How do I make a richTextBox have a transparent background / show the image beneath it? (C#, Windows Forms App)

Hi @dorian tester , Welcome to Microsoft Q&A,

Sorry, there is no official plugin for the WinForms project that supports transparent background color.

If you can use WPF, you can use transparent background color in it.

Best Regards,

Jiale

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If the OS is Windows 8 or higher, you can make the child window a layered window. To do this, add an application manifest and enable Windows 8.

      <!-- Windows 8 -->
      <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}" />

https://zcusa.951200.xyz/en-us/answers/questions/1433265/adding-transparent-form-in-panel

public partial class Form1 : Form
{

    public Form1()
    {
        InitializeComponent();
        richTextBox1.HandleCreated += RichTextBox1_HandleCreated;
    }

    private void RichTextBox1_HandleCreated(object? sender, EventArgs e)
    {
        IntPtr hWnd = richTextBox1.Handle;
        int exStyle = GetWindowLong(hWnd, GWL_EXSTYLE);
        SetWindowLong(hWnd, GWL_EXSTYLE, exStyle | WS_EX_LAYERED);
        SetLayeredWindowAttributes(hWnd, ColorTranslator.ToWin32(richTextBox1.BackColor), 0, LWA_COLORKEY);
    }

    const int WS_EX_LAYERED = 0x00080000;
    const int GWL_EXSTYLE = -20;
    const int LWA_COLORKEY = 0x01;
    const int LWA_ALPHA = 0x02;

    [DllImport("user32.dll", SetLastError = true)]
    private static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern int GetWindowLong(IntPtr hWnd, int nIndex);

    [DllImport("user32.dll", SetLastError = true)]
    private static extern bool SetLayeredWindowAttributes(
        IntPtr hWnd, int crKey, byte bAlpha, int dwFlags);
}