#948 – Complete Example of Limiting TextBox Input

If you want to limit text allowed as input to a TextBox, a full strategy for checking text being input should include handling the PreviewKeyDown and PreviewTextInput events, as well as implementing a pasting handler.  Below is a full example that limits text input to alphabetic characters only.

        <TextBox Name="txtMyText" Margin="5" Height="100"
                 TextWrapping="Wrap"
                 AcceptsReturn="True"
                 VerticalScrollBarVisibility="Auto"
                 PreviewTextInput="TextBox_PreviewTextInput"
                 PreviewKeyDown="TextBox_PreviewKeyDown"/>

        public MainWindow()
        {
            this.InitializeComponent();
            DataObject.AddPastingHandler(txtMyText, PasteHandler);
        }

        private bool IsAlphabetic(string s)
        {
            Regex r = new Regex(@"^[a-zA-Z]+$");

            return r.IsMatch(s);
        }

        private void TextBox_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            // Prohibit non-alphabetic
            if (!IsAlphabetic(e.Text))
                e.Handled = true;
        }

        private void TextBox_PreviewKeyDown(object sender, KeyEventArgs e)
        {
            // Prohibit space
            if (e.Key == Key.Space)
                e.Handled = true;
        }

        private void PasteHandler(object sender, DataObjectPastingEventArgs e)
        {
            TextBox tb = sender as TextBox;
            bool textOK = false;

            if (e.DataObject.GetDataPresent(typeof(string)))
            {
                // Allow pasting only alphabetic
                string pasteText = e.DataObject.GetData(typeof(string)) as string;
                if (IsAlphabetic(pasteText))
                    textOK = true;
            }

            if (!textOK)
                e.CancelCommand();
        }