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

I eventually got to the bottom of this. I moved the locks into the getter / setter methods of the bitmap property and implemented a “deep copy” method to free up the property as soon as possible.

private static object syncRoot = new Object();
private Bitmap _bm;
private Bitmap bm
{
    get
    {
        lock (syncRoot)
            return this._bm.DeepClone();
    }
    set
    {
        lock (syncRoot)
        {
            this._bm = value.DeepClone();
        }
    }
}

The DeepClone() extension method was cribbed from an answer to another question:

public static T DeepClone<T>(this T a)
{
    using (MemoryStream stream = new MemoryStream())
    {
        BinaryFormatter formatter = new BinaryFormatter();
        formatter.Serialize(stream, a);
        stream.Position = 0;
        return (T)formatter.Deserialize(stream);
    }
}