after.cs

public class GuidConstraint : IRouteConstraint
{
  public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
  {
    if (!values.ContainsKey(parameterName))
      return false;

    string stringValue = values[parameterName] as string;
    if (string.IsNullOrEmpty(stringValue))
      return false;

    Guid guidValue;
    return Guid.TryParse(stringValue, out guidValue) && (guidValue != Guid.Empty);
  }
}

before.cs

public class GuidConstraint : IRouteConstraint {

public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
{
    if (values.ContainsKey(parameterName))
    {
        string stringValue = values[parameterName] as string;

        if (!string.IsNullOrEmpty(stringValue))
        {
            Guid guidValue;

            return Guid.TryParse(stringValue, out guidValue) && (guidValue != Guid.Empty);
        }
    }

    return false;
}}