In my previous post I was using Extension Method to “reuse” code when validating string arguments and throwing custom exception if string was null or empty.
Here is that example:

namespace UnityTest.Core
{
    public static class ArgumentValidation
    {
        public static void IsEmptyOrNull(this string value, string parameterName)
        {
            if (String.IsNullOrEmpty(value))
                throw new CoreException(String.Format("{0} is null or empty",parameterName));
        }
    }
}

To use it in you class you have to set
using UnityTest.Core;

public List<UnityTest.Models.Menu> GetUserMenuBL(string userName)
        {
            //will check for null or string.empty and
            // throw custom exception if that is true.
            userName.IsEmptyOrNull("userName");
            try
            {
                return DALayer.GetUserMenuDB(userName);
            }
            catch (CoreException)
            { throw; }
            catch (Exception)
            { throw; }
        }

More information and advanced example can be found here