How to prevent InvalidOperationException when using System.Drawing.Graphics?

Question

I am using the System.Drawing.Graphics.DrawLines(Pen pen, PointF[] points) method in a multithreaded application, but the System.Drawing.Graphics isn't shared between threads.

Why it keeps throwing the System.InvalidOperationException: The object is currently in use elsewhere ?

Answer 1

Simple answer: don't do that. Only access the GUI on the GUI thread.

Answer 2

The problem was: I was using the same System.Drawing.Pen instance for all threads. I had to clone it for every thread in order to solve the problem.

var pens = new Pen[0];
lock (this._pens)
{
    pens = (Pen[])this._pens.Select(a => (Pen) a.Clone()).ToArray();
}

Even the lock is essential in order to solve this problem