how to disable a textbox when it has some data and not disable if it has no data in c#

The other answers are not enough. You should trigger the check of the content somehow. The only way to do it is handling the event

myTextBox.TextChanged += delegate(object sender, EventArgs e) {
    TextBox textBox = (TextBox)sender;
    textBox.Enabled = string.IsNullOrEmpty(textBox.Text);
};

Now, let me tell you: this is a trap. As soon as you delete the text, your text box cannot be edited anymore. Are you not afraid :-)?

I'm not sure you want to trigger this stuff in this way, at least I gave you the idea. After all, you can re-enable the text box using some other control with textBox.Enabled = true. I hope (as much as I can see from your Question), some updates are coming from the database; in this case getting non-empty data can re-enable the text box as you need.

—SA

textBox.Enable = (string.IsNullOrEmpty(textBox.Text));

Doesn't textbox.Enabled = false do it? Am I missing something here?