Replace StringBuilder with TextWriter in JsonFormatter

csharp/src/Google.Protobuf.Test/GeneratedMessageTest.cs

@@ -710,5 +710,14 @@ namespace Google.Protobuf
             Assert.AreEqual("{ \"singleForeignMessage\": { \"c\": 16, \"@cInHex\": \"10\" } }", message.ToString());
             Assert.AreEqual("{ \"singleForeignMessage\": { \"c\": 16 } }", JsonFormatter.Default.Format(message));
         }
+
+        [Test]
+        public void CustomDiagnosticMessage_DirectToTextWriterCall()
+        {
+            var message = new ForeignMessage { C = 31 };
+            var writer = new StringWriter();
+            JsonFormatter.Default.Format(message, writer);
+            Assert.AreEqual("{ \"c\": 31 }", writer.ToString());
+        }
     }
 }

csharp/src/Google.Protobuf/Collections/MapField.cs

@@ -34,6 +34,7 @@ using Google.Protobuf.Reflection;
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Text;
 using Google.Protobuf.Compatibility;
@@ -474,9 +475,9 @@ namespace Google.Protobuf.Collections
         /// </summary>
         public override string ToString()
         {
-            var builder = new StringBuilder();
-            JsonFormatter.Default.WriteDictionary(builder, this);
-            return builder.ToString();
+            var writer = new StringWriter();
+            JsonFormatter.Default.WriteDictionary(writer, this);
+            return writer.ToString();
         }
 
         #region IDictionary explicit interface implementation

csharp/src/Google.Protobuf/Collections/RepeatedField.cs

@@ -33,6 +33,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.IO;
 using System.Text;
 
 namespace Google.Protobuf.Collections
@@ -474,9 +475,9 @@ namespace Google.Protobuf.Collections
         /// </summary>
         public override string ToString()
         {
-            var builder = new StringBuilder();
-            JsonFormatter.Default.WriteList(builder, this);
-            return builder.ToString();
+            var writer = new StringWriter();
+            JsonFormatter.Default.WriteList(writer, this);
+            return writer.ToString();
         }
 
         /// <summary>

csharp/src/Google.Protobuf/WellKnownTypes/FieldMaskPartial.cs

@@ -33,6 +33,7 @@
 using System;
 using System.Collections;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using System.Text;
 
@@ -57,19 +58,19 @@ namespace Google.Protobuf.WellKnownTypes
             var firstInvalid = paths.FirstOrDefault(p => !ValidatePath(p));
             if (firstInvalid == null)
             {
-                var builder = new StringBuilder();
-                JsonFormatter.WriteString(builder, string.Join(",", paths.Select(JsonFormatter.ToCamelCase)));
-                return builder.ToString();
+                var writer = new StringWriter();
+                JsonFormatter.WriteString(writer, string.Join(",", paths.Select(JsonFormatter.ToCamelCase)));
+                return writer.ToString();
             }
             else
             {
                 if (diagnosticOnly)
                 {
-                    var builder = new StringBuilder();
-                    builder.Append("{ \"@warning\": \"Invalid FieldMask\", \"paths\": ");
-                    JsonFormatter.Default.WriteList(builder, (IList) paths);
-                    builder.Append(" }");
-                    return builder.ToString();
+                    var writer = new StringWriter();
+                    writer.Write("{ \"@warning\": \"Invalid FieldMask\", \"paths\": ");
+                    JsonFormatter.Default.WriteList(writer, (IList)paths);
+                    writer.Write(" }");
+                    return writer.ToString();
                 }
                 else
                 {