Draw lines on a PictureBox

My question is related to Stack Overflow question Draw lines on a picturebox using mouse clicks in C#, but when the mouse button is up, the drawn line disappears. How do I fix this?

private void GainBox_MouseDn(object sender, MouseEventArgs e)
{
    mouse_dn = true;
}

private void GainBox_MouseMv(object sender, MouseEventArgs e)
{
    //Line drawn from lookup table
 开发者_Go百科   if (mouse_dn)
    {
        img = new Bitmap(256, 256);

        //Get the coordinates (x, y) for line from lookup table.

        for (x = x1; x < x2; x++)
            img.SetPixel(x, y, Color.Red);

        //Same for y coordinate
    }
    GainBox.Refresh();
}

private void GainBox_MouseUp(object sender, MouseEventArgs e)
{
    mouse_dn = false;
}

Here is a small complete program that does draw lines based on points (in this case, it follows the mouse). I think you can rework that into what you need.

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }


    // Variable that will hold the point from which to draw the next line
    Point latestPoint;


    private void GainBox_MouseDown(object sender, MouseEventArgs e)
    {
        if ((e.Button & MouseButtons.Left) == MouseButtons.Left)
        {
            // Remember the location where the button was pressed
            latestPoint = e.Location;
        }
    }

    private void GainBox_MouseMove(object sender, MouseEventArgs e)
    {
        if ((e.Button & MouseButtons.Left) == MouseButtons.Left)
        {
            using (Graphics g = GainBox.CreateGraphics())
            {
                // Draw next line and...
                g.DrawLine(Pens.Red, latestPoint, e.Location);

                // ... Remember the location
                latestPoint = e.Location;
            }
        }
    }
}

One problem in your solution is that you are drawing on a temporary bitmap, but the image in that bitmap is never transferred to your PictureBox. In the solution presented here, there isn't any extra bitmap needed.

Use Graphics Object to Drawline

e.g.

Graphics gfx = GainBox.CreateGraphics();
gfx.Drawline([Your Parameters here]);

gainbox.refresh() should stay inside the if (mouse_dn) clause.