There might be duplicated enum values when allow_alias is true. Add PreferredAlias into OriginalNameAttribute to remove the duplication (#2727)

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

@@ -66,6 +66,18 @@ namespace Google.Protobuf
             AssertJson("{ 'c': 0 }", formatter.Format(new ForeignMessage()));
         }
 
+        [Test]
+        public void EnumAllowAlias()
+        {
+            var message = new TestEnumAllowAlias
+            {
+                Value = TestEnumWithDupValue.Foo2,
+            };
+            var actualText = JsonFormatter.Default.Format(message);
+            var expectedText = "{ 'value': 'FOO1' }";
+            AssertJson(expectedText, actualText);
+        }
+
         [Test]
         public void AllSingleFields()
         {

csharp/src/Google.Protobuf.Test/TestProtos/UnittestProto3.cs

@@ -85,70 +85,72 @@ namespace Google.Protobuf.TestProtos {
             "ZXN0TXV0dWFsUmVjdXJzaW9uQRIzCgJiYhgBIAEoCzInLnByb3RvYnVmX3Vu",
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-            "ZF9tZXNzYWdlGMgBIAEoCzIzLnByb3RvYnVmX3VuaXR0ZXN0LlRlc3RGaWVs",
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-            "CgRkYXRhGAEgASgEIhsKC0Jvb2xNZXNzYWdlEgwKBGRhdGEYASABKAgicwoJ",
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-            "ASgJSAASNgoLZm9vX21lc3NhZ2UYAyABKAsyHy5wcm90b2J1Zl91bml0dGVz",
-            "dC5UZXN0QWxsVHlwZXNIAEIFCgNmb28iqgMKD1Rlc3RQYWNrZWRUeXBlcxIY",
-            "CgxwYWNrZWRfaW50MzIYWiADKAVCAhABEhgKDHBhY2tlZF9pbnQ2NBhbIAMo",
-            "A0ICEAESGQoNcGFja2VkX3VpbnQzMhhcIAMoDUICEAESGQoNcGFja2VkX3Vp",
-            "bnQ2NBhdIAMoBEICEAESGQoNcGFja2VkX3NpbnQzMhheIAMoEUICEAESGQoN",
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-            "UlNFX0IQpucDEg8KCFNQQVJTRV9DELKxgAYSFQoIU1BBUlNFX0QQ8f//////",
-            "////ARIVCghTUEFSU0VfRRC03vz///////8BEgwKCFNQQVJTRV9HEAIymQEK",
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-            "+AEBqgIaR29vZ2xlLlByb3RvYnVmLlRlc3RQcm90b3NiBnByb3RvMw=="));
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+            "LlByb3RvYnVmLlRlc3RQcm90b3NiBnByb3RvMw=="));
       descriptor = pbr::FileDescriptor.FromGeneratedCode(descriptorData,
           new pbr::FileDescriptor[] { global::Google.Protobuf.TestProtos.UnittestImportProto3Reflection.Descriptor, },
           new pbr::GeneratedClrTypeInfo(new[] {typeof(global::Google.Protobuf.TestProtos.ForeignEnum), typeof(global::Google.Protobuf.TestProtos.TestEnumWithDupValue), typeof(global::Google.Protobuf.TestProtos.TestSparseEnum), }, new pbr::GeneratedClrTypeInfo[] {
@@ -162,6 +164,7 @@ namespace Google.Protobuf.TestProtos {
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestRecursiveMessage), global::Google.Protobuf.TestProtos.TestRecursiveMessage.Parser, new[]{ "A", "I" }, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestMutualRecursionA), global::Google.Protobuf.TestProtos.TestMutualRecursionA.Parser, new[]{ "Bb" }, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestMutualRecursionB), global::Google.Protobuf.TestProtos.TestMutualRecursionB.Parser, new[]{ "A", "OptionalInt32" }, null, null, null),
+            new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestEnumAllowAlias), global::Google.Protobuf.TestProtos.TestEnumAllowAlias.Parser, new[]{ "Value" }, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestCamelCaseFieldNames), global::Google.Protobuf.TestProtos.TestCamelCaseFieldNames.Parser, new[]{ "PrimitiveField", "StringField", "EnumField", "MessageField", "RepeatedPrimitiveField", "RepeatedStringField", "RepeatedEnumField", "RepeatedMessageField" }, null, null, null),
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestFieldOrderings), global::Google.Protobuf.TestProtos.TestFieldOrderings.Parser, new[]{ "MyString", "MyInt", "MyFloat", "SingleNestedMessage" }, null, null, new pbr::GeneratedClrTypeInfo[] { new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.TestFieldOrderings.Types.NestedMessage), global::Google.Protobuf.TestProtos.TestFieldOrderings.Types.NestedMessage.Parser, new[]{ "Oo", "Bb" }, null, null, null)}),
             new pbr::GeneratedClrTypeInfo(typeof(global::Google.Protobuf.TestProtos.SparseEnumMessage), global::Google.Protobuf.TestProtos.SparseEnumMessage.Parser, new[]{ "SparseEnum" }, null, null, null),
@@ -206,8 +209,8 @@ namespace Google.Protobuf.TestProtos {
     [pbr::OriginalName("FOO1")] Foo1 = 1,
     [pbr::OriginalName("BAR1")] Bar1 = 2,
     [pbr::OriginalName("BAZ")] Baz = 3,
-    [pbr::OriginalName("FOO2")] Foo2 = 1,
-    [pbr::OriginalName("BAR2")] Bar2 = 2,
+    [pbr::OriginalName("FOO2", PreferredAlias = false)] Foo2 = 1,
+    [pbr::OriginalName("BAR2", PreferredAlias = false)] Bar2 = 2,
   }
 
