Assign result of a method to a variable in script task using C#

Hi, I am using vs 2017 ssdt and would like to assign result of a method to a variable in script task using C#. I am trying to do this outside of main method. I can read the SSIS variable values in the main method but not being able to access it outside of 

public partial class ScriptMain : Microsoft.SqlServer.Dts.Tasks.ScriptTask.VSTARTScriptObjectModelBase
{
        public void Main()
	{
          //Do something
        }

        private class DiffClass
        {
         private Microsoft.SqlServer.Dts.Runtime.Variables DtsVariables;
         public string MakeURL()
         {
          return DtsVariables["User::URLBase"].Value.ToString() + "/default.aspx"; // how to assign this value to the below variable
         }

         string urlbase = MakeURL(); //Return results of MakeURL() to the string urlbase
        }
}

Thank you.

SQLEnthusiast