How to draw a Rectangle in any direction after MouseDown using C#?

Question

How can I draw a rectangle in all directions? Current code:

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    // Starting point of the selection:
    if (e.Button == MouseButtons.Left)
    {
        _selecting = true;
        _selection = new Rectangle(new Point(e.X, e.Y), new Size());
    }
}

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    // Update the actual size of the selection:
    if (_selecting)
    {
        _selection.Width = e.X - _selection.X;
        _selection.Height = e.Y - _selection.Y;
        pictureBox1.Refresh(); // redraw picturebox

    }
}

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left && _selecting)
    {
        _selecting = false;
    }
}

This allows me to MouseDown and then draw down and to the right, but I am unable to draw in any other direction beyond the anchor point. How can I draw a rectangle in any direction like eg Microsoft Paint?

I first tried:

_selection.Width = Math.Abs(e.X - _selection.X);
_selection.Height = Math.Abs(e.Y - _selection.Y);

But this creates a funny mirror effect (which is not what I want). I then tried a simple shift:

_selection.X = _selection.Left - 5;

This did what I expected and moved a static rectangle 5 units left, so I thought it would be a simple matter of continuously shifting the anchor point during the Paint event:

private void UpdateRectange(Point newPos)
{
    var width = newPos.X - _selection.X;
    var height = newPos.Y - _selection.Y;
    _selection.Width = Math.Abs(width);
    _selection.Height = Math.Abs(height);
    if (width < 0 && height > 0) // move down (+) and left (-)
    {
        //_selection.X = _selection.Left + width;
        _selection.Offset(width, 0);
    }
    uxScreenGrab.Refresh(); // redraw picturebox
}

But this resulted in pushing a vertical line across the screen towards the left, when moving to the left of the original anchor point.The width does not properly update for some reason.

Answer 1

The solution proposed in the question looks good:

_selection.Width = Math.Abs(e.X - initiallySelectedX);
_selection.Height = Math.Abs(e.Y - initiallySelectedY);

...but You have to move the origin of the rectangle when (eX - initiallySelectedX) < 0

So probably You want to add something like this to Your code:

var diffX = e.x - initiallySelectedX;
if (diffX < 0) _selection.X = initiallySelectedX - diffX;
var diffY = e.y - initiallySelectedY;
if (diffY < 0) _selection.Y = initiallySelectedY - diffY;

Where initiallySelectedX and initiallySelectedY are variables set onMouseDown . this is only a rough idea. The idea behind is that the Width and Height of the Rectangle cannot be NEGATIVE !!

Answer 2

Here's another snippet to draw a selection rectangle on a PictureBox :

//...
private Point startPoint;
private Point endPoint;
private readonly Pen P1 = new Pen(Color.SteelBlue, 2) {
    Alignment = PenAlignment.Center, DashStyle = DashStyle.Dash};
//...

Set the startPoint in the MouseDown event and reset the endPoint variables:

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        startPoint = new Point(e.X, e.Y);
        endPoint = Point.Empty;
    }
}

Set the endPoint and call Invalidate() method in the MouseMove event:

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    var p = new Point(e.X, e.Y);

    if (e.Button == MouseButtons.Left &&
        !p.Equals(startPoint))
    {
        endPoint = p;
        pictureBox1.Invalidate();
    }
}

Also call Invalidate() in the MouseUp event to remove the selection rectangle:

private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
{
    pictureBox1.Invalidate();
}

Draw the selection rectangle:

private void pictureBox1_Paint(object sender, PaintEventArgs e)
{
    if(MouseButtons == MouseButtons.Left &&
        !startPoint.Equals(endPoint) &&
        !endPoint.Equals(Point.Empty))
    {
        var g = e.Graphics;
        var rect = Rectangle.Empty;

        if(startPoint.X < endPoint.X)
        {
            rect.X = startPoint.X;
            rect.Width = endPoint.X - startPoint.X;
        }
        else
        {
            rect.X = endPoint.X;
            rect.Width = startPoint.X - endPoint.X;
        }
        if(startPoint.Y < endPoint.Y)
        {
            rect.Y = startPoint.Y;
            rect.Height = endPoint.Y - startPoint.Y;
        }
        else
        {
            rect.Y = endPoint.Y;
            rect.Height = startPoint.Y - endPoint.Y;
        }
        g.DrawRectangle(P1, rect);
    }
}

And don't forget to clean up:

private void YourForm_FormClosing(object sender, FormClosingEventArgs e)
{
    P1.Dispose();
}

Keep it simple.

Related Posts

^{How to call a method that uses PaintEventArgs and coordinates variables}

Answer 3

UPDATE:

Keeping track of the starting point on the MouseDown event allows the anchor point to correctly update:

private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
{
    // Starting point of the selection:
    if (e.Button == MouseButtons.Left)
    {
        _selecting = true;
        _selection = new Rectangle(new Point(e.X, e.Y), new Size());
        _startingPoint = e.Location;
    }
}

private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
{
    // Update the actual size of the selection:
    if (_selecting)
    {
        UpdateRectange(e.Location);
    }
}

private void UpdateRectange(Point newPos)
{
    var diffX = newPos.X - _startingPoint.X;
    var diffY = newPos.Y - _startingPoint.Y;
    var newSize = new Size(Math.Abs(diffX), Math.Abs(diffY));
    if (diffX > 0 && diffY > 0)
    {
        _selection = new Rectangle(_startingPoint, newSize);
    }
    else if (diffX < 0 && diffY < 0)
    {
        _selection = new Rectangle(newPos, newSize);
    }
    else if (diffX > 0 && diffY < 0)
    {
        _selection = new Rectangle(new Point(_startingPoint.X, _startingPoint.Y + diffY), newSize);
    }
    else
    {
        _selection = new Rectangle(new Point(_startingPoint.X + diffX, _startingPoint.Y), newSize);
    }
    uxScreenGrab.Invalidate();
}

UPDATE 2:

Re-wrote UpdateRectangle to simply move the anchor point, instead of instantiating at every call:

private void UpdateRectange(Point newPos)
{
    var diffX = newPos.X - _startingPoint.X;
    var diffY = newPos.Y - _startingPoint.Y;
    var newSize = new Size(Math.Abs(diffX), Math.Abs(diffY));
    if (diffX > 0 && diffY > 0)
    {
        _selection.Size = newSize;
    }
    else if (diffX < 0 && diffY < 0)
    {
        _selection.Location = newPos;
        _selection.Size = newSize;
    }
    else if (diffX > 0 && diffY < 0)
    {
        _selection.Y = _startingPoint.Y + diffY;
        _selection.Size = newSize;
    }
    else
    {
        _selection.X = _startingPoint.X + diffX;
        _selection.Size = newSize;
    }
    uxScreenGrab.Invalidate();
}