Provide simple access to descriptor declarations in C#

This is primarily for access to comments, which would be expected to be available in a protoc plugin.

The implementation has two fiddly aspects:

- We use a Lazy<T> to avoid building the map before cross-linking. An alternative would be to crosslink at the end of the constructor, and remove the calls to CrossLink elsewhere. This would be generally better IMO, but deviate from the Java code.
- The casts to IReadOnlyList<DescriptorBase> are unfortunate. They'll always work, because these lists are always ReadOnlyCollection<T> for a descriptor type... but we can't use IList<DescriptorBase> as that's not covariant, and it's annoyingly fiddly to change the field to be of type ReadOnlyCollection<T>.

1 files changed, 15 insertions, 0 deletions

diff --git a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
index dbb6768b..03fd63ce 100644
--- a/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
+++ b/csharp/src/Google.Protobuf/Reflection/MessageDescriptor.cs
@@ -115,6 +115,21 @@ namespace Google.Protobuf.Reflection
         /// </summary>
         public override string Name => Proto.Name;
 
+        internal override IReadOnlyList<DescriptorBase> GetNestedDescriptorListForField(int fieldNumber)
+        {
+            switch (fieldNumber)
+            {
+                case DescriptorProto.FieldFieldNumber:
+                    return (IReadOnlyList<DescriptorBase>) fieldsInDeclarationOrder;
+                case DescriptorProto.NestedTypeFieldNumber:
+                    return (IReadOnlyList<DescriptorBase>) NestedTypes;
+                case DescriptorProto.EnumTypeFieldNumber:
+                    return (IReadOnlyList<DescriptorBase>) EnumTypes;
+                default:
+                    return null;
+            }
+        }
+
         internal DescriptorProto Proto { get; }
 
         /// <summary>