A marquee control in C#

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Overview

This application demonstrates how to create a "string loop" in C#. A common use of "string loops" is in web advertisements, where the slogan or product name appears to be revolving in a circle. The code creates a rectangle in the middle of the client area that is just large enough to fit the entire message, minus any trailing white space. Clicking on the form or resizing it will cause the "string loop" to restart.

Points to Notice

Rather than use a string to represent the message ("Catch Me If You Can…" ) it is far more efficient and intuitive to use the StringBuilder class, found in the System.Text namespace. StringBuilder allows you to directly manipulate a string, rather than having to make copies of a regular string. The manipulation of the StringBuilder’s internal string is quite straightforward:

private void DrawMovingText()
{
    Graphics grfx = CreateGraphics();
    Font font = new Font( "Courier New", 20, FontStyle.Bold );
    string spaces = " ";
    // 
    // StringBuilder is used to allow for efficient manipulation of 
    // one string, rather than generating many separate strings
    //
    StringBuilder str = 
        new StringBuilder( "Catch Me If You Can..." + spaces );

    Rectangle rect = CreateRect( grfx, str, font );

    int numCycles = str.Length * 3 + 1;
    for( int i = 0; i < numCycles; ++i )
    {
        grfx.FillRectangle( Brushes.White, rect );
        grfx.DrawString( str.ToString(), font, Brushes.Red, rect );
        // relocate the first char to the end of the string
        str.Append( str[0] );
        str.Remove( 0, 1 );
        // pause for visual effect
        Thread.Sleep( 150 );
    }
    grfx.Dispose();
}

To avoid having DrawMovingText() eat up all of the CPU's attention it is placed on a worker thread. Every time the user clicks on the form or resizes it, the thread is killed and reborn. Obviously, if you wanted to alter the speed at which the letters "move" you would simply adjust the number of milliseconds that the thread sleeps for.

//
// All of these events will restart the thread that draws 
// the text
//
private void Form_Load( object sender, System.EventArgs e )
{
    StartThread();
}
private void Form_MouseUp(object sender, 
                          System.Windows.Forms.MouseEventArgs e)
{
    StartThread();
}
private void Form_Resize( object sender, System.EventArgs e )
{
    StartThread();
}
private void label_Click(object sender, System.EventArgs e)
{
    StartThread();
}
private void StartThread()
{
    thread.Abort();
    // give the thread time to die
    Thread.Sleep( 100 ); 
    Invalidate();
    thread = new Thread(new ThreadStart(DrawMovingText));
    thread.Start();
}

The final point of interest is how the size and location of the rectangle that contains the message are determined. The Graphics class has a MeasureString method that will give you the width or height of the string that you are passing in. Those are used for the width and height of the rectangle. You then use those values in tandem with the dimensions of the form to determine the coordinates of where the rectangle will originate from.

private Rectangle CreateRect
( Graphics grfx, StringBuilder str, Font font )
{
    // + 5 to allow last char to fit
    int w = (int)grfx.MeasureString(str.ToString(), font).Width + 5; 
    int h = (int)grfx.MeasureString( str.ToString(), font ).Height;
    int x = (int)this.Width / 2 - w / 2;
    int y = (int)this.Height / 2 - h;
    return new Rectangle( x, y, w, h );
}