How do I get the underlying type of a Generic List?

The Type.GetGenericArguments() method will let you determine the underlying type of the elements of a list object at runtime. See this article for more information.

http://msdn.microsoft.com/en-us/library/system.type.getgenericarguments.aspx

For example:

if (t.IsGenericType)

{
    // If this is a generic type, display the type arguments.
    //
    Type[] typeArguments = t.GetGenericArguments();
    Console.WriteLine("\tList type arguments ({0}):", typeArguments.Length);

    foreach (Type tParam in typeArguments)
    {
        // If this is a type parameter, display its
        // position.
        //

        if (tParam.IsGenericParameter)
        {
            Console.WriteLine("\t\t{0}\t(unassigned - parameter position {1})",tParam,
  tParam.GenericParameterPosition);
        }
        else
        {
            Console.WriteLine("\t\t{0}", tParam);
        }
    }
}