   /// <summary>
@@ -2939,6 +2942,123 @@ namespace Google.Protobuf.TestProtos {
 
   }
 
+  public sealed partial class TestEnumAllowAlias : pb::IMessage<TestEnumAllowAlias> {
+    private static readonly pb::MessageParser<TestEnumAllowAlias> _parser = new pb::MessageParser<TestEnumAllowAlias>(() => new TestEnumAllowAlias());
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public static pb::MessageParser<TestEnumAllowAlias> Parser { get { return _parser; } }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public static pbr::MessageDescriptor Descriptor {
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[10]; }
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    pbr::MessageDescriptor pb::IMessage.Descriptor {
+      get { return Descriptor; }
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public TestEnumAllowAlias() {
+      OnConstruction();
+    }
+
+    partial void OnConstruction();
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public TestEnumAllowAlias(TestEnumAllowAlias other) : this() {
+      value_ = other.value_;
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public TestEnumAllowAlias Clone() {
+      return new TestEnumAllowAlias(this);
+    }
+
+    /// <summary>Field number for the "value" field.</summary>
+    public const int ValueFieldNumber = 1;
+    private global::Google.Protobuf.TestProtos.TestEnumWithDupValue value_ = 0;
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public global::Google.Protobuf.TestProtos.TestEnumWithDupValue Value {
+      get { return value_; }
+      set {
+        value_ = value;
+      }
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public override bool Equals(object other) {
+      return Equals(other as TestEnumAllowAlias);
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public bool Equals(TestEnumAllowAlias other) {
+      if (ReferenceEquals(other, null)) {
+        return false;
+      }
+      if (ReferenceEquals(other, this)) {
+        return true;
+      }
+      if (Value != other.Value) return false;
+      return true;
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public override int GetHashCode() {
+      int hash = 1;
+      if (Value != 0) hash ^= Value.GetHashCode();
+      return hash;
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public override string ToString() {
+      return pb::JsonFormatter.ToDiagnosticString(this);
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public void WriteTo(pb::CodedOutputStream output) {
+      if (Value != 0) {
+        output.WriteRawTag(8);
+        output.WriteEnum((int) Value);
+      }
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public int CalculateSize() {
+      int size = 0;
+      if (Value != 0) {
+        size += 1 + pb::CodedOutputStream.ComputeEnumSize((int) Value);
+      }
+      return size;
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public void MergeFrom(TestEnumAllowAlias other) {
+      if (other == null) {
+        return;
+      }
+      if (other.Value != 0) {
+        Value = other.Value;
+      }
+    }
+
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
+    public void MergeFrom(pb::CodedInputStream input) {
+      uint tag;
+      while ((tag = input.ReadTag()) != 0) {
+        switch(tag) {
+          default:
+            input.SkipLastField();
+            break;
+          case 8: {
+            value_ = (global::Google.Protobuf.TestProtos.TestEnumWithDupValue) input.ReadEnum();
+            break;
+          }
+        }
+      }
+    }
+
+  }
+
   /// <summary>
   /// Test message with CamelCase field names.  This violates Protocol Buffer
   /// standard style.
@@ -2950,7 +3070,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[10]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[11]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -3243,7 +3363,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[11]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[12]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -3607,7 +3727,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[12]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[13]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -3727,7 +3847,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[13]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[14]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -3844,7 +3964,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[14]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[15]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -3953,7 +4073,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[15]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[16]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4070,7 +4190,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[16]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[17]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4190,7 +4310,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[17]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[18]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4307,7 +4427,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[18]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[19]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4424,7 +4544,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[19]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[20]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4541,7 +4661,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[20]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[21]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4658,7 +4778,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[21]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[22]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4778,7 +4898,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[22]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[23]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -4990,7 +5110,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[23]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[24]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -5377,7 +5497,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[24]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[25]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -5760,7 +5880,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[25]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[26]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -5989,7 +6109,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[26]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[27]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6112,7 +6232,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[27]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[28]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6201,7 +6321,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[28]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[29]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6290,7 +6410,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[29]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[30]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6379,7 +6499,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[30]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[31]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6468,7 +6588,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[31]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[32]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
@@ -6557,7 +6677,7 @@ namespace Google.Protobuf.TestProtos {
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]
     public static pbr::MessageDescriptor Descriptor {
-      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[32]; }
+      get { return global::Google.Protobuf.TestProtos.UnittestProto3Reflection.Descriptor.MessageTypes[33]; }
     }
 
