Object reference not set to an instance of an object: When using Session in WebMethod

The exception "Object reference not set to an instance of an object" could occur when you are trying to use a session variable within a WebMethod in a ASP.NET WebService.

An example would be as below


[WebMethod]
    public string HelloWorldGetName() {

        Session["name"] = "xx";
        string a = Session["name"].ToString();
        return a;

    }

This method will throw the above exception. To avoid the error you need to enable session state inside WebMethod


[WebMethod(EnableSession=true)]
    public string HelloWorldGetName() {

        Session["name"] = "xx";
        string a = Session["name"].ToString();
        return a;

    }

If you do not want to use Sessions inside the WebMethod then leave it as default. Default setting is EnableSession=false.

If you Enable Session state WebMethod would be bit more time consuming when it comes to the method execution.