Why are some textboxes not accepting Control + A shortcut to select all by default

You might be looking for the ShortcutsEnabled property. Setting it to true would allow your text boxes to implement the Ctrl+A shortcut (among others). From the documentation:

Use the ShortcutsEnabled property to enable or disable the following shortcut key combinations:

• CTRL+Z

• CTRL+E

• CTRL+C

• CTRL+Y

• CTRL+X

• CTRL+BACKSPACE

• CTRL+V

• CTRL+DELETE

• CTRL+A

• SHIFT+DELETE

• CTRL+L

• SHIFT+INSERT

• CTRL+R

However, the documentation states:

The TextBox control does not support the CTRL+A shortcut key when the Multiline property value is true.

You will probably have to use another subclass of TextBoxBase, such as RichTextBox, for that to work.

Indeed CTRL + A will not work unless you add something like this:

  private void textBox1_KeyDown(object sender, KeyEventArgs e)
  {
      if (e.Control && (e.KeyCode == Keys.A))
      {
          if (sender != null)
               ((TextBox)sender).SelectAll();
          e.Handled = true;
      }
  }

This answer worked for me in a similar question (which isn't marked as accepted)

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    const int WM_KEYDOWN = 0x100;
    var keyCode = (Keys) (msg.WParam.ToInt32() &
                          Convert.ToInt32(Keys.KeyCode));
    if ((msg.Msg == WM_KEYDOWN && keyCode == Keys.A) 
        && (ModifierKeys == Keys.Control) 
        && txtYourTextBox.Focused)
    {
        txtYourTextBox.SelectAll();
        return true;
    }            
    return base.ProcessCmdKey(ref msg, keyData);
}

Original Post: How can I allow ctrl+a with TextBox in winform?