Fix the code. (The DrawCircle part.)
namespace bresenhamCircle
{
public partial class Form1 : Form
{
private int x1, y1, x2, y2;

public Form1()
{
InitializeComponent();


}
private void pictureBox1_Click(object sender, EventArgs e)
{
if (x1 != 0 && y1 != 0)
{
x2 = (int)((MouseEventArgs)e).X;
y2 = (int)((MouseEventArgs)e).Y;

DrawCircle(x1, y1, x2, y2);

x1 = 0;
y1 = 0;
x2 = 0;
y2 = 0;
}
else
{
x1 = (int)((MouseEventArgs)e).X;
y1 = (int)((MouseEventArgs)e).Y;
}
}

private void DrawCircle(int x1, int y1, int x2, int y2)

{
pictureBox1.Image = (Image)new Bitmap(pictureBox1.Width, pictureBox1.Height);
Graphics g = Graphics.FromImage(pictureBox1.Image);
int r = 0; //radius ???

int x = x1;
int y = y1;
x = -r; // ???
y = 0; //???
int F = 1 - r;
int delFs = 3;
int delFd = 5 - 2 * r;
while (x + y < 0)
{
g.FillRectangle(Brushes.Black, x, y, 1, 1);
if (F > 0)
{
//Диагональное смещение.
F += delFd;
x++;
y++;
delFs += 2;
delFd += 4;
}
else
{
//Вертикальное смешение.
F += delFs;
y++;
delFs += 2;
delFd += 2;
}
}
}

}
}