);
+                first = false;
+            }            
+            return !first;
+        }
+
+        // Converted from src/google/protobuf/util/internal/utility.cc ToCamelCase
+        // TODO: Use the new field in FieldDescriptor.
+        internal static string ToCamelCase(string input)
+        {
+            bool capitalizeNext = false;
+            bool wasCap = true;
+            bool isCap = false;
+            bool firstWord = true;
+            StringBuilder result = new StringBuilder(input.Length);
+
+            for (int i = 0; i < input.Length; i++, wasCap = isCap)
+            {
+                isCap = char.IsUpper(input[i]);
+                if (input[i] == '_')
+                {
+                    capitalizeNext = true;
+                    if (result.Length != 0)
+                    {
+                        firstWord = false;
+                    }
+                    continue;
+                }
+                else if (firstWord)
+                {
+                    // Consider when the current character B is capitalized,
+                    // first word ends when:
+                    // 1) following a lowercase:   "...aB..."
+                    // 2) followed by a lowercase: "...ABc..."
+                    if (result.Length != 0 && isCap &&
+                        (!wasCap || (i + 1 < input.Length && char.IsLower(input[i + 1]))))
+                    {
+                        firstWord = false;
+                    }
+                    else
+                    {
+                        result.Append(char.ToLowerInvariant(input[i]));
+                        continue;
+                    }
+                }
+                else if (capitalizeNext)
+                {
+                    capitalizeNext = false;
+                    if (char.IsLower(input[i]))
+                    {
+                        result.Append(char.ToUpperInvariant(input[i]));
+                        continue;
+                    }
+                }
+                result.Append(input[i]);
+            }
+            return result.ToString();
+        }
+        
+        private static void WriteNull(StringBuilder builder)
+        {
+            builder.Append("null");
+        }
+
+        private static bool IsDefaultValue(IFieldAccessor accessor, object value)
+        {
+            if (accessor.Descriptor.IsMap)
+            {
+                IDictionary dictionary = (IDictionary) value;
+                return dictionary.Count == 0;
+            }
+            if (accessor.Descriptor.IsRepeated)
+            {
+                IList list = (IList) value;
+                return list.Count == 0;
+            }
+            switch (accessor.Descriptor.FieldType)
+            {
+                case FieldType.Bool:
+                    return (bool) value == false;
+                case FieldType.Bytes:
+                    return (ByteString) value == ByteString.Empty;
+                case FieldType.String:
+                    return (string) value == "";
+                case FieldType.Double:
+                    return (double) value == 0.0;
+                case FieldType.SInt32:
+                case FieldType.Int32:
+                case FieldType.SFixed32:
+                case FieldType.Enum:
+                    return (int) value == 0;
+                case FieldType.Fixed32:
+                case FieldType.UInt32:
+                    return (uint) value == 0;
+                case FieldType.Fixed64:
+                case FieldType.UInt64:
+                    return (ulong) value == 0;
+                case FieldType.SFixed64:
+                case FieldType.Int64:
+                case FieldType.SInt64:
+                    return (long) value == 0;
+                case FieldType.Float:
+                    return (float) value == 0f;
+                case FieldType.Message:
+                case FieldType.Group: // Never expect to get this, but...
