c# - why - Extension Methods not Recognized

why we use extension method in c# (6)

What is necessary to have an extension method honored when it exists in an imported assembly? I built one in a class library project but it is not recognized in my web project which references the library. All the other classes and methods in the library are honored and visible but this extension method is not. The extension method is visible when used within the library.

Are you sure the extension method is made public?

For an example implementation that helped me:

(Note the this keyword that has already been mentioned).

    /// <summary>
    /// Convert current bytes to string
    /// </summary>
    /// <param name="bytes">Current byte[]</param>
    /// <returns>String version of current bytes</returns>
    public static string StringValue(this byte[] currentBytes)
        return string.Concat(Array.ConvertAll(bytes, b => b.ToString("X2")));

For anyone wondering, I had this same problem none of the answers worked. Turns out it was because the using statement of the assembly was aliased:

using ex = MyApp.Example

Removing the alias worked, but I decided instead to add a duplicate, non-aliased using, which also fixed the problem:

using MyApp.Example
using ex = MyApp.Example

I had this problem using an extension method on enums in solutions referenced each other as shown below. Intellisense worked for the extension method in the ApplicationUI project and it even ran without compile or run-time errors. But the method simply didn't work. Also, the immediate window assured me that BusinessObjectLib.MyEnum did not contain a method with the name of my extension method, and no extension method could be found.

GenericLib - project where extension method on generic enums is defined
BusinessObjectLib - project where enums were defined, references GenericLib
ApplicationUI - project referencing both GenericLib and BusinessObjectLib

Even though, the Solution Explorer looked OK viewing all projects from ApplicationUI, when I opened the BusinessObjectLib project, I could see its reference to GenericLib was broken for some reason. (Like all of our code, VS probably has bugs as well?). First, I worked in the BusinessObjectLib project opened directly in VS, removing the ref, then removing the project, then restoring both in the opposite order. Then I renamed the ApplicationUI.sou file and forced it to be rebuilt. I was able to fix this problem through these actions, but only the .sou file rename seemed to do the trick. The immediate window still continues to give me the same error, but at least the run-time code works again. I am using this exact pattern in several other projects without having the kind of problem I'm having here.

In my case, the Extension method was in an external reference which was referencing a different version of a component. I synchronized versions on both projects and it worked.

Make sure if using templates, your template declaration matches what is declared in the method signature with, "this"..


SomeClass<string, string> test = new SomeClass<string, string>();

extensionMethod<key, val>(this SomeClass<key, Lazy<val>>, string val)

the extension method will not show up because of the lazy wrapper.