Q:

Fill a drawn triangle using lines

i need to generate a new method to fill the triangle in the below code and call it separately
any advise please?
public void draw(Graphics g, Pen blackPen)
    {
        double xDiff, yDiff, xMid, yMid;

        xDiff = oppPt.X - keyPt.X;
        yDiff = oppPt.Y - keyPt.Y;
        xMid = (oppPt.X + keyPt.X) / 2;
        yMid = (oppPt.Y + keyPt.Y) / 2;

        // draw triangle
        g.DrawLine(blackPen, (int)keyPt.X, (int)keyPt.Y, (int)(xMid + yDiff / 2), (int)(yMid - xDiff / 2));
        g.DrawLine(blackPen, (int)(xMid + yDiff / 2), (int)(yMid - xDiff / 2), (int)oppPt.X, (int)oppPt.Y);
        g.DrawLine(blackPen, (int)keyPt.X, (int)keyPt.Y, oppPt.X, oppPt.Y);

    }

the method should take both of these arguments
public void fillTriangle(Graphics g, Brush redBrush)
    {

    }

A:

Use a single function for the drawing, and for reduced complexity and consistency use a GraphicsPath object.
void DrawGraphics(Graphics g, Pen pen, Brush brush)
{
    float xDiff=oppPt.X-keyPt.X;
    float yDiff=oppPt.Y-keyPt.Y;
    float xMid=(oppPt.X+keyPt.X)/2;
    float yMid=(oppPt.Y+keyPt.Y)/2;

    // Define path with the geometry information only
    var path = new GraphicsPath();
    path.AddLines(new PointF[] {
        keyPt,
        new PointF(xMid + yDiff/2, yMid-xDiff/2),
        oppPt,
    });
    path.CloseFigure();

    // Fill Triangle
    g.FillPath(brush, path);

    // Draw Triangle
    g.DrawPath(pen, path);
}

The result is as seen below:

