How do create a Generic Object using Reflection

Question

First let me say, that what I want to do is get the value of a property in a generic class that may be overriden by class that inherits from it. Think of it in the base class as a default value, that the inheritor of the class can override to set their own Default value.

I have tried to use reflection directly on the type, using the System.Reflection.FieldInfo.GetValue but this does not work for classes with generic types. So I think that I need to instantiate the class to be able to see what the value is.

The "types" I have I retrieved by reading the Dlls in the bin and using Reflection to find the types that inherit from my interface.

I am using .NET 4.5

here is documentation that seems like it explains exactly what I need to do http://msdn.microsoft.com/en-us/library/b8ytshk6.aspx

In this documentation the only difference I can see is how we got our types, I looked in the bin for types and they simply called typeof(), Since types are so complex it seems this may be a likely mis-match but I cannot see what is missing(if anything)

foreach (var item in types)
{
    var ts = item.GetField("DefaultTimeToExpire");
    Type[] typeArguments = item.GetGenericArguments();
    if (ts != null)
    {
        var t = item.MakeGenericType(typeArguments);
        var obj = Activator.CreateInstance(t);
        var timespan = obj.DefaultTimeToExpire;
        subscriberInfos.Add(new Tuple<string, Type, TimeSpan>(item.Name, item, timespan));
    }
}

I am calling GetField to look for Items that have a field "DefaultTimeToExpire" so far this part works well to find the type I need. Next I call GetGenericArguments which returns an expected array of the type Arguments. then I call MakeGenericType and finally Create instance wich gives me the error message

"Cannot create an instance of BusinessLogic.TestSubscriberXXX`1[Message] because Type.ContainsGenericParameters is true."

This looks like exactly what I am supposed to do. Thanks

Answer 1

In order to instantiate a generic type, you need to know the actual values (types) that should be substituted for its type parameters. The GetGenericArguments() method, being a form of reflection, only gives you the type arguments, not their actual values. The values are up to you... that is the entire point of generics.

If item is a type like List<T> then item.GetGenericArguments() will return an array containing a fake "type" representing the type parameter T (with its IsGenericParameter property set to true). Therefore, passing that parameter type back into item.MakeGenericType() will simply create another open generic type equivalent to the original. To close the generic type so that it can be instantiated you need to provide an actual (non-parameter) type argument, such as int.

For example, typeof(List<>).MakeGenericType(typeof(int)) will return typeof(List<int>), while typeof(List<>).MakeGenericType(typeof(List<>).GetGenericArguments()) will simply return typeof(List<>) again. This is what is happening in your code.

I'm sorry if that is a bit opaque, I don't know how else to explain it. The bottom line is that a type like List<T> is only useful if you have a type you want to substitute in place of T.