如何根据鼠标的移动绘制矩形？

Question

I found sample code here for drawing on a form:

http://msdn.microsoft.com/en-us/library/aa287522(v=vs.71).aspx

As a followup to this requirement (discovering which controls are beneath a rectangle described by the user dragging the mouse):

There seems to be a mismatch between the location of my controls and the location of my MouseDown and -Up events

...I want to provide the user instant/constant feedback about just what they are about to select (when/if they release the mouse button). I want to not just draw a line following the mouse's movement, but draw the rectangle that is being described by their mousewrangling efforts.

I'm thinking the MouseMove event, coupled with code from the two links above, could do the trick, but is that fired too often/would that have a malevolent impact on performance? If so, what would be a preferable event to hook, or would a timer be the way to go here?

UPDATE

This code, adapted from John's example below (the only difference is the StackOverflow-inducing calls to base.* are commented out, and I changed the color from red to black (no reference to Stendahl intended)), works except that previously drawn rectangles display again after releasing the mouse. IOW, the first rectangle draws perfectly - it disappears with the mouse up click (as intended). However, when I describe a second rectangle by depressing the left mouse key and dragging down and to the right, the first rectangle displays again! And this continues to happen - every previously drawn rectangle is remembered and brought back to the fore when a new rectangle is being drawn.

public partial class Form1 : Form { private Point? _start; private Rectangle _previousBounds;

public Form1()
{
    InitializeComponent();
}

private void Form1_MouseDown(object sender, MouseEventArgs e)
{
    _start = e.Location;
    //base.OnMouseDown(e); 

}

private void Form1_MouseMove(object sender, MouseEventArgs e)
{
    if (_start.HasValue)
        DrawFrame(e.Location);

    //base.OnMouseMove(e); 

}

private void Form1_MouseUp(object sender, MouseEventArgs e)
{
    ReverseFrame();
    _start = null;
    //base.OnMouseUp(e); 
}

private void ReverseFrame() 
{ 
    ControlPaint.DrawReversibleFrame(_previousBounds, Color.Black, FrameStyle.Dashed); 
} 

private void DrawFrame(Point end) 
{ 
    ReverseFrame(); 

    var size = new Size(end.X - _start.Value.X, end.Y - _start.Value.Y); 
    _previousBounds = new Rectangle(_start.Value, size); 
    _previousBounds = this.RectangleToScreen(_previousBounds); 
    ControlPaint.DrawReversibleFrame(_previousBounds, Color.Black, FrameStyle.Dashed); 
} 

}

Answer 1

ControlPaint.DrawReversibleFrame() will do what you want. Performance is not generally a problem - just keep it small and clean.

-- EDIT: Added a code sample. StackOverflowException indicates something is wrong - but without seeing yours, can't answer directly.

private Point? _start;
private Rectangle _previousBounds;

protected override void OnMouseDown(MouseEventArgs e)
{
    _start = e.Location;
    base.OnMouseDown(e);
}

protected override void OnMouseMove(MouseEventArgs e)
{
    if( _start.HasValue ) {
        ReverseFrame();
        DrawFrame(e.Location);
    }

    base.OnMouseMove(e);
}

protected override void OnMouseUp(MouseEventArgs e)
{
    ReverseFrame();
    _start = null;
    base.OnMouseUp(e);
}

private void ReverseFrame()
{
    ControlPaint.DrawReversibleFrame(_previousBounds, Color.Red, FrameStyle.Dashed);

}
private void DrawFrame(Point end)
{
    ReverseFrame();

    var size = new Size(end.X - _start.Value.X, end.Y - _start.Value.Y);
    _previousBounds = new Rectangle(_start.Value, size);
    _previousBounds = this.RectangleToScreen(_previousBounds);
    ControlPaint.DrawReversibleFrame(_previousBounds, Color.Red, FrameStyle.Dashed);
}