How to make a TextBox accept only the letters S and N?

In the .xaml file of your Window , place the following code. What matters here is the KeyDown="txtTest_KeyDown" event:

<TextBox x:Name="txtTest" KeyDown="txtTest_KeyDown" HorizontalAlignment="Left" Height="23" Margin="239,148,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="282"/>

Now, in the% w of% of your% w%, create the% w% method as follows:

private void txtTest_KeyDown(object sender, KeyEventArgs e)
{
    if(e.Key == Key.S || e.Key == Key.N)
    {
        e.Handled = false;
    }
    else
    {
        e.Handled = true;
    }
}

What happens immediately is to intercept the key pressed, using the KeyDown event, and check which one it was. If the key corresponds to S or N the implementation should return e.Handled = false; , otherwise return e.Handled = true; .

Unfortunately this is not enough, since the space key is not intercepted. On the other hand there is the possibility that the text can be entered via paste .

So, to deal with all possibilities, you should treat the following events:

• PreviewTextInput
• PreviewKeyDown
• Paste

and restrict the number of accepted characters to 1.

The "best" way to implement is to write a custom TextBox that you can use anywhere.

public class S_N_TextBox : TextBox
{

    public S_N_TextBox()
    {
        DataObject.AddPastingHandler(this, PastingHandler);
        MaxLength = 1;
    }

    private void PastingHandler(object sender, DataObjectPastingEventArgs e)
    {
        var pasteText = e.DataObject.GetData(typeof(string)) as string;
        //Garante que o texto inserido via paste é válido
        if (!IsValid(pasteText))
        {
            e.CancelCommand();
        }
    }

    protected override void OnPreviewTextInput(TextCompositionEventArgs e)
    {
        if (IsValid(e.Text))
        {
            //A documentação recomenda que o método base seja chamado.
            base.OnPreviewTextInput(e);
            return;
        }
        e.Handled = true;

    }

    protected override void OnPreviewKeyDown(KeyEventArgs e)
    {
        //Não permite o uso do espaço
        e.Handled = e.Key == Key.Space;
    }

    //Determina que texto pode ser aceite
    protected virtual bool IsValid(string input) => input == "S" || input == "N";
}

This class can be used as the basis for other restrictions that you want a TextBlox to have. You only need to change the IsValid () method.