C# 4 and .NET 4

OC42 ❘ ChaPTer 48 GrAphics with Gdi+
In most cases, this will work. However, there could be a problem if your application (or even some other
application with a high priority) is doing some computationally intensive work at the moment the user
double-clicks. It just might happen in that case that the OnDoubleClick() event handler does not get called
until perhaps half a second or so after the user has double-clicked. You do not want such delays because
they usually annoy users intensely, but even so, occasionally it does happen and sometimes for reasons
beyond the control of your application (a slow computer, for instance). The trouble is that half a second
is easily enough time for the mouse to be moved halfway across the screen, in which case your call to
Control.MousePosition will return the completely wrong location!
A better approach here is to rely on one of the many overlaps between mouse event meanings. The first part
of double-clicking a mouse involves pressing the left button down. This means that if OnDoubleClick() is
called, you know that OnMouseDown() has also just been called, with the mouse at the same location. You
can use the OnMouseDown() override to record the position of the mouse, ready for OnDoubleClick(). This
is the approach taken in CapsEditor:
protected override void OnMouseDown(MouseEventArgs e)
{
base.OnMouseDown(e);
mouseDoubleClickPosition = new Point(e.X, e.Y);
}
Now look at the OnDoubleClick() override. There is quite a bit more work to do here:
protected override void OnDoubleClick(EventArgs e)
{
int i = PageCoordinatesToLineIndex(mouseDoubleClickPosition);
if (i >= 0)
{
TextLineInformation lineToBeChanged =
(TextLineInformation)documentLines[i];
lineToBeChanged.Text = lineToBeChanged.Text.ToUpper();
Graphics dc = this.CreateGraphics();
uint newWidth =(uint)dc.MeasureString(lineToBeChanged.Text,
mainFont).Width;
if (newWidth > lineToBeChanged.Width)
lineToBeChanged.Width = newWidth;
if (newWidth+2*margin > this.documentSize.Width)
{
documentSize.Width = (int)newWidth;
AutoScrollMinSize = this.documentSize;
}
Rectangle changedRectangle = new Rectangle(
LineIndexToPageCoordinates(i),
new Size((int)newWidth,
(int)this.lineHeight));
Invalidate(changedRectangle);
}
base.OnDoubleClick(e);
}
code download CapsEditor.sln
You start off by calling PageCoordinatesToLineIndex() to work out which line of text the mouse pointer
was hovering over when the user double-clicked. If this call returns -1, then you weren’t over any text, so
there is nothing to do — except, of course, call the base class version of OnDoubleClick() to let Windows
do any default processing.
Assuming that you have identified a line of text, you can use the string.ToUpper() method to convert
it to uppercase. That was the easy part. The hard part is figuring out what needs to be redrawn where.
Fortunately, because this example is simple, there are not too many combinations. You can assume that
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