Question

I’m having a problem instantiating an anonymous type in my code.

For some reason, TResponse response = default(TResponse); returns null, even though TResponse has a constructor for it.

Am I being dumb?!

Class:

public class MyClass
{
  public MyResponse GetResponse(MyRequest request)
  {
    return Service<MyRequest, MyResponse>.MakeRequest(
      request,
      delegate() {
        return AnotherService.GetRequest(request);
      }
    );
  }
}

Service class

public static class Service<TRequest, TResponse>
  where TRequest : IRequest
  where TResponse : IResponse
{
  public delegate TResponse UseDelegate();

  public TResponse MakeRequest(TRequest request, UseDelegate codeBlock)
  {
    TResponse response = default(TResponse); // <-- Returns nulll

    response = codeBlock();

    return response;
  }
}

Answer 1

As Brandon has said, default returns null for any reference type.

However, I don’t see why you’re using it at all – the value you’ve assigned will be overwritten by the return value of codeBlock() anyway. In other words, you can change your MakeRequest method to just:

public TResponse MakeRequest(TRequest request, UseDelegate codeBlock)
{
    TResponse response = codeBlock();
    return response;
}

or even:

public TResponse MakeRequest(TRequest request, UseDelegate codeBlock)
{
    return codeBlock();
}

I’m assuming that in reality there’s some more code there… but if you really want to call a parameterless constructor, you can constrain TResponse with:

where TResponse : IResponse, new()

and then use:

TResponse response = new TResponse();

That way you’ll get a compile-time guarantee that TResponse has a parameterless constructor; just using Activator.CreateInstance(typeof(TResponse)) without a constraint on TResponse would work, but would delay finding out about the problem where you tried to use response type which didn’t have a parameterless constructor.

Furthermore, I don’t see any anonymous types in your code – the only way an anonymous type would have a parameterless constructor would be if you used:

new {}

which would be somewhat pointless.