Determining whether a Type is an Anonymous Type

Question

In C# 3.0, is it possible to determine whether an instance of Type represents an Anonymous Type?

Answer 1

In methadata and CLR there is no such terms as anonymous types. Anonymous types are solely compiler feature.

Answer 2

Might be helpful to know why you want to know this. If you execute the following:

var myType = new { Name = "Bill" };
Console.Write( myType.GetType().Name  );

...you would see something like "<>f__AnonymousType0`1" output as the type name. Depending on your requirements, you may be able to assume that a type starting with <>, containing "AnonymousType" and a back quote character is what you're looking for.

Answer 3

It seems anonymous types get a DebuggerDisplayAttribute put on them where Type = "<Anonymous Type>".

Edit: But only when you compile in Debug mode. Darn.

Answer 4

Even though an anonymous type is an ordinary type, you can use some heuristics:

public static class TypeExtension {

    public static Boolean IsAnonymousType(this Type type) {
        Boolean hasCompilerGeneratedAttribute = type.GetCustomAttributes(typeof(CompilerGeneratedAttribute), false).Count() > 0;
        Boolean nameContainsAnonymousType = type.FullName.Contains("AnonymousType");
        Boolean isAnonymousType = hasCompilerGeneratedAttribute && nameContainsAnonymousType;

        return isAnonymousType;
    }
}

Another good heuristic to be used is if the class name is a valid C# name (anonymous type are generated with no valid C# class names - use regular expression for this).

Answer 5

There is no C# language construct which allows you to say "Is this an anonymous type". You can use a simple heuristic to approximate if a type is an anonymous type, but it's possible to get tricked by people hand coding IL, or using a language where such characters as > and < are valid in identifiers.

public static class TypeExtensions {
  public static bool IsAnonymousType(this Type t) {
    var name = t.Name;
    if ( name.Length < 3 ) {
      return false;
    }
    return name[0] == '<' 
        && name[1] == '>' 
        && name.IndexOf("AnonymousType", StringComparison.Ordinal) > 0;
}