Why can't I use concatenation on a public static string

FYI I spent about 30 minutes searching for the answer. If I missed it on stackoverflow, i'm sorry. This seems like an easy answer, however none of my colleagues know either.

I have am working with an existing library. I'm trying to maintain integration with a current system, while adding the ability to change some hard-coded values. I've refactored the code to utilize the ConfigurationManager so I can use parametrized web deployment.

My question is this.. Why, when I access Constants.CourseMillRegisterURL, I only get back part of the variable? The part that I get back is the piece read from web.config. I expect to get a complete URL containing both variables concat'd but I only get my web.config value "userlogin.jsp".

I've tried also coding it so that the values are concat'd in the privates, but it doesn't work that way either. I really want to stay with static because the whole library refers to this class using code like

string theUrl = Constants.CoursMillUrl + Constants.CourseMillRegisterUrl

Each variable returns the following:

• Constants.CourseMillUrl = "http://www.valuefromwebconfig.com/cm6/cm0670"
• Constants.CourseMillRegisterUrl = "valuefromwebconfig.jsp"
• Constants.CourseMillLoginUrl = "othervaluefromwebconfig.jsp"

Why are my values not

• Constants.CourseMillUrl = "http://www.valuefromwebconfig.com/cm6/cm0670"
• Constants.CourseMillRegisterUrl = ""http://www.valuefromwebconfig.com/cm6/cm0670/valuefromwebconfig.jsp"
• Constants.CourseMillLoginUrl = "http://www.valuefromwebconfig.com/cm6/cm0670/othervaluefromwebconfig.jsp"

My code is below.

namespace STTI.CourseMill.Library
{
    #region

    using System.Configuration;

    #endregion

    public static class Constants
    {
        // prod 

        #region Static Fields

        public static string CourseMillRegisterURL = CourseMillURL + courseMillRegisterURL;

        public static string CourseMillURL = courseMillURL;

        public static string CourseMillUserLoginURL = CourseMillURL + courseMillUserLoginURL;

        #endregion

        #region Properties

        private static string courseMillRegisterURL
        {
            get
            {
                string output = ConfigurationManager.AppSettings["CourseMillRegisterUrl"];
                if (output == null)
                {
                    output = "sttilogin.jsp?d=t";
                }

                return output;
            }
        }

        private static string courseMillURL
        {
            get
            {
                string output = ConfigurationManager.AppSettings["CourseMillURL"];
                if (output == null)
                {
                    output = "http://hardcodedvalue/cm6/cm0670";
                }

                if (!output.EndsWith("/"))
                {
                    output += "/";
                }

                return output;
            }
        }

        private static string courseMillUserLoginURL
        {
            get
            {
                string output = ConfigurationManager.AppSettings["CourseMillLoginUrl"];
                if (output == null)
                {
                    output = "sttilogin.jsp?d=t";
                }

                return output;
            }
        }

        #endregion
    }
}