Создание просто Silverlight - приложения для Windows Phone 7

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using Microsoft.Phone.Controls;
namespace FirstPhoneApplication
{
    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();
        }

        private void PlusButton_Click(object sender, RoutedEventArgs e)
        {

        }

        private void MinusButton_Click(object sender, RoutedEventArgs e)
        {
        }
        private void MultiplicationButton_Click(object sender, RoutedEventArgs e)
        {
        }

        private void DivideButton_Click(object sender, RoutedEventArgs e)
        {
        }

        private void EquallyButton_Click(object sender, RoutedEventArgs e)
        {
        }
    
     
    }
}

private double a;
        private double b;
        private double c;
        private string opertaiontype;
        private void PlusButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                a = Convert.ToDouble(ResultTextBox.Text);
                opertaiontype = " Plus" ;
                ResultTextBox.Text = " " ;
            }
            catch { MessageBox.Show(" Введен не числовой элемент." );}
        }

        private void MinusButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                a = Convert.ToDouble(ResultTextBox.Text);
                opertaiontype = " Minus" ;
                ResultTextBox.Text = " " ;
            }
            catch { MessageBox.Show(" Введен не числовой элемент." );}
        }

        private void MultiplicationButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                a = Convert.ToDouble(ResultTextBox.Text);
                opertaiontype = " Multiplication" ;
                ResultTextBox.Text = " " ;
            }
            catch { MessageBox.Show(" Введен не числовой элемент." );}
        }

        private void DivideButton_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                a = Convert.ToDouble(ResultTextBox.Text);
                opertaiontype = " Dividide" ;
                ResultTextBox.Text = " " ;
            }
            catch { MessageBox.Show(" