Remove dead HasValue code for ExtensionValue and add null-checks to ExtensionSet.Set

csharp/src/Google.Protobuf/ExtensionSet.cs

@@ -115,6 +115,8 @@ namespace Google.Protobuf
         /// </summary>
         public static void Set<TTarget, TValue>(ref ExtensionSet<TTarget> set, Extension<TTarget, TValue> extension, TValue value) where TTarget : IExtendableMessage<TTarget>
         {
+            ProtoPreconditions.CheckNotNullUnconstrained(value, nameof(value));
+
             IExtensionValue extensionValue;
             if (set == null)
             {

csharp/src/Google.Protobuf/ExtensionValue.cs

@@ -47,7 +47,6 @@ namespace Google.Protobuf
 
     internal sealed class ExtensionValue<T> : IExtensionValue
     {
-        private bool hasValue;
         private T field;
         private FieldCodec<T> codec;
 
@@ -59,10 +58,6 @@ namespace Google.Protobuf
 
         public int CalculateSize()
         {
-            if (!hasValue)
-            {
-                return 0;
-            }
             return codec.CalculateSizeWithTag(field);
         }
 
@@ -70,7 +65,6 @@ namespace Google.Protobuf
         {
             return new ExtensionValue<T>(codec)
             {
-                hasValue = hasValue,
                 field = field is IDeepCloneable<T> ? (field as IDeepCloneable<T>).Clone() : field
             };
         }
@@ -82,7 +76,6 @@ namespace Google.Protobuf
 
             return other is ExtensionValue<T>
                 && codec.Equals((other as ExtensionValue<T>).codec)
-                && hasValue.Equals((other as ExtensionValue<T>).hasValue)
                 && Equals(field, (other as ExtensionValue<T>).field);
             // we check for equality in the codec since we could have equal field values however the values could be written in different ways
         }
@@ -92,7 +85,6 @@ namespace Google.Protobuf
             unchecked
             {
                 int hash = 17;
-                hash = hash * 31 + hasValue.GetHashCode();
                 hash = hash * 31 + field.GetHashCode();
                 hash = hash * 31 + codec.GetHashCode();
                 return hash;
@@ -101,7 +93,6 @@ namespace Google.Protobuf
 
         public void MergeFrom(CodedInputStream input)
         {
-            hasValue = true;
             codec.ValueMerger(input, ref field);
         }
 
@@ -109,24 +100,18 @@ namespace Google.Protobuf
         {
             if (value is ExtensionValue<T>)
             {
-                var extensionValue = value as ExtensionValue<T>;
-                if (extensionValue.hasValue)
-                {
-                    hasValue |= codec.FieldMerger(ref field, extensionValue.field);
-                }
+                var extensionValue = value as ExtensionValue<T>;
+                codec.FieldMerger(ref field, extensionValue.field);
             }
         }
 
         public void WriteTo(CodedOutputStream output)
         {
-            if (hasValue)
+            output.WriteTag(codec.Tag);
+            codec.ValueWriter(output, field);
+            if (codec.EndTag != 0)
             {
-                output.WriteTag(codec.Tag);
-                codec.ValueWriter(output, field);
-                if (codec.EndTag != 0)
-                {
-                    output.WriteTag(codec.EndTag);
-                }
+                output.WriteTag(codec.EndTag);
             }
         }
 
@@ -134,15 +119,19 @@ namespace Google.Protobuf
 
         public void SetValue(T value)
         {
-            hasValue = true;
             field = value;
         }
 
-        public bool HasValue => hasValue;
-
         public bool IsInitialized()
         {
-            return HasValue && field is IMessage && (field as IMessage).IsInitialized();
+            if (field is IMessage)
+            {
+                return (field as IMessage).IsInitialized();
+            }
+            else
+            {
+                return true;
+            }
         }
     }
 

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

@@ -254,11 +254,8 @@ namespace Google.Protobuf.Reflection
                 if (extensionValue is ExtensionValue<T>)
                 {
                     ExtensionValue<T> single = extensionValue as ExtensionValue<T>;
-                    if (single.HasValue)
-                    {
-                        value = single.GetValue();
-                        return true;
-                    }
+                    value = single.GetValue();
+                    return true;
                 }
                 else if (extensionValue is RepeatedExtensionValue<T>)
                 {
@@ -279,11 +276,8 @@ namespace Google.Protobuf.Reflection
                         var typeArgs = typeInfo.GenericTypeArguments;
                         if (typeArgs.Length == 1 && typeArgs[0].GetTypeInfo().IsEnum)
                         {
-                            if ((bool)typeInfo.GetDeclaredProperty(nameof(ExtensionValue<T>.HasValue)).GetValue(extensionValue))
-                            {
-                                value = (T)typeInfo.GetDeclaredMethod(nameof(ExtensionValue<T>.GetValue)).Invoke(extensionValue, EmptyParameters);
-                                return true;
-                            }
+                            value = (T)typeInfo.GetDeclaredMethod(nameof(ExtensionValue<T>.GetValue)).Invoke(extensionValue, EmptyParameters);
+                            return true;
                         }
                     }
                     else if (type.GetGenericTypeDefinition() == typeof(RepeatedExtensionValue<>))