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diff --git a/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs
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+++ b/csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs
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+#region Copyright notice and license
+// Protocol Buffers - Google's data interchange format
+// Copyright 2016 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.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+
+namespace Google.Protobuf.WellKnownTypes
+{
+    // Manually-written partial class for the FieldMask well-known type.
+    public partial class FieldMask : ICustomDiagnosticMessage
+    {
+        /// <summary>
+        /// Converts a timestamp  specified in seconds/nanoseconds to a string.
+        /// </summary>
+        /// <remarks>
+        /// If the value is a normalized duration in the range described in <c>field_mask.proto</c>,
+        /// <paramref name="diagnosticOnly"/> is ignored. Otherwise, if the parameter is <c>true</c>,
+        /// a JSON object with a warning is returned; if it is <c>false</c>, an <see cref="InvalidOperationException"/> is thrown.
+        /// </remarks>
+        /// <param name="paths">Paths in the field mask</param>
+        /// <param name="diagnosticOnly">Determines the handling of non-normalized values</param>
+        /// <exception cref="InvalidOperationException">The represented duration is invalid, and <paramref name="diagnosticOnly"/> is <c>false</c>.</exception>
+        internal static string ToJson(IList<string> paths, bool diagnosticOnly)
+        {
+            var firstInvalid = paths.FirstOrDefault(p => !ValidatePath(p));
+            if (firstInvalid == null)
+            {
+                var writer = new StringWriter();
+                JsonFormatter.WriteString(writer, string.Join(",", paths.Select(JsonFormatter.ToCamelCase)));
+                return writer.ToString();
+            }
+            else
+            {
+                if (diagnosticOnly)
+                {
+                    var writer = new StringWriter();
+                    writer.Write("{ \"@warning\": \"Invalid FieldMask\", \"paths\": ");
+                    JsonFormatter.Default.WriteList(writer, (IList)paths);
+                    writer.Write(" }");
+                    return writer.ToString();
+                }
+                else
+                {
+                    throw new InvalidOperationException($"Invalid field mask to be converted to JSON: {firstInvalid}");
+                }
+            }
+        }
+
+        /// <summary>
+        /// Camel-case converter with added strictness for field mask formatting.
+        /// </summary>
+        /// <exception cref="InvalidOperationException">The field mask is invalid for JSON representation</exception>
+        private static bool ValidatePath(string input)
+        {
+            for (int i = 0; i < input.Length; i++)
+            {
+                char c = input[i];
+                if (c >= 'A' && c <= 'Z')
+                {
+                    return false;
+                }
+                if (c == '_' && i < input.Length - 1)
+                {
+                    char next = input[i + 1];
+                    if (next < 'a' || next > 'z')
+                    {
+                        return false;
+                    }
+                }
+            }
+            return true;
+        }
+
+        /// <summary>
+        /// Returns a string representation of this <see cref="FieldMask"/> for diagnostic purposes.
+        /// </summary>
+        /// <remarks>
+        /// Normally the returned value will be a JSON string value (including leading and trailing quotes) but
+        /// when the value is non-normalized or out of range, a JSON object representation will be returned
+        /// instead, including a warning. This is to avoid exceptions being thrown when trying to
+        /// diagnose problems - the regular JSON formatter will still throw an exception for non-normalized
+        /// values.
+        /// </remarks>
+        /// <returns>A string representation of this value.</returns>
+        public string ToDiagnosticString()
+        {
+            return ToJson(Paths, true);
+        }
+    }
+}