+                    return value == null;
+                default:
+                    throw new ArgumentException("Invalid field type");
+            }
+        }
+        
+        private void WriteValue(StringBuilder builder, object value)
+        {
+            if (value == null)
+            {
+                WriteNull(builder);
+            }
+            else if (value is bool)
+            {
+                builder.Append((bool) value ? "true" : "false");
+            }
+            else if (value is ByteString)
+            {
+                // Nothing in Base64 needs escaping
+                builder.Append('"');
+                builder.Append(((ByteString) value).ToBase64());
+                builder.Append('"');
+            }
+            else if (value is string)
+            {
+                WriteString(builder, (string) value);
+            }
+            else if (value is IDictionary)
+            {
+                WriteDictionary(builder, (IDictionary) value);
+            }
+            else if (value is IList)
+            {
+                WriteList(builder, (IList) value);
+            }
+            else if (value is int || value is uint)
+            {
+                IFormattable formattable = (IFormattable) value;
+                builder.Append(formattable.ToString("d", CultureInfo.InvariantCulture));
+            }
+            else if (value is long || value is ulong)
+            {
+                builder.Append('"');
+                IFormattable formattable = (IFormattable) value;
+                builder.Append(formattable.ToString("d", CultureInfo.InvariantCulture));
+                builder.Append('"');
+            }
+            else if (value is System.Enum)
+            {
+                WriteString(builder, value.ToString());
+            }
+            else if (value is float || value is double)
+            {
+                string text = ((IFormattable) value).ToString("r", CultureInfo.InvariantCulture);
+                if (text == "NaN" || text == "Infinity" || text == "-Infinity")
+                {
+                    builder.Append('"');
+                    builder.Append(text);
+                    builder.Append('"');
+                }
+                else
+                {
+                    builder.Append(text);
+                }
+            }
+            else if (value is IMessage)
+            {
+                IMessage message = (IMessage) value;
+                if (message.Descriptor.IsWellKnownType)
+                {
+                    WriteWellKnownTypeValue(builder, message.Descriptor, value, true);
+                }
+                else
+                {
+                    WriteMessage(builder, (IMessage) value);
+                }
+            }
+            else
+            {
+                throw new ArgumentException("Unable to format value of type " + value.GetType());
+            }
+        }
+
+        /// <summary>
+        /// Central interception point for well-known type formatting. Any well-known types which
+        /// don't need special handling can fall back to WriteMessage. We avoid assuming that the
+        /// values are using the embedded well-known types, in order to allow for dynamic messages
+        /// in the future.
+        /// </summary>
+        private void WriteWellKnownTypeValue(StringBuilder builder, MessageDescriptor descriptor, object value, bool inField)
+        {
+            // Currently, we can never actually get here, because null values are always handled by the caller. But if we *could*,
+            // this would do the right thing.
+            if (value == null)
+            {
+                WriteNull(builder);
+                return;
+            }
+            // For wrapper types, the value will either be the (possibly boxed) "native" value,
+            // or the message itself if we're formatting it at the top level (e.g. just calling ToString on the object itself).
+            // If it's the message form, we can extract the value first, which *will* be the (possibly boxed) native value,
+            // and then proceed, writing it as if we were definitely in a field. (We never need to wrap it in an extra string...
+            // WriteValue will do the right thing.)
+            if (descriptor.IsWrapperType)
+            {
+                if (value is IMessage)
+                {
+                    var message = (IMessage) value;
+                    value = message.Descriptor.Fields[WrappersReflection.WrapperValueFieldNumber].Accessor.GetValue(message);
+                }
+                WriteValue(builder, value);
+                return;
+            }
+            if (descriptor.FullName == Timestamp.Descriptor.FullName)
+            {
+                MaybeWrapInString(builder, value, WriteTimestamp, inField);
+                return;
+            }
+            if (descriptor.FullName == Duration.Descriptor.FullName)
+            {
+                MaybeWrapInString(builder, value, WriteDuration, inField);
+                return;
+            }
+            if (descriptor.FullName == FieldMask.Descriptor.FullName)
+            {
+                MaybeWrapInString(builder, value, WriteFieldMask, inField);
+                return;
+            }
+            if (descriptor.FullName == Struct.Descriptor.FullName)
+            {
+                WriteStruct(builder, (IMessage) value);
+                return;
+            }
+            if (descriptor.FullName == ListValue.Descriptor.FullName)
+            {
+                var fieldAccessor = descriptor.Fields[ListValue.ValuesFieldNumber].Accessor;
+                WriteList(builder, (IList) fieldAccessor.GetValue((IMessage) value));
+                return;
+            }
+            if (descriptor.FullName == Value.Descriptor.FullName)
+            {
+                WriteStructFieldValue(builder, (IMessage) value);
+                return;
+            }
+            if (descriptor.FullName == Any.Descriptor.FullName)
+            {
+                WriteAny(builder, (IMessage) value);
+                return;
+            }
+            WriteMessage(builder, (IMessage) value);
+        }
+
+        /// <summary>
+        /// Some well-known types end up as string values... so they need wrapping in quotes, but only
+        /// when they're being used as fields within another message.
