Reflection over Type Constraints

Question

In class and method definitions, it's possible to add type constraints like where T : IFoo.

Is it possible to reflect over those constraints with System.Type or MethodInfo? I haven't found anything so far; any help will be appreciated.

Answer 1

You can iterate through the generic parameters to the type, and for each parameter, you can ask for the constraint types.

You do this using:

• Type.GetGenericArguments of Type to find the generic arguments to the type, ie. Type<T>, you would find T.
• Type.GetGenericParameterConstraints gives you the base types that each such parameter is constrained against, you call this on the arguments you find from the above method.

Take a look at this code, which you can run through LINQPad:

void Main()
{
    Type type = typeof(TestClass<>);
    foreach (var parm in type.GetGenericArguments())
    {
        Debug.WriteLine(parm.Name);
        parm.GetGenericParameterConstraints().Dump();
    }
}

public class TestClass<T>
    where T : Stream
{
}

The output is:

T

Type [] (1 item)  
typeof (Stream)

To find other constraints, such as new(), you can use the .GenericParameterAttributes flags enum, example:

void Main()
{
    Type type = typeof(TestClass<>);
    foreach (var parm in type.GetGenericArguments())
    {
        Debug.WriteLine(parm.Name);
        parm.GetGenericParameterConstraints().Dump();
        parm.GenericParameterAttributes.Dump();
    }
}

public class TestClass<T>
    where T : new()
{
}

Which outputs:

T

Type [] (1 item)  
typeof (Stream)

DefaultConstructorConstraint

Answer 2

You can use the GetGenericParameterConstraints() method to do that.

Answer 3

Using a previously found System.Type you can use GetGenericParameterConstraints().

Here's an excellent MSDN article on Generics and Reflection.