Snipped from this ng:
"I came across a reference on a web site
(
http://www.personalmicrocosms.com/ht...htextbox_lines )
that said to speed up access to a rich text box's lines that you needed to
use a "foreach" loop instead of a "for" loop. This made absolutely no sense
to me, but the author had posted his code and timing results. The "foreach"
(a VB and other languages construct) was 0.01 seconds to access 1000 lines
in
rich text box, whereas the "for" loop (a traditional C++ construct) was an
astounding 25 seconds (on a not very fast PC).
I recreated a test file using the partial source code posted by the author
and verified that there is a SIGNIFICANT performance difference between the
two constructs (although on my PC is was 0.01 seconds vs 3.6 seconds - still
a noticeable delay). Unfortunately, there was no explanation as to why this
was the case and I couldn't see anything as to why one loop construct would
be different. Looking at the generated IL code with Lutz Roeder's Reflector
tool, I see that the real culprit is not the loop structure but the
get_Lines() function that is pulled out of the loop in the "foreach" loop
and
not in the "for" loop code. Which, leads to me post this question about the
differences in complier code generation/optimization and is there any
setting
that can change this.
Interestingly, this is true for both Debug and Release builds. The compiler
generated code that called that function twice for each pass of the loop
(once for the loop index check and then again for the length calculation).
Pulling out unneccessary function calls is pretty basic optimization, and I
surprised that the compiler didn't detect this.
With the IDE's intellisense and auto completion features, the "for" loop
construct shown in the code below seems like something that someone might
actually code up, and of course who would have figured out that the
get_Lines
method would be so performance intensive.
Makes me wonder if there are any other gotchas like this.
Thanks, Mike L.
----------------------------------------------------------------------------
----------------
//Simple windows form with a richtextbox control, initialized w/1000 lines
of text (e.g., "line #101", etc).
private void ForLoopButton_Click(object sender, System.EventArgs e)
{
Cursor.Current = Cursors.WaitCursor;
int Len = 0;
int Start = Environment.TickCount;
for (int i = 0; i < TheRichTextBox.Lines.Length; i++)
{
Len += TheRichTextBox.Lines[i].Length;
}
int ElapsedTime = Environment.TickCount - Start;
ResultsTextBox.Clear();
RsultsTextBox.Text = "for loop\r\n\r\nElapsed time = " + ((double)
ElapsedTime / (double) 1000.0).ToString() + " seconds\r\n\r\nResult = " +
Len.ToString();
Cursor.Current = Cursors.Arrow;
}
private void ForEachLoopButton_Click(object sender, System.EventArgs e)
{
Cursor.Current = Cursors.WaitCursor;
int Len = 0;
int Start = Environment.TickCount;
foreach (String Line in TheRichTextBox.Lines)
{
Len += Line.Length;
}
int ElapsedTime = Environment.TickCount - Start;
ResultsTextBox.Clear();
ResultsTextBox.Text = "foreach loop\r\n\r\nElapsed time = " + ((double)
ElapsedTime / (double) 1000.0).ToString() + " seconds\r\n\r\nResult = " +
Len.ToString();
Cursor.Current = Cursors.Arrow;
}
private void ForLoopButton2_Click(object sender, System.EventArgs e)
{
//Performance results now same as ForEachLoopButton_Click with the changes
made.
Cursor.Current = Cursors.WaitCursor;
int Len = 0;
int Start = Environment.TickCount;
string[] lines = TheTextBox.Lines;
for (int i = 0; i < lines.Length; i++)
{
Len += lines[i].Length;
}
int ElapsedTime = Environment.TickCount - Start;
ResultsTextBox.Clear();
RsultsTextBox.Text = "for loop\r\n\r\nElapsed time = " + ((double)
ElapsedTime / (double) 1000.0).ToString() + " seconds\r\n\r\nResult = " +
Len.ToString();
Cursor.Current = Cursors.Arrow;
}"
"Othman Kinani" <Ot**********@discussions.microsoft.com> wrote in message
news:15**********************************@microsof t.com...
"an*******@discussions.microsoft.com" wrote:
Is there a performance difference between this:
\\\
Dim i As Integer
For i = 0 to myObject.Controls.Count - 1
myObject.Controls(i) = ...
Next
///
and this:
\\\
Dim ctl As Control
For Each ctl In myObject.Controls
ctl = ...
Next
///
Or is For Each just "prettier"?
Thanks,
Eric
