在Graphics.Drawing中使用未分配的局部变量

Question

So I'm trying to write a program that draws a circle by drawing a line every 1 degree and changes color by incrementing through RGB (Started by Random).

Here is the code that I have so far

public static void drawCircle(int iRandR, int iRandG, int iRandB)
{
    for (int x = 0; x < 359; ++x)
    {
        double dXAngle = 0;
        double dYAngle = 0;
        dXAngle = 300 + Math.Cos(x) + 200;
        dYAngle = 300 + Math.Sin(x) + 200;
        Pen pPen = new Pen(Color.FromArgb(255, iRandR, iRandG, iRandB));
        Graphics gDraw;
        int iXAngle = Convert.ToInt32(dXAngle);
        int iYAngle = Convert.ToInt32(dYAngle);
        gDraw.DrawLine(pPen, 300, 300, iXAngle, iYAngle); //Error called here
    }

}
private void drawCircleButton_Click(object sender, EventArgs e)
{
    int genRandR = 0;
    int genRandG = 0;
    int genRandB = 0;
    Random rRand = new Random();
    genRandR = rRand.Next(0, 255);
    genRandG = rRand.Next(0, 255);
    genRandB = rRand.Next(0, 255);
    drawCircle(genRandR, genRandG, genRandB);
    drawCircleButton.Hide();
}

The only problem is that the compiler has a problem with the above line and it throws "Use of unassigned local variable 'gDraw'" I did some googling and a lot of other examples looked like mine but I can't figure out why mine was throwing this error.

Any help would be greatly appreciated.

Answer 1

You need to pass the desired Graphics instance as a parameter from somewhere. The Graphics class, as explained on MSDN, encapsulates a GDI+ drawing surface. It's used to abstract various drawing operations to different display devices (your screen, a bitmap, metafile, or even printing).

public static void DrawCircle(Graphics gDraw, int iRandR, int iRandG, int iRandB)
{
    ...
}

You are probably calling it from a Paint event handler, or something similar, where you have access to the Graphics object you want to draw to. Ie:

protected override void OnPaint(PaintEventArgs p)
{
     var graphics = p.Graphics;
     DrawCircle(graphics, ...);
}