How do you write a C# Extension Method for a Generically Typed Class

Question

This should hopefully be a simple one.

I would like to add an extension method to the System.Web.Mvc.ViewPage< T > class.

How should this extension method look?

My first intuitive thought is something like this:

namespace System.Web.Mvc
{
    public static class ViewPageExtensions
    {
        public static string GetDefaultPageTitle(this ViewPage<Type> v)
        {
            return "";
        }
    }
}

Solution

The general solution is this answer.

The specific solution to extending the System.Web.Mvc.ViewPage class is my answer below, which started from the general solution.

The difference is in the specific case you need both a generically typed method declaration AND a statement to enforce the generic type as a reference type.

Answer 1

Thanks leddt. Doing that yielded the error:

The type 'TModel' must be a reference type in order to use it as parameter 'TModel' in the generic type or method

which pointed me to this page, which yielded this solution:

namespace System.Web.Mvc
{
    public static class ViewPageExtensions
    {
        public static string GetDefaultPageTitle<T>(this ViewPage<T> v) 
          where T : class
        {
            return "";
        }
    }
}

Answer 2

I don't have VS installed on my current machine, but I think the syntax would be:

namespace System.Web.Mvc
{
    public static class ViewPageExtensions
    {
        public static string GetDefaultPageTitle<T>(this ViewPage<T> v)
        {
            return "";
        }
    }
}

Answer 3

It just needs the generic type specifier on the function:

namespace System.Web.Mvc
{
    public static class ViewPageExtensions
    {
        public static string GetDefaultPageTitle<Type>(this ViewPage<Type> v)
        {
            return "";
        }
    }
}

Edit: Just missed it by seconds!

Answer 4

namespace System.Web.Mvc
{
    public static class ViewPageExtensions
    {
        public static string GetDefaultPageTitle<T>(this ViewPage<T> view)
            where T : class
        {
            return "";
        }
    }
}

You may also need/wish to add the "new()" qualifier to the generic type (i.e. "where T : class, new()" to enforce that T is both a reference type (class) and has a parameterless constructor.