Windows Forms: How to copy paste plain text in RichTextBox in C#

Dragging a RichTextBox control from your Visual Studio toolbox to your form designer.

By default, Instead of pasting the text with format. We can extract the plain text, then add it into the RichTextBox control.

Clicking on RichTextBox control, then select properties.

Adding a KeyDown event handler allows you to paste text into a RichTextBox control.

private void richTextBox_KeyDown(object sender, KeyEventArgs e)
{
    if (e.Control && e.KeyCode == Keys.V)
    {
        richTextBox.Text += (string)Clipboard.GetData("Text");
        e.Handled = true;
    }
}

Every time you copy data, it will be stored in the Clipboard. To get plain text to can call GetData("Text") method.