无法在WPF中将焦点设置在TextBox上

Question

I can't set focus on my TextBox , I have this code:

private void txbUid_KeyDown(object sender, KeyEventArgs e)
{
  if (e.Key != Key.Tab) return; // here control doesn't loose focus

  string stringUid = txbUid.Text;
  long uid;

  if (!TryParseUid(stringUid, out uid))
  {
    //string errMsg = $"Niepoprawny kod UID: {stringUid}";
    //lblError.Text = errMsg;
    //this.LogError(errMsg);
    //txbUid.Text = "";
    //txbUid.Focus();
    return; // here control looses focus
  }
}

As you can see, I tried commenting out particular lines to see whether on of them is causing the problem. But nothing has worked.

Answer 1

All this was caused by pressing tab button.

If it wasn't pressed, control stay focused as it should - and that was misleading.

When I pressed tab, then by default, the next control got focus, ie I handled the event, but it was passed further to containing control, which set focus on other control. Thus unexpected behaviour.

The solution was to set e.Handled , instead of using Focus() method (after which further processing of an event set focus to other control anyway), to true in the event, so pressing tab event wasn't processed any further.

Example code:

private void txbUid_KeyDown(object sender, KeyEventArgs e)
{
  if (e.Key != Key.Tab) return; // here control doesn't loose focus

  string stringUid = txbUid.Text;
  long uid;

  if (!TryParseUid(stringUid, out uid))
  {
    // no need to comment out anything
    string errMsg = $"Niepoprawny kod UID: {stringUid}";
    lblError.Text = errMsg;
    this.LogError(errMsg);
    txbUid.Text = "";
    // mark event as handled
    e.Handled = true;
    return;
  }
}

Answer 2

The Tab character will be handled internally by the TextBox . You can try setting e.Handled = true before returning. If that doesn't work, you need to handle PreviewKeyDown instead (and don't forget to set the Handled property).