Question

In the code below, the StylusPointCollection (SPC) changes according to whether I declare the StylusPoint variable inside or outside the method (i.e. if I comment the internal definition and uncomment the commented lines) . I don’t understand why this is.

//StylusPoint spXY;

private void DrawBinaryTree(int depth, StylusPoint pt, double length, double theta)
{        
    if (depth > 0)
    {    
        StylusPoint spXY = new StylusPoint(pt.X + length * Math.Cos(theta) * Math.Cos(a), pt.Y + length * Math.Sin(theta) * Math.Sin(a));

        //spXY = new StylusPoint(pt.X + length * Math.Cos(theta) * Math.Cos(a), pt.Y + length * Math.Sin(theta) * Math.Sin(a));

        SPC.Add(pt);
        SPC.Add(spXY);

        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta + deltaTheta);
        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta - deltaTheta);
    }
}

I have tried and failed to come up with a more simple example using LinqPad.

Answer 1

The difference stems from the fact that you make two recursive calls to the function trying to pass the variable in question to both. When you declare the variable outside the function, it gets modified by each call to DrawBinaryTree. When you declare the variable local, each call to DrawBinaryTree gets its own copy of the variable that other calls cannot modify

With a local:

private void DrawBinaryTree(int depth, StylusPoint pt, double length, double theta)
{        
    if (depth > 0)
    {    
        StylusPoint spXY = new StylusPoint(pt.X + length * Math.Cos(theta) * Math.Cos(a), pt.Y + length * Math.Sin(theta) * Math.Sin(a));

        SPC.Add(pt);
        SPC.Add(spXY);

        //spXY has the values you just set above 
        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta + deltaTheta);
        //since spXY is local, it still has the values you set above in this
        //call to the function (recursive calls have not modified it)
        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta - deltaTheta);
    }
}

With a global:

StylusPoint spXY;

private void DrawBinaryTree(int depth, StylusPoint pt, double length, double theta)
{        
    if (depth > 0)
    {    
        spXY = new StylusPoint(pt.X + length * Math.Cos(theta) * Math.Cos(a), pt.Y + length * Math.Sin(theta) * Math.Sin(a));

        SPC.Add(pt);
        SPC.Add(spXY);

        //spXY has the values you just set above 
        //(assuming there are no other functions running on other threads that modify it)
        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta + deltaTheta);
        //spXY now has the values from the last recursive call to DrawBinaryTree!
        DrawBinaryTree(depth - 1, spXY, length * lengthScale, theta - deltaTheta);
    }
}