ASP.Net AJAX “Unknown web method, Parameter name: methodName”

09 February 2016

Scenario

You have some backend code in an .asmx or .aspx file to handle AJAX requests. When you call them from the frontend with a perfectly formed jQuery request, you get the error:

Unknown web method _____.
Parameter name: methodName

Exception Details: System.ArgumentException: Unknown web method _____.

Causes

If you search for this one, most results will tell you to make it a public static class. However, that wasn’t necessary in my case and wasn’t the problem.

My Solution

I made my .asmx service by copying+pasting an existing one. This screwed me up, because the .asmx file still contained a reference to the .asmx.cs file from the original, not the copy.

• Browse the directory with your service files in it. Open the .asmx file in a simple editor like Notepad++

• Verify that the CodeBehind and Class properties reference the correct file

Once I fixed the Class property, my code worked.