Avoid creating UnknownFieldSets unless we really have to.

src/AddressBook/AddressBookProtos.cs

@@ -246,19 +246,27 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
           }
           
           public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-            pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+            pb::UnknownFieldSet.Builder unknownFields = null;
             while (true) {
               uint tag = input.ReadTag();
               switch (tag) {
                 case 0: {
+                  if (unknownFields != null) {
                     this.UnknownFields = unknownFields.Build();
+                  }
                   return this;
                 }
                 default: {
-                  if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+                  if (pb::WireFormat.IsEndGroupTag(tag)) {
+                    if (unknownFields != null) {
                       this.UnknownFields = unknownFields.Build();
+                    }
                     return this;
                   }
+                  if (unknownFields == null) {
+                    unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+                  }
+                  ParseUnknownField(input, unknownFields, extensionRegistry, tag);
                   break;
                 }
                 case 10: {
@@ -268,6 +276,9 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
                 case 16: {
                   int rawValue = input.ReadEnum();
                   if (!global::System.Enum.IsDefined(typeof(global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneType), rawValue)) {
+                    if (unknownFields == null) {
+                      unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+                    }
                     unknownFields.MergeVarintField(2, (ulong) rawValue);
                   } else {
                     Type = (global::Google.ProtocolBuffers.Examples.AddressBook.Person.Types.PhoneType) rawValue;
@@ -510,19 +521,27 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 10: {
@@ -799,19 +818,27 @@ namespace Google.ProtocolBuffers.Examples.AddressBook {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 10: {

src/ProtoGen/EnumFieldGenerator.cs

@@ -54,6 +54,9 @@ namespace Google.ProtocolBuffers.ProtoGen {
       // TODO(jonskeet): Make a more efficient way of doing this
       writer.WriteLine("int rawValue = input.ReadEnum();");
       writer.WriteLine("if (!global::System.Enum.IsDefined(typeof({0}), rawValue)) {{", TypeName);
+      writer.WriteLine("  if (unknownFields == null) {"); // First unknown field - create builder now
+      writer.WriteLine("    unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);");
+      writer.WriteLine("  }");
       writer.WriteLine("  unknownFields.MergeVarintField({0}, (ulong) rawValue);", Number);
       writer.WriteLine("} else {");
       writer.WriteLine("  {0} = ({1}) rawValue;", PropertyName, TypeName);

src/ProtoGen/MessageGenerator.cs

@@ -374,21 +374,29 @@ namespace Google.ProtocolBuffers.ProtoGen {
       writer.WriteLine();
       writer.WriteLine("public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {");
       writer.Indent();
-      writer.WriteLine("pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);");
+      writer.WriteLine("pb::UnknownFieldSet.Builder unknownFields = null;");
       writer.WriteLine("while (true) {");
       writer.Indent();
       writer.WriteLine("uint tag = input.ReadTag();");
       writer.WriteLine("switch (tag) {");
       writer.Indent();
       writer.WriteLine("case 0: {"); // 0 signals EOF / limit reached
+      writer.WriteLine("  if (unknownFields != null) {");
       writer.WriteLine("    this.UnknownFields = unknownFields.Build();");
+      writer.WriteLine("  }");
       writer.WriteLine("  return this;");
       writer.WriteLine("}");
       writer.WriteLine("default: {");
-      writer.WriteLine("  if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {");
+      writer.WriteLine("  if (pb::WireFormat.IsEndGroupTag(tag)) {");
+      writer.WriteLine("    if (unknownFields != null) {");
       writer.WriteLine("      this.UnknownFields = unknownFields.Build();");
+      writer.WriteLine("    }");
       writer.WriteLine("    return this;"); // it's an endgroup tag
       writer.WriteLine("  }");
+      writer.WriteLine("  if (unknownFields == null) {"); // First unknown field - create builder now
+      writer.WriteLine("    unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);");
+      writer.WriteLine("  }");
+      writer.WriteLine("  ParseUnknownField(input, unknownFields, extensionRegistry, tag);");
       writer.WriteLine("  break;");
       writer.WriteLine("}");
       foreach (FieldDescriptor field in sortedFields) {

