I found a bug using the generated Csharp MergeFrom method

[cluo0618]

Hi
I found a bug using the generated Csharp MergeFrom method. If my data contains mapfield, mergefrom will add other mapfield directly. If there are duplicates, it will always report : ArgumentException: Key already exists in map. I think the merge of the map should be changed to containskey and then directly replace the element.

`public void MergeFrom(LineComponent other) {

  if (other == null) {
    return;
  }
  if (other.target_ != null) {
    if (target_ == null) {
      Target = new global::Protocol.LineInfoComponent(); 
    }
    Target.MergeFrom(other.Target);
  }
  lines_.Add(other.lines_);
  _unknownFields = pb::UnknownFieldSet.MergeFrom(_unknownFields, other._unknownFields);
}`

this lines is a mapfield type,using add directly on the opposite mapfield type will always report an error.

[jskeet]

Right, it looks like we should be using a specific merge method for a MapField<,>. The language guide specifically says:

When parsing from the wire or when merging, if there are duplicate map keys the last key seen is used. When parsing a map from text format, parsing may fail if there are duplicate keys.

I'll come up with a unit test to reproduce this, then fix it. It's likely to need both a generator and a support library change.

[cluo0618]

thanks