How can I use ref to modify this class variable in C#?

Question

I'm trying to use the ref keyword in C# to modify a class variable passed to a delegate function. I want the delegate function to be able to modify the values stored in container for the parent and its two children. What happens right now is that a delegate function can modify the parent (since I pass a reference directly to container[parent]) but not the children, since I have to process them first and thus pass a reference to leftChild and rightChild.

• Is it possible to have leftChild be a reference to container[leftChildIndex], so that the delegate function can modify the value stored in container? (same with right child)

  private void traversePostOrder(Modify operation, int parentIndex) {
      if (parentIndex < size) {
          int leftChildIndex = getLeftChildIndex(parentIndex);
          int rightChildIndex = getRightChildIndex(parentIndex);
          T parent = container[parentIndex];
          T leftChild = default(T);
          T rightChild = default(T);
  
          Library.Diagnostics.Message.logMessage("P: " + parent, 2);
  
          if (leftChildIndex < container.Length) {
              traversePostOrder(operation, leftChildIndex);
              leftChild = container[leftChildIndex];
          }
  
          if (rightChildIndex < container.Length) {
              traversePostOrder(operation, rightChildIndex);
              rightChild = container[rightChildIndex];
          }
  
          operation(ref container[parentIndex], ref leftChild, ref rightChild);
      }
  }
  

Answer 1

The problem is where you define them:

T leftChild = default(T);
T rightChild = default(T);

You pass a reference to those objects, and they get destroyed right after the end of the method because they're local variables.
Try sending the objects directly.

private void traversePostOrder(Modify operation, int parentIndex) {
    if (parentIndex < size) {
        int leftChildIndex = getLeftChildIndex(parentIndex);
        int rightChildIndex = getRightChildIndex(parentIndex);
        T parent = container[parentIndex];
        bool leftChildModified = false;
        bool rightChildModified = false;

        Library.Diagnostics.Message.logMessage("P: " + parent, 2);

        if (leftChildIndex < container.Length) {
            traversePostOrder(operation, leftChildIndex);
            leftChildModified = true;
        }

        if (rightChildIndex < container.Length) {
            traversePostOrder(operation, rightChildIndex);
            rightChildModified = true;
        }

        if(leftChildModified && rightChildModified)
        {
            operation(ref container[parentIndex], ref container[leftChildIndex], ref container[rightChildIndex]);
        }
        else if(leftChildModified)
        {
            operation(ref container[parentIndex], ref container[leftChildIndex], ref Default(T));
        }
        else if(rightChildModified)
        {
            operation(ref container[parentIndex], ref Default(T), ref container[rightChildIndex]);
        }
        else
        {
            operation(ref container[parentIndex], ref default(T), ref default(T));
        }
    }
}

Answer 2

What you're looking for is pointers, and C# doesn't expose them - luckily.

You can simply assign the values back to the class variable:

operation(ref container[parentIndex], ref leftChild, ref rightChild);
container[leftChildIndex] = leftChild;
container[rightChildIndex] = rightChild;