+        /// </summary>
+        private void MaybeWrapInString(StringBuilder builder, object value, Action<StringBuilder, IMessage> action, bool inField)
+        {
+            if (inField)
+            {
+                builder.Append('"');
+                action(builder, (IMessage) value);
+                builder.Append('"');
+            }
+            else
+            {
+                action(builder, (IMessage) value);
+            }
+        }
+
+        private void WriteTimestamp(StringBuilder builder, IMessage value)
+        {
+            // TODO: In the common case where this *is* using the built-in Timestamp type, we could
+            // avoid all the reflection at this point, by casting to Timestamp. In the interests of
+            // avoiding subtle bugs, don't do that until we've implemented DynamicMessage so that we can prove
+            // it still works in that case.
+            int nanos = (int) value.Descriptor.Fields[Timestamp.NanosFieldNumber].Accessor.GetValue(value);
+            long seconds = (long) value.Descriptor.Fields[Timestamp.SecondsFieldNumber].Accessor.GetValue(value);
+
+            // Even if the original message isn't using the built-in classes, we can still build one... and then
+            // rely on it being normalized.
+            Timestamp normalized = Timestamp.Normalize(seconds, nanos);
+            // Use .NET's formatting for the value down to the second, including an opening double quote (as it's a string value)
+            DateTime dateTime = normalized.ToDateTime();
+            builder.Append(dateTime.ToString("yyyy'-'MM'-'dd'T'HH:mm:ss", CultureInfo.InvariantCulture));
+            AppendNanoseconds(builder, Math.Abs(normalized.Nanos));
+            builder.Append('Z');
+        }
+
+        private void WriteDuration(StringBuilder builder, IMessage value)
+        {
+            // TODO: Same as for WriteTimestamp
+            int nanos = (int) value.Descriptor.Fields[Duration.NanosFieldNumber].Accessor.GetValue(value);
+            long seconds = (long) value.Descriptor.Fields[Duration.SecondsFieldNumber].Accessor.GetValue(value);
+
+            // Even if the original message isn't using the built-in classes, we can still build one... and then
+            // rely on it being normalized.
+            Duration normalized = Duration.Normalize(seconds, nanos);
+
+            // The seconds part will normally provide the minus sign if we need it, but not if it's 0...
+            if (normalized.Seconds == 0 && normalized.Nanos < 0)
+            {
+                builder.Append('-');
+            }
+
+            builder.Append(normalized.Seconds.ToString("d", CultureInfo.InvariantCulture));
+            AppendNanoseconds(builder, Math.Abs(normalized.Nanos));
+            builder.Append('s');
+        }
+
+        private void WriteFieldMask(StringBuilder builder, IMessage value)
+        {
+            IList paths = (IList) value.Descriptor.Fields[FieldMask.PathsFieldNumber].Accessor.GetValue(value);
+            AppendEscapedString(builder, string.Join(",", paths.Cast<string>().Select(ToCamelCase)));
+        }
+
+        private void WriteAny(StringBuilder builder, IMessage value)
+        {
+            if (ReferenceEquals(this, diagnosticFormatter))
+            {
+                WriteDiagnosticOnlyAny(builder, value);
+                return;
+            }
+
+            string typeUrl = (string) value.Descriptor.Fields[Any.TypeUrlFieldNumber].Accessor.GetValue(value);
+            ByteString data = (ByteString) value.Descriptor.Fields[Any.ValueFieldNumber].Accessor.GetValue(value);
+            string typeName = GetTypeName(typeUrl);
+            MessageDescriptor descriptor = settings.TypeRegistry.Find(typeName);
+            if (descriptor == null)
+            {
+                throw new InvalidOperationException($"Type registry has no descriptor for type name '{typeName}'");
+            }
+            IMessage message = descriptor.Parser.ParseFrom(data);
+            builder.Append("{ ");
+            WriteString(builder, AnyTypeUrlField);
+            builder.Append(NameValueSeparator);
+            WriteString(builder, typeUrl);
+
+            if (descriptor.IsWellKnownType)
+            {
+                builder.Append(PropertySeparator);
+                WriteString(builder, AnyWellKnownTypeValueField);
+                builder.Append(NameValueSeparator);
