Question

I am attempting to use a delegate in a way which I have never seen described in my various books.

My questions are:

Is it possible to use delegates in this way? and

If so, how should I alter the code to make use of the delegate?

Specifically I want one function to call another function from a choice of two possible functions.

class Profile
{
   private List<verticalCurve> allVCs;
   // create allVCs in the constructor

   private double nonTrivialFunctionToFindTheRightVCin_allVCs
                      (double lengthAlong, getSwitchForProfile aDel)
   { // about thirty lines of code which I want to reuse }


   public double getElevation(double distanceAlongPfl)
   {
         // compiler error on the following line:
      getSwitchForProfile myDelEL = 
            new verticalCurve.getSwitchForProfile(verticalCurve.getElevation);

      return nonTrivialFunctionToFindTheRightVCin_allVCs
                 (distanceAlongPfl, myDelEL);
   }

   public double getSlope(double distanceAlongPfl)
   {
         // compiler error on the following line:
      getSwitchForProfile myDelSL = 
            new verticalCurve.getSwitchForProfile(verticalCurve.getSlope);

      return nonTrivialFunctionToFindTheRightVCin_allVCs
                 (distanceAlongPfl, myDelSL);
   }

}  // end class Profile

class verticalCurve
{
   private double elevation;
   private double slope;

   static internal delegate double getSwitchForProfiles(double distanceAlongPfl);

   public double getElevation(double distanceAlong)
   { computeElevation then return elevation; }

   public slope getSlope(double distanceAlong)
   { compute slope then return slope;}
}  // end class verticalCurve

The compiler error states

An object reference is required for the non-static field, method, or property ‘Profile.verticalCurve.getElevation(distanceAlong)’

It appears that my problem is that at the moment I assign a method to the delegate, I do not yet know which instance of verticalCurve it will be called on. But I can’t make verticalCurve.getElevation static because it has to know which verticalCurve it is on.

Sorry for the long setup to the questions. I did try to simplify it, but it seems irreducible beyond this point.

Thanks in advance for your help.

• Paul Schrum

Answer 1

It sounds like you want to create a delegate which takes the object to call it on as a parameter:

delegate double DistanceFunction(VerticalCurve curve, double distance);

void SomeFunction(DistanceFunction func) {
    double result = func(someCurve, 42);
}

SomeFunction((c, dist) => c.GetSlope(dist));