为什么我的C＃扩展方法在此控制台应用程序中不起作用？

Question

I have the console app:

namespace LicenceCreator
{
    class Program
    {  
        static void Main(string[] args)
        {
        }    

        public static string TrimTextAndAppendDots(
            this string stringArg, int maxLengthArg)
        {
            string retString;

            if (stringArg.Length > maxLengthArg)
            {
                retString = stringArg.Substring(0, maxLengthArg) + "...";
            }
            else
            {
                retString = stringArg;
            }

            return retString;
        }
    }
}

But for some reason the extension method isnt picked up when I call it from a string in the main method any ideas why?

Answer 1

I'm guessing that by "isn't picked up", you mean "the compiler complains that it can't find it". Then:

The extension method must be in a "static" class and you must have a "using" directive pointing to the namespace of that class. Do you?

If it's "the compiler doesn't complain but the string doesn't change" then maybe you are calling it as

 myString.TrimTextAndAppendDots(10);

instead of

 myString = myString.TrimTextAndAppendDots(10);

Answer 2

Extension methods must be declared within a static class.

• Is extension method's class static?

In order to call an extension method, declaring class' namespace must be referenced:

• Have you added class assembly reference?
• Have you declared the corresponding "using" statement for extension methods class' namespace?

---

UPDATE:

Answering to your comment, I find my answer correct anyway, but now we can add:

• Is your "Program" class static? ;)

Answer 3

Extensions have to be on a static class, just like this:

public static class StringExtension
{
    public static string TrimTextAndAppendDots(this string stringArg, int maxLengthArg)
    {
        string retString = "";

        if (stringArg.Length > maxLengthArg)
        {
            retString = stringArg.Substring(0, maxLengthArg) + "...";
        }
        else
        {
            retString = stringArg;
        }

        return retString;
    }
}

And finally you have to declare StringExtension with using statement wherever you want to use said extension methods