Object already in use elsewhere c# even if using the lock instruction

Question

Hi

I try to draw onto bitmap inside a picturebox from multiple tasks. This is a simplified version of my main code. The purpose of this code is to draw onto a picturebox from multiple Tasks, since I will need to catch and draw with lines the position of my mouse on the pb and also graphically the readings from a sensor through a serial port. I keep on getting the error message: Object already in use elsewhere. Following other posts I have already tried to use the lock instruction, but I keep on getting this error message.

namespace TEst2  
{  
    public partial class Form1 : Form  
    {  
        Bitmap bmp;  
        Graphics g;  
  
        public Form1()  
        {  
            InitializeComponent();  
  
            bmp = new Bitmap(pictureBox1.Width, pictureBox1.Height);  
              
            g = Graphics.FromImage(bmp);  
        }  
  
        private void pictureBox1_Click(object sender, EventArgs e)  
        {  
              
            pictureBox1.Image = bmp;  
            Task.Run( () => Draw());  
            Task.Run( () => DrawMouse());  
            Task.Run( () => DrawIt());  
        }  
  
        private void DrawIt()  
        {  
            while (true)  
            {  
                pictureBox1.Image = (Bitmap)bmp.Clone();  
            }  
                  
        }  
  
        private void DrawMouse()  
        {  
            while (true)  
            {  
                try  
                {  
                    lock (bmp)  
                    {  
                        BitmapData bmData2 = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat);  
                        g.DrawLine(new Pen(Color.Green, 2), 50, 0, 50, 50);  
                        bmp.UnlockBits(bmData2);  
  
                    }  
                }  
                catch (Exception ex)  
                {  
                    MessageBox.Show(ex.Message);  
                }  
            }  
        }  
        private void Draw()  
        {  
  
            while (true)  
            {  
  
                try  
                {  
                    lock (bmp)  
                    {  
                        BitmapData bmData2 = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp.Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat);  
                        g.DrawLine(new Pen(Color.Red, 2), 0, 0, 50, 50);  
                        bmp.UnlockBits(bmData2);  
  
                    }  
                }  
                catch (Exception ex)  
                {  
                    MessageBox.Show(ex.Message);  
                }  
            }  
        }  
    }  
}  
  

I have noticed and also read in quiet a few posts that the problem seems to be rising, when using the pictureBox1.Image = bmp command. Thanks in advance for your help

Answer 1

Hi @ms-jaft , Welcome to Microsoft Q&A.

You put g.DrawLine(new Pen(Color.Red, 2), 0, 0, 50, 50); in BitmapData bmData2 = bmp.LockBits(new Rectangle(0, 0, bmp.Width, bmp. Height), System.Drawing.Imaging.ImageLockMode.ReadOnly, bmp.PixelFormat); and bmp.UnlockBits(bmData2); will directly cause an error.

You also shouldn't use multiple while(true) loops.

You can try using drawimage instead of picturebox.image=bmp;

Best Regards,
Jiale

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