C#

protected override void OnMouseMove(System.Windows.Forms.MouseEventArgs e)
{
    //========================================
    //You may want to add some validation here.

    this.Left = Cursor.Position.X;
    this.Top = Cursor.Position.Y;

}

protected override void OnMouseMove(System.Windows.Forms.MouseEventArgs e)
{

    this.Left = Cursor.Position.X - this.Left/2;
    this.Top =  Cursor.Position.Y - this.Top/2;

}

private bool mouseDown = false;
private Point mouseDownLocation = Point.Empty;

protected override void OnMouseDown(System.Windows.Forms.MouseEventArgs e)
{
	if( (e.Button & MouseButtons.Left) > 0 )
	{
		mouseDown = true;
		mouseDownLocation = PointToClient(Cursor.Position);
	}
}

protected override void OnMouseUp(System.Windows.Forms.MouseEventArgs e)
{
	if( (e.Button & MouseButtons.Left) > 0 )
	{
		mouseDown = false;
		mouseDownLocation = Point.Empty;
	}
}

protected override void OnMouseMove(System.Windows.Forms.MouseEventArgs e)
{
	if( (mouseDown) )
	{
		// Get the current position
		Point currLocation = PointToClient(Cursor.Position);
		Point offset = currLocation;

		offset.Offset(-mouseDownLocation.X, -mouseDownLocation.Y);

		Point newLocation = Location;

		newLocation.Offset(offset.X, offset.Y);

		Location = newLocation;
	}
}

if( (File.GetAttributes(filename) & FileAttributes.ReadOnly) == FileAttributes.ReadOnly )
	File.SetAttributes( filename , FileAttributes.Normal );

if(file.Attributes == FileAttributes.ReadOnly)
{
   // The file is read only!
}