c# - Locking does not prevent InvalidOperationException: Object is currently in use elsewhere

Question

I was under the impression that lock() would prevent multiple threads from accessing an object simultaneously.

But, an InvalidOperationException (Object is currently in use elsewhere) is still frequently thrown by the following code:

lock (this)
{
    localCopy = (Bitmap)this.bm.Clone();
    int x, y;
    float pX = this.p.x;
    int width = localCopy.Width;
    x = (int)Math.Round((double)(pX * (float)width));
    if (x >= localCopy.Width) x = localCopy.Width - 1;
    y = (int)Math.Round((double)(this.p.y * (float)localCopy.Height));
    if (y >= localCopy.Height) y = localCopy.Height - 1;
    colourPixel = localCopy.GetPixel(x, y);
}

Some things to note:

• I split up the calculation of x to isolate the cause of the exception. It appears to come from accessing the bitmap.
• I tried creating a local copy of the bitmap, but this just causes the same exception. I've tried Clone()ing and creating a new Bitmap. Neither works.
• I've tried locking on this (as seen) and on the bitmap object. Neither works.

Am I trying to use lock() in a way I'm not supposed to? Have I misunderstood it's purpose? How can I prevent InvalidOperationExceptions?

Answer 1

Probably try using an object for locking purpose rather than locking "this".

A class level variable

private static object syncRoot = new Object();

and when you are using

lock (syncRoot)
{
....
}