GetGenericTypeDefinitionImplementation.cs

public static Type GetGenericTypeDefinitionImplementation(this Type type, Type definition)
{
    if (definition.IsInterface)
    {
        return type.GetGenericInterfaceDefinitionImplementation(definition);
    }
    else
    {
        return type.GetGenericClassDefinitionImplementation(definition);
    }
}

public static Type GetGenericInterfaceDefinitionImplementation(this Type type, Type interfaceDefinition)
{
    if (type.IsInterface && type.ImplementsGenericDefinition(interfaceDefinition))
    {
        return type;
    }

    var interfaces = type.GetInterfaces();

    foreach (var @interface in interfaces)
    {
        if (@interface.ImplementsGenericDefinition(interfaceDefinition))
        {
            return @interface;
        }
    }

    return null;
}

public static Type GetGenericClassDefinitionImplementation(this Type type, Type classDefinition)
{
    if (type == typeof(object))
    {
        return null;
    }

    if (type.ImplementsGenericDefinition(classDefinition))
    {
        return type;
    }

    return type.BaseType.GetGenericClassDefinitionImplementation(classDefinition);
}

public static bool ImplementsGenericDefinition(this Type type, Type definition)
{
    return type.IsConstructedGenericType && type.GetGenericTypeDefinition() == definition;
}