Jef Claes

On software and life

21 May 2011

Checking for anonymous types

Because I blogged about anonymous types last month, I thought following method would also make an interesting post.

private static bool IsAnonymousType(Type type) {
    Debug.Assert(type != null, "Type should not be null");

    // HACK: The only way to detect anonymous types right now.
    return Attribute.IsDefined(type, typeof(CompilerGeneratedAttribute), false)
            && type.IsGenericType 
            && type.Name.Contains("AnonymousType")
            && (type.Name.StartsWith("<>", StringComparison.OrdinalIgnoreCase) ||
                type.Name.StartsWith("VB$", StringComparison.OrdinalIgnoreCase))
            && (type.Attributes & TypeAttributes.NotPublic) == TypeAttributes.NotPublic;

For a type to be anonymous:

  • It should be marked with the CompilerGenerated attribute
  • It should be a generic type
  • Its name should contain “AnonymousType”
  • Its name should start with “<>” or “VB$”
  • It shouldn’t be publicly accessible

A little fun fact is that the VB and C# compiler generate different type names. The C# compiler makes the type name start with “<>” and the VB compiler uses “VB$”. Both smart safeguards, because the compiler doesn’t allow us to use “<>” or “$” while defining type names. I find the C# way a tad more elegant though.

I stumbled upon this beauty while browsing the [ASP.NET MVC source] ( (System.Web.Helpers.ObjectVisitor). Because there is no direct way to detect anonymous types yet, I’m pretty sure this is the best implementation out there.