Firstly, if you expect us to help you with errors then you need to provide us with the error messages and where they occur in the code.
That said, you should absolutely not be doing what you're doing the way you're doing it. If you want a prompt displayed when the
TextBox is empty then that functionality is built into Windows. It's officially known as a cue banner and it doesn't interact with the
Text property directly, so there's no handling of events to change the
Text under specific circumstances. There are three main ways that you could implement this and I will present them in descending order of dodginess. Firstly, just put it straight into your form:
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(
IntPtr hWnd,
int msg,
int wParam,
[MarshalAs(UnmanagedType.LPWStr)] string lParam);
private const int EM_SETCUEBANNER = 0x1501;
private void Form1_Load(object sender, EventArgs e)
{
SendMessage(textBox1.Handle,
EM_SETCUEBANNER,
0,
"Type Here");
}
Secondly, you could create an extension method:
public static class TextBoxExtensions
{
private const int EM_SETCUEBANNER = 0x1501;
public static void SetCueBannerText(this TextBox source, string text)
{
SendMessage(source.Handle,
EM_SETCUEBANNER,
0,
text);
}
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(
IntPtr hWnd,
int msg,
int wParam,
[MarshalAs(UnmanagedType.LPWStr)] string lParam);
}
and then call that in your form:
private void Form1_Load(object sender, EventArgs e)
{
textBox1.SetCueBannerText("Type Here");
}
Finally, you can build it into a custom control:
public class CueBannerTextBox : TextBox
{
private const int EM_SETCUEBANNER = 0x1501;
private string cueBannerText;
public event EventHandler CueBannerTextChanged;
[Category("Appearance")]
public string CueBannerText
{
get => cueBannerText;
set
{
if (cueBannerText != value)
{
cueBannerText = value;
SendMessage(Handle,
EM_SETCUEBANNER,
0,
cueBannerText);
OnCueBannerTextChanged(EventArgs.Empty);
}
}
}
protected virtual void OnCueBannerTextChanged(EventArgs e)
{
CueBannerTextChanged?.Invoke(this, e);
}
[DllImport("user32.dll", CharSet = CharSet.Auto)]
private static extern int SendMessage(
IntPtr hWnd,
int msg,
int wParam,
[MarshalAs(UnmanagedType.LPWStr)] string lParam);
}
and then add that to your form instead of a regular
TextBox. Once you've added that class and built your project, the new control will appear in the
Toolbox automatically, or you can build it into a library and then add it to the
Toolbox for all projects. If you already have regular
TextBoxes on the form and don't want to have to delete them, you can edit the design code file by handle and simply change the type in code. You will then have access to the
CueBannerText property in the designer and, when it is set, the text will apear at design time too.
For the record, there is an
EM_GETCUEBANNER message too, so you don't have to store the text in the control and you probably shouldn't, because it will be inaccurate if an
EM_SETCUEBANNER message is sent externally. I just couldn't be bothered doing that here and now. Consider it am exercise for you if you make the right choice and go with the custom control.
