为什么我的自定义控件文本框的onpaint覆盖方法永远不会被调用？

Question

I have created a customized textbox which has border in red color. I then launch my application but this OnPaint never gets called.

My code is this:

public partial class UserControl1 : TextBox
    {
        public string disable, disableFlag;
        public string Disable 
        { 
            get 
            { 

                return disable;
            } 
            set 
            { 
                disable = value;
                disableFlag = disable;
                //OnPaint(PaintEventArgs.Empty);
            } 
        }

        public UserControl1()
        {
            InitializeComponent();
        }


        protected override void OnPaint(PaintEventArgs e)
        {
            base.OnPaint(e);
            this.Text = "testing 1";
            if (disable == "true")
            {

                Pen redPen = new Pen(Color.Red);
                Graphics graphics = this.CreateGraphics();
                graphics.DrawRectangle(redPen, this.Location.X,
                              this.Location.Y, this.Width, this.Height);
                //  base.DrawFrame(e.Graphics);
            }
        }
    }

Please let me know what is the problem (Here is the snapshot of the winform http://prntscr.com/ceq7x5 )?

Answer 1

You shouldn't create a new Graphics object. Use e.Graphics.DrawRectangle to be able to draw on the control using the already existing Graphics object like this:

Pen redPen = new Pen(Color.Red);
e.Graphics.DrawRectangle(redPen, this.Location.X,
                          this.Location.Y, this.Width, this.Height);

Also, to repeat my comment here about the Disabled flag. There's no point adding a custom Disable flag. Use the Enabled property that is already provided by the Windows Forms TextBox control.

Edit: Please notice that the above code doesn't work in TextBox's case, because it handles drawing differently. TextBox is basically just a wrapper around the native Win32 TextBox so you need to listen to quite a few messages that tell it to repaint itself. You also need to get the handle to the device context and transform it to a managed Graphics object to be able to draw.

Take a look at this article for code and explanations on how to draw on top of TextBox. Especially part 2. Drawing Onto the TextBox on the bottom of the page.