src/ProtoGen/RepeatedEnumFieldGenerator.cs

@@ -79,6 +79,9 @@ namespace Google.ProtocolBuffers.ProtoGen {
       // TODO(jonskeet): Make a more efficient way of doing this
       writer.WriteLine("int rawValue = input.ReadEnum();");
       writer.WriteLine("if (!global::System.Enum.IsDefined(typeof({0}), rawValue)) {{", TypeName);
+      writer.WriteLine("  if (unknownFields == null) {"); // First unknown field - create builder now
+      writer.WriteLine("    unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);");
+      writer.WriteLine("  }");
       writer.WriteLine("  unknownFields.MergeVarintField({0}, (ulong) rawValue);", Number);
       writer.WriteLine("} else {");
       writer.WriteLine("  Add{0}(({1}) rawValue);", PropertyName, TypeName);

src/ProtocolBuffers.Test/TestProtos/UnitTestEmbedOptimizeForProtoFile.cs

@@ -214,19 +214,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 10: {

src/ProtocolBuffers.Test/TestProtos/UnitTestImportProtoFile.cs

@@ -193,19 +193,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 8: {

src/ProtocolBuffers.Test/TestProtos/UnitTestMessageSetProtoFile.cs

@@ -205,19 +205,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
           }
@@ -376,19 +384,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 10: {
@@ -594,19 +610,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 120: {
@@ -789,19 +813,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 202: {
@@ -1027,19 +1059,27 @@ namespace Google.ProtocolBuffers.TestProtos {
           }
           
           public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-            pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+            pb::UnknownFieldSet.Builder unknownFields = null;
             while (true) {
               uint tag = input.ReadTag();
               switch (tag) {
                 case 0: {
+                  if (unknownFields != null) {
                     this.UnknownFields = unknownFields.Build();
+                  }
                   return this;
                 }
                 default: {
-                  if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+                  if (pb::WireFormat.IsEndGroupTag(tag)) {
+                    if (unknownFields != null) {
                       this.UnknownFields = unknownFields.Build();
+                    }
                     return this;
                   }
+                  if (unknownFields == null) {
+                    unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+                  }
+                  ParseUnknownField(input, unknownFields, extensionRegistry, tag);
                   break;
                 }
                 case 16: {
@@ -1227,19 +1267,27 @@ namespace Google.ProtocolBuffers.TestProtos {
       }
       
       public override Builder MergeFrom(pb::CodedInputStream input, pb::ExtensionRegistry extensionRegistry) {
-        pb::UnknownFieldSet.Builder unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+        pb::UnknownFieldSet.Builder unknownFields = null;
         while (true) {
           uint tag = input.ReadTag();
           switch (tag) {
             case 0: {
+              if (unknownFields != null) {
                 this.UnknownFields = unknownFields.Build();
+              }
               return this;
             }
             default: {
-              if (!ParseUnknownField(input, unknownFields, extensionRegistry, tag)) {
+              if (pb::WireFormat.IsEndGroupTag(tag)) {
+                if (unknownFields != null) {
                   this.UnknownFields = unknownFields.Build();
+                }
                 return this;
               }
+              if (unknownFields == null) {
+                unknownFields = pb::UnknownFieldSet.CreateBuilder(this.UnknownFields);
+              }
+              ParseUnknownField(input, unknownFields, extensionRegistry, tag);
               break;
             }
             case 11: {

src/ProtocolBuffers/GeneratedBuilder.cs

@@ -158,10 +158,12 @@ namespace Google.ProtocolBuffers {
     }
 
     public override TBuilder MergeUnknownFields(UnknownFieldSet unknownFields) {
+      if (unknownFields != UnknownFieldSet.DefaultInstance) {
         TMessage result = MessageBeingBuilt;
         result.SetUnknownFields(UnknownFieldSet.CreateBuilder(result.UnknownFields)
             .MergeFrom(unknownFields)
             .Build());
+      }
       return ThisBuilder;
     }
 

src/ProtocolBuffers/WireFormat.cs

@@ -91,6 +91,10 @@ namespace Google.ProtocolBuffers {
       return (WireType) (tag & TagTypeMask);
     }
 
+    public static bool IsEndGroupTag(uint tag) {
+      return (WireType)(tag & TagTypeMask) == WireType.EndGroup;
+    }
+
     /// <summary>
     /// Given a tag value, determines the field number (the upper 29 bits).
     /// </summary>