How do you write a C# Extension Method for a Gener

2019-01-24 09:25发布

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.

7条回答
甜甜的少女心
2楼-- · 2019-01-24 09:43

Glenn Block has a good example of implementing a ForEach extension method to IEnumerable<T>.

From his blog post:

public static class IEnumerableUtils
{
    public static void ForEach<T>(this IEnumerable<T> collection, Action<T> action)
    {
        foreach(T item in collection)
            action(item);
    }
}
查看更多
啃猪蹄的小仙女
3楼-- · 2019-01-24 09:47

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 "";
        }
    }
}
查看更多
干净又极端
4楼-- · 2019-01-24 09:49

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 "";
        }
    }
}
查看更多
Summer. ? 凉城
5楼-- · 2019-01-24 09:58

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!

查看更多
萌系小妹纸
6楼-- · 2019-01-24 09:59
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.

查看更多
We Are One
7楼-- · 2019-01-24 10:01

If you want the extension to only be available for the specified type you simply just need to specify the actual type you will be handling

something like...

public static string GetDefaultPageTitle(this ViewPage<YourSpecificType> v)
{
  ...
}

Note intellisense will then only display the extension method when you declare your (in this case) ViewPage with the matching type.

Also, best not to use the System.Web.Mvc namespace, I know its convenient to not have to include your namespace in the usings section, but its far more maintainable if you create your own extensions namespace for your extension functions.

查看更多
登录 后发表回答