How Can I Make a C# Panel Receive Keyboard Input and Display a Focus Rectangle?

Improve C# Panel user control: solve focus problem

In graphics programs using C#, Panels that require keyboard input often encounter some problems. A common problem is that the Panel cannot obtain focus, resulting in the failure to trigger the KeyUp/KeyDown/KeyPress and GotFocus/LostFocus events.

In order to enhance the functionality of Panel, a more elegant solution is to modify the Panel base class as follows:

1. Enable optionality:

   <code class="language-csharp"> SetStyle(ControlStyles.Selectable, true);
    TabStop = true;</code>

2. Click the mouse to force focus:

   <code class="language-csharp"> protected override void OnMouseDown(MouseEventArgs e) {
        this.Focus();
        base.OnMouseDown(e);
    }</code>

3. Rewrite input key processing:

   <code class="language-csharp"> protected override bool IsInputKey(Keys keyData) {
        if (keyData == Keys.Up || keyData == Keys.Down) return true;
        if (keyData == Keys.Left || keyData == Keys.Right) return true;
        return base.IsInputKey(keyData);
    }</code>

4. Custom focus visual effects:

   <code class="language-csharp"> protected override void OnEnter(EventArgs e) {
        this.Invalidate();
        base.OnEnter(e);
    }
    protected override void OnLeave(EventArgs e) {
        this.Invalidate();
        base.OnLeave(e);
    }</code>

5. Show focus rectangle:

   <code class="language-csharp"> protected override void OnPaint(PaintEventArgs pe) {
        base.OnPaint(pe);
        if (this.Focused) {
            var rc = this.ClientRectangle;
            rc.Inflate(-2, -2);
            ControlPaint.DrawFocusRectangle(pe.Graphics, rc);
        }
    }</code>

With these modifications, Panel can both be selected and receive keyboard input. The code provided ensures that the Panel gets focus when clicked and responds to the up, down, left and right arrow keys. Additionally, when a Panel gains focus, it displays a focus rectangle around it, thereby enhancing the user experience.

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