This interface ensures that all the child controls of your control get a unique ID.
This way your custom control can keep track of its constituent controls.

Source URL: http://www.bipinjoshi.net/articles/31cf8d3d-7f71-46f4-b847-3eea47a7f2f5.aspx 

Use this test:

 if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(Nullable<>)) {…}

 Source: http://msdn2.microsoft.com/en-us/library/ms366789.aspx

Or

Using Reflection to Determine whether an Type is Nullable And Get the underlying Type