Btukfyl

Here is my webmethod

[WebMethod]

    public string CheckService(string name, ref string msg)

    {

        return "Hello" + name;

    }

and this is my ajax call

$(document).ready(function () {

                $.ajax({

                    type: "POST",

                    url: "<%= ResolveUrl("integrator.asmx/CheckService") %>",

                    data: '{name: "zakki",msg:"" }',

                    contentType: "application/json; charset=utf-8",

                    dataType: "json",

                    async: false,

                    success: function (data1) {

                        console.log('', data1.d);

                    }

                });

            });

It raises an error

Cannot convert object of type 'System.String' to type 'System.String&'

The short answer is: You cannot use pass-by-reference (with ref/out keywords) for parameters in a method marked with [WebMethod] attribute.

Here is the reason that explains why & added for second System.String type in exception message, taken from this reference:

The key to decoding the source of this message is the "&" located in
the end of the text. This is the syntax used when passing a parameter
to a function BYREF (By-Reference) & used in
C-Notation to indicate "pass the AddressOf" variable name. Even if you coded
the WebService in C#/VB.NET, Microsoft will convert/compile it in C type
notation when it outputs it as an assembly.

Therefore, all parameters inside a method which uses [WebMethod] attribute must use pass-by-value, by removing ref keyword:

[WebMethod]

public string CheckService(string name, string msg)

{

    // return string here

}