<p>What is the best way to block certain input keys from being used in a TextBox with out blocking special keystrokes such as <kbd>Ctrl</kbd>-<kbd>V</kbd>/<kbd>Ctrl</kbd>-<kbd>C</kbd>?</p>
<p>For example, only allowing the user to enter a subset of characters or numerics such as  A or B or C and nothing else.</p>

You could use the TextChanged event for the textbox.
```
```
private void txtInput_TextChanged(object sender, EventArgs e)
    {
        if (txtInput.Text.ToUpper() == "A" || txtInput.Text.ToUpper() == "B")
        {
            //invalid entry logic here
        }
    }
```
```