How to restrict the input in TextBox only until the edge of the box?

Question

I want to restrict the input in the TextBox only to the edge of the box? I tried it with MaxLength property but I didn't get the result I wanted. Are there other options?

Answer 1

From comments, a test by comparing text lenght with TextBox client width (I tested with renderScope.ActualWidth (Reflection) which seems accurate) =>

     <TextBox x:Name="textBox1" HorizontalAlignment="Left" Height="27" Margin="79,133,0,0" VerticalAlignment="Top" Width="225" PreviewKeyDown="textBox1_PreviewKeyDown"/>

Main code :

    public MainWindow()
    {
        InitializeComponent();
        DataObject.AddPastingHandler(textBox1, OnPaste);
    }

    private KeyConverter kc = new KeyConverter();

    private void textBox1_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        TextBox tb = (TextBox)sender;
        string sText = tb.Text;
        string sKey = kc.ConvertToString(e.Key);
        bool bCapsLock = Keyboard.IsKeyToggled(Key.CapsLock);
        bool bShift = (Keyboard.Modifiers & ModifierKeys.Shift) == ModifierKeys.Shift;
        bool bCharacter = false;
        if ((!bCapsLock && !bShift) || (bCapsLock && bShift))
        {
            sKey = sKey.ToLower();
        }
        if (sKey != null && sKey.Length == 1)
        {
            if (char.IsLetterOrDigit(sKey[0]))
            {
                sText += sKey[0];
                bCharacter = true;
            }
        }
        // System.Windows.Controls.TextBoxView
        var renderScope = tb.GetFieldValue<FrameworkElement>("_renderScope") as IServiceProvider;
        var nActualWidthView = (double)renderScope.InvokeMethod("get_ActualWidth");
        var sz = MeasureText(tb, sText);
        if (bCharacter && sz.Width >= nActualWidthView)
        {
            e.Handled = true;
            Console.Beep(1000, 10);
        }
    }        

    private void OnPaste(object sender, DataObjectPastingEventArgs e)
    {
        e.Handled = true;
        e.CancelCommand();
    }

    // Adapted from MSAGL
    public Size MeasureText(TextBox ctrl, string text)
    {
        FormattedText formattedText = new FormattedText(
            text,
            System.Globalization.CultureInfo.CurrentCulture,
            FlowDirection.LeftToRight,
            new Typeface(ctrl.FontFamily, ctrl.FontStyle, ctrl.FontWeight, ctrl.FontStretch),
            ctrl.FontSize,
            new SolidColorBrush(Colors.Black),
            VisualTreeHelper.GetDpi(this).PixelsPerDip);
        return new Size(formattedText.Width, formattedText.Height);
    }   

Utility class for Reflection :

// https://sudonull.com/post/12432-Bug-when-working-TextBoxGetLineText-in-NET-WPF
public static class ReflectionExtensions
{
    public static T GetFieldValue<T>(this object obj, string name)
    {
        var bindingFlags = System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance;
        var field = obj.GetType().GetField(name, bindingFlags);
        if (field == null)
            field = obj.GetType().BaseType.GetField(name, bindingFlags);
        return (T)field?.GetValue(obj);
    }
    public static object InvokeMethod(this object obj, string methodName, params object[] methodParams)
    {
        var methodParamTypes = methodParams?.Select(p => p.GetType()).ToArray() ?? new Type[] { };
        var bindingFlags = System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Public | System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.Static;
        System.Reflection.MethodInfo method = null;
        var type = obj.GetType();
        while (method == null && type != null)
        {
            method = type.GetMethod(methodName, bindingFlags, Type.DefaultBinder, methodParamTypes, null);
            var intfs = type.GetInterfaces();
            if (method != null)
                break;
            foreach (var intf in intfs)
            {
                method = intf.GetMethod(methodName, bindingFlags, Type.DefaultBinder, methodParamTypes, null);
                if (method != null)
                    break;
            }
            type = type.BaseType;
        }
        return method?.Invoke(obj, methodParams);
    }
}