这对我有用:

[WebMethod]
[ScriptMethod(ResponseFormat = ResponseFormat.Json)]
public void ReturnExactValueFromWebMethod(string AuthCode)
{
    string r = "return my exact response without ASP.NET added junk";
    HttpContext.Current.Response.BufferOutput = true;
    HttpContext.Current.Response.Write(r);
    HttpContext.Current.Response.Flush();
}