+                WriteWellKnownTypeValue(builder, descriptor, message, true);
+            }
+            else
+            {
+                WriteMessageFields(builder, message, true);
+            }
+            builder.Append(" }");
+        }
+
+        private void WriteDiagnosticOnlyAny(StringBuilder builder, IMessage value)
+        {
+            string typeUrl = (string) value.Descriptor.Fields[Any.TypeUrlFieldNumber].Accessor.GetValue(value);
+            ByteString data = (ByteString) value.Descriptor.Fields[Any.ValueFieldNumber].Accessor.GetValue(value);
+            builder.Append("{ ");
+            WriteString(builder, AnyTypeUrlField);
+            builder.Append(NameValueSeparator);
+            WriteString(builder, typeUrl);
+            builder.Append(PropertySeparator);
+            WriteString(builder, AnyDiagnosticValueField);
+            builder.Append(NameValueSeparator);
+            builder.Append('"');
+            builder.Append(data.ToBase64());
+            builder.Append('"');
+            builder.Append(" }");
+        }
+
+        internal static string GetTypeName(String typeUrl)
+        {
+            string[] parts = typeUrl.Split('/');
+            if (parts.Length != 2 || parts[0] != TypeUrlPrefix)
+            {
+                throw new InvalidProtocolBufferException($"Invalid type url: {typeUrl}");
+            }
+            return parts[1];
+        }
+
+        /// <summary>
+        /// Appends a number of nanoseconds to a StringBuilder. Either 0 digits are added (in which
+        /// case no "." is appended), or 3 6 or 9 digits.
+        /// </summary>
+        private static void AppendNanoseconds(StringBuilder builder, int nanos)
+        {
+            if (nanos != 0)
+            {
+                builder.Append('.');
+                // Output to 3, 6 or 9 digits.
+                if (nanos % 1000000 == 0)
+                {
+                    builder.Append((nanos / 1000000).ToString("d", CultureInfo.InvariantCulture));
+                }
+                else if (nanos % 1000 == 0)
+                {
+                    builder.Append((nanos / 1000).ToString("d", CultureInfo.InvariantCulture));
+                }
+                else
+                {
+                    builder.Append(nanos.ToString("d", CultureInfo.InvariantCulture));
+                }
+            }
+        }
+
+        private void WriteStruct(StringBuilder builder, IMessage message)
+        {
+            builder.Append("{ ");
+            IDictionary fields = (IDictionary) message.Descriptor.Fields[Struct.FieldsFieldNumber].Accessor.GetValue(message);
+            bool first = true;
+            foreach (DictionaryEntry entry in fields)
+            {
+                string key = (string) entry.Key;
+                IMessage value = (IMessage) entry.Value;
+                if (string.IsNullOrEmpty(key) || value == null)
+                {
+                    throw new InvalidOperationException("Struct fields cannot have an empty key or a null value.");
+                }
+
+                if (!first)
+                {
+                    builder.Append(PropertySeparator);
+                }
+                WriteString(builder, key);
+                builder.Append(NameValueSeparator);
+                WriteStructFieldValue(builder, value);
+                first = false;
+            }
+            builder.Append(first ? "}" : " }");
+        }
+
+        private void WriteStructFieldValue(StringBuilder builder, IMessage message)
+        {
+            var specifiedField = message.Descriptor.Oneofs[0].Accessor.GetCaseFieldDescriptor(message);
+            if (specifiedField == null)
+            {
+                throw new InvalidOperationException("Value message must contain a value for the oneof.");
+            }
+
+            object value = specifiedField.Accessor.GetValue(message);
+            
+            switch (specifiedField.FieldNumber)
+            {
+                case Value.BoolValueFieldNumber:
+                case Value.StringValueFieldNumber:
+                case Value.NumberValueFieldNumber:
+                    WriteValue(builder, value);
+                    return;
+                case Value.StructValueFieldNumber:
+                case Value.ListValueFieldNumber:
+                    // Structs and ListValues are nested messages, and already well-known types.