     [global::System.Diagnostics.DebuggerNonUserCodeAttribute]

csharp/src/Google.Protobuf/JsonFormatter.cs

@@ -835,6 +835,9 @@ namespace Google.Protobuf
             // TODO: Consider adding functionality to TypeExtensions to avoid this difference.
             private static Dictionary<object, string> GetNameMapping(System.Type enumType) =>
                 enumType.GetFields(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static)
+                    .Where(f => (f.GetCustomAttributes(typeof(OriginalNameAttribute), false)
+                                 .FirstOrDefault() as OriginalNameAttribute)
+                                 ?.PreferredAlias ?? true)
                     .ToDictionary(f => f.GetValue(null),
                                   f => (f.GetCustomAttributes(typeof(OriginalNameAttribute), false)
                                         .FirstOrDefault() as OriginalNameAttribute)
@@ -844,6 +847,8 @@ namespace Google.Protobuf
             private static Dictionary<object, string> GetNameMapping(System.Type enumType) =>
                 enumType.GetTypeInfo().DeclaredFields
                     .Where(f => f.IsStatic)
+                    .Where(f => f.GetCustomAttributes<OriginalNameAttribute>()
+                                 .FirstOrDefault()?.PreferredAlias ?? true)
                     .ToDictionary(f => f.GetValue(null),
                                   f => f.GetCustomAttributes<OriginalNameAttribute>()
                                         .FirstOrDefault()

csharp/src/Google.Protobuf/Reflection/OriginalNameAttribute.cs

@@ -46,6 +46,11 @@ namespace Google.Protobuf.Reflection
         /// </summary>
         public string Name { get; set; }
 
+        /// <summary>
+        /// If the name is preferred in the .proto file.
+        /// </summary>
+        public bool PreferredAlias { get; set; }
+
         /// <summary>
         /// Constructs a new attribute instance for the given name.
         /// </summary>
@@ -53,6 +58,8 @@ namespace Google.Protobuf.Reflection
         public OriginalNameAttribute(string name)
         {
             Name = ProtoPreconditions.CheckNotNull(name, nameof(name));
+            PreferredAlias = true;
         }
+	
     }
-}
+}

src/google/protobuf/compiler/csharp/csharp_enum.cc

@@ -65,6 +65,7 @@ void EnumGenerator::Generate(io::Printer* printer) {
                  "name", descriptor_->name());
   printer->Indent();
   std::set<string> used_names;
+  std::set<int> used_number;
   for (int i = 0; i < descriptor_->value_count(); i++) {
       WriteEnumValueDocComment(printer, descriptor_->value(i));
       string original_name = descriptor_->value(i)->name();
@@ -76,10 +77,18 @@ void EnumGenerator::Generate(io::Printer* printer) {
           << ") in " << descriptor_->name() << "; adding underscore to distinguish";
         name += "_";
       }
-      printer->Print("[pbr::OriginalName(\"$original_name$\")] $name$ = $number$,\n",
-         "original_name", original_name,
-         "name", name,
-         "number", SimpleItoa(descriptor_->value(i)->number()));         
+      int number = descriptor_->value(i)->number();
+      if (!used_number.insert(number).second) {
+          printer->Print("[pbr::OriginalName(\"$original_name$\", PreferredAlias = false)] $name$ = $number$,\n",
+             "original_name", original_name,
+             "name", name,
+             "number", SimpleItoa(number));
+      } else {
+          printer->Print("[pbr::OriginalName(\"$original_name$\")] $name$ = $number$,\n",
+             "original_name", original_name,
+             "name", name,
+             "number", SimpleItoa(number));
+      }
   }
   printer->Outdent();
   printer->Print("}\n");

src/google/protobuf/unittest_proto3.proto

@@ -200,6 +200,9 @@ message TestMutualRecursionB {
   int32 optional_int32 = 2;
 }
 
+message TestEnumAllowAlias {
+  TestEnumWithDupValue value = 1;
+}
 
 // Test an enum that has multiple values with the same number.
 enum TestEnumWithDupValue {