扩展方法不适用于int吗？

Question

public static class NumberExtensions
{
    static string Format(this int integer) // Write one for int too, just incase :')
    {
        return string.Format("{0:n0}", integer);
    }
}

Calling the following code tells me the method Format doesn't exist.

idItemsFinished.Format()

Here is the variable

var idItemsFinished = Program.IdItemsProcessed;

It's simply just an int.

public int IdItemsProcessed;

Answer 1

Adding a public access specifier to the extension method should fix the issue.

public static class NumberExtentions
{
    public static string Format(this int integer) 
    {
        return string.Format("{0:n0}", integer);
    }
}

See Dot Net Fiddle

Answer 2

To answer the question in the subject - yes, extension methods do work on int , there's nothing special about int . In fact, they'll work on integer literals as well - 5.Format() should work fine.

You've left out what I suspect is the reason for your problem, which is namespaces. There might be dozens of places in your code and especially your dependent libraries that add extension methods to int and you don't want to necessarily import them all (leading to potential conflicts), so you have to add a using statement to your code that points to the namespace of NumberExtentions .

This is no different than adding a reference to a class, except that Visual Studio's intellisense is usually pretty good in highlighting an unknown class and offering to add a using for it for you, but it won't be as helpful with extension methods.

Simply add a using statement and you'll probably be good to go.