+                    var nestedMessage = (IMessage) specifiedField.Accessor.GetValue(message);
+                    WriteWellKnownTypeValue(builder, nestedMessage.Descriptor, nestedMessage, true);
+                    return;
+                case Value.NullValueFieldNumber:
+                    WriteNull(builder);
+                    return;
+                default:
+                    throw new InvalidOperationException("Unexpected case in struct field: " + specifiedField.FieldNumber);
+            }
+        }
+
+        internal void WriteList(StringBuilder builder, IList list)
+        {
+            builder.Append("[ ");
+            bool first = true;
+            foreach (var value in list)
+            {
+                if (!CanWriteSingleValue(value))
+                {
+                    continue;
+                }
+                if (!first)
+                {
+                    builder.Append(PropertySeparator);
+                }
+                WriteValue(builder, value);
+                first = false;
+            }
+            builder.Append(first ? "]" : " ]");
+        }
+
+        internal void WriteDictionary(StringBuilder builder, IDictionary dictionary)
+        {
+            builder.Append("{ ");
+            bool first = true;
+            // This will box each pair. Could use IDictionaryEnumerator, but that's ugly in terms of disposal.
+            foreach (DictionaryEntry pair in dictionary)
+            {
+                if (!CanWriteSingleValue(pair.Value))
+                {
+                    continue;
+                }
+                if (!first)
+                {
+                    builder.Append(PropertySeparator);
+                }
+                string keyText;
+                if (pair.Key is string)
+                {
+                    keyText = (string) pair.Key;
+                }
+                else if (pair.Key is bool)
+                {
+                    keyText = (bool) pair.Key ? "true" : "false";
+                }
+                else if (pair.Key is int || pair.Key is uint | pair.Key is long || pair.Key is ulong)
+                {
+                    keyText = ((IFormattable) pair.Key).ToString("d", CultureInfo.InvariantCulture);
+                }
+                else
+                {
+                    if (pair.Key == null)
+                    {
+                        throw new ArgumentException("Dictionary has entry with null key");
+                    }
+                    throw new ArgumentException("Unhandled dictionary key type: " + pair.Key.GetType());
+                }
+                WriteString(builder, keyText);
+                builder.Append(NameValueSeparator);
+                WriteValue(builder, pair.Value);
+                first = false;
+            }
+            builder.Append(first ? "}" : " }");
+        }
+
+        /// <summary>
+        /// Returns whether or not a singular value can be represented in JSON.
+        /// Currently only relevant for enums, where unknown values can't be represented.
+        /// For repeated/map fields, this always returns true.
+        /// </summary>
+        private bool CanWriteSingleValue(object value)
+        {
+            if (value is System.Enum)
+            {
+                return System.Enum.IsDefined(value.GetType(), value);
+            }
+            return true;
+        }
+
+        /// <summary>
+        /// Writes a string (including leading and trailing double quotes) to a builder, escaping as required.
+        /// </summary>
+        /// <remarks>
+        /// Other than surrogate pair handling, this code is mostly taken from src/google/protobuf/util/internal/json_escaping.cc.
+        /// </remarks>
+        private void WriteString(StringBuilder builder, string text)
+        {
+            builder.Append('"');
+            AppendEscapedString(builder, text);
+            builder.Append('"');
+        }
+
+        /// <summary>
+        /// Appends the given text to the string builder, escaping as required.
+        /// </summary>
+        private void AppendEscapedString(StringBuilder builder, string text)
+        {
+            for (int i = 0; i < text.Length; i++)
+            {
+                char c = text[i];
+                if (c < 0xa0)
+                {
+                    builder.Append(CommonRepresentations[c]);
+                    continue;
+                }
+                if (char.IsHighSurrogate(c))
+                {
+                    // Encountered first part of a surrogate pair.
+                    // Check that we have the whole pair, and encode both parts as hex.
+                    i++;
+                    if (i == text.Length || !char.IsLowSurrogate(text[i]))
+                    {
+                        throw new ArgumentException("String contains low surrogate not followed by high surrogate");
+                    }
+                    HexEncodeUtf16CodeUnit(builder, c);
+                    HexEncodeUtf16CodeUnit(builder, text[i]);
+                    continue;
+                }
+                else if (char.IsLowSurrogate(c))
+                {
+                    throw new ArgumentException("String contains high surrogate not preceded by low surrogate");
+                }
+                switch ((uint) c)
+                {
+                    // These are not required by json spec
+                    // but used to prevent security bugs in javascript.
