using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
namespace TestApplication {
 /// <summary>
 /// Interaction logic for MainWindow.xaml
 /// </summary>
 public partial class MainWindow: Window {
 public MainWindow() {
 InitializeComponent();
 }

 private void TextBox_PreviewTextInput(object sender, TextCompositionEventArgs e) {
 if (e.Text != "." && IsNumber(e.Text) == false) {
 e.Handled = true;
 } else if (e.Text == ".") {
 if (((TextBox) sender).Text.IndexOf(e.Text) > -1) {
 e.Handled = true;
 }
 }
 }
 private bool IsNumber(string Text) {
 int output;
 return int.TryParse(Text, out output);
 }
 }
}