I'm trying to using an aggregation projection stage to convert input documents to output documents with calculated properties. Using a C# expression with a definite type, e.g. 

var projection = Builders<TaskEntity>.Projection
    .Expression(e => new TaskHeader
    {
        Id = e.Id,
        Name = e.Name,
        LinkCount = e.Links.Length
    });

results in 

System.FormatException: Element 'Links' does not match any field or property of class TaskHeader.

whereas defining the projection step using a BsonDocument as below works as expected

var projection = new BsonDocumentProjectionDefinition<TaskEntity, TaskHeader>(
    BsonDocument.Parse(@"{
      Name: 1,
      LinkCount: { $size: ""$Links"" }
    }"));

Is there some way to build valid typed projections via C# expression tree when the output property names don't match the input property names? I thought this may have been fixed by the V3 Linq provider but that seems to give the same exception.