+                    case 0xfeff:  // Zero width no-break space
+                    case 0xfff9:  // Interlinear annotation anchor
+                    case 0xfffa:  // Interlinear annotation separator
+                    case 0xfffb:  // Interlinear annotation terminator
+
+                    case 0x00ad:  // Soft-hyphen
+                    case 0x06dd:  // Arabic end of ayah
+                    case 0x070f:  // Syriac abbreviation mark
+                    case 0x17b4:  // Khmer vowel inherent Aq
+                    case 0x17b5:  // Khmer vowel inherent Aa
+                        HexEncodeUtf16CodeUnit(builder, c);
+                        break;
+
+                    default:
+                        if ((c >= 0x0600 && c <= 0x0603) ||  // Arabic signs
+                            (c >= 0x200b && c <= 0x200f) ||  // Zero width etc.
+                            (c >= 0x2028 && c <= 0x202e) ||  // Separators etc.
+                            (c >= 0x2060 && c <= 0x2064) ||  // Invisible etc.
+                            (c >= 0x206a && c <= 0x206f))
+                        {
+                            HexEncodeUtf16CodeUnit(builder, c);
+                        }
+                        else
+                        {
+                            // No handling of surrogates here - that's done earlier
+                            builder.Append(c);
+                        }
+                        break;
+                }
+            }
+        }
+
+        private const string Hex = "0123456789abcdef";
+        private static void HexEncodeUtf16CodeUnit(StringBuilder builder, char c)
+        {
+            builder.Append("\\u");
+            builder.Append(Hex[(c >> 12) & 0xf]);
+            builder.Append(Hex[(c >> 8) & 0xf]);
+            builder.Append(Hex[(c >> 4) & 0xf]);
+            builder.Append(Hex[(c >> 0) & 0xf]);
+        }
+
+        /// <summary>
+        /// Settings controlling JSON formatting.
+        /// </summary>
+        public sealed class Settings
+        {
+            /// <summary>
+            /// Default settings, as used by <see cref="JsonFormatter.Default"/>
+            /// </summary>
+            public static Settings Default { get; }
+
+            // Workaround for the Mono compiler complaining about XML comments not being on
+            // valid language elements.
+            static Settings()
+            {
+                Default = new Settings(false);
+            }
+
+            /// <summary>
+            /// Whether fields whose values are the default for the field type (e.g. 0 for integers)
+            /// should be formatted (true) or omitted (false).
+            /// </summary>
+            public bool FormatDefaultValues { get; }
+
+            /// <summary>
+            /// The type registry used to format <see cref="Any"/> messages.
+            /// </summary>
+            public TypeRegistry TypeRegistry { get; }
+
+            // TODO: Work out how we're going to scale this to multiple settings. "WithXyz" methods?
+
+            /// <summary>
+            /// Creates a new <see cref="Settings"/> object with the specified formatting of default values
+            /// and an empty type registry.
+            /// </summary>
+            /// <param name="formatDefaultValues"><c>true</c> if default values (0, empty strings etc) should be formatted; <c>false</c> otherwise.</param>
+            public Settings(bool formatDefaultValues) : this(formatDefaultValues, TypeRegistry.Empty)
+            {
+            }
+
+            /// <summary>
+            /// Creates a new <see cref="Settings"/> object with the specified formatting of default values
+            /// and type registry.
+            /// </summary>
+            /// <param name="formatDefaultValues"><c>true</c> if default values (0, empty strings etc) should be formatted; <c>false</c> otherwise.</param>
+            /// <param name="typeRegistry">The <see cref="TypeRegistry"/> to use when formatting <see cref="Any"/> messages.</param>
+            public Settings(bool formatDefaultValues, TypeRegistry typeRegistry)
+            {
+                FormatDefaultValues = formatDefaultValues;
+                TypeRegistry = Preconditions.CheckNotNull(typeRegistry, nameof(typeRegistry));
+            }
+        }
+    }
+}