How to conditionally invoke a generic method with constraints? [duplicate]

Answer 1

You can use a helper class and dynamic type to skip compile-time checks:

sealed class CountDuplicatesFastCaller
{
    public int Call<T>(IList<T> list) where T : IComparable<T>
    {
        return CountDuplicatesFast(list);
    }
}

public static int CountDuplicates<T>(IList<T> list)
{
    if (typeof (IComparable<T>).IsAssignableFrom(typeof (T)))
    {
        return ((dynamic) new CountDuplicatesFastCaller()).Call(list);
    }
    else
    {
        /* use the slow algorithm */
    }
}

This should be faster than pure reflection because of DLR caching mechanisms.

Answer 2

Here's a way to do it using dynamic:

if (typeof(IComparable<T>).IsAssignableFrom(typeof(T)))
{
    return CountDuplicatesFast((dynamic)list);
}

Or with reflection:

if (typeof(IComparable<T>).IsAssignableFrom(typeof(T)))
{
    var method = typeof(MyType).GetMethod("CountDuplicatesFast");
    var generic = method.MakeGenericMethod(typeof(T));
    return (int)generic.Invoke(null, new object[] { list });
}

I don't think that there's a way to do this statically (i.e. without reflection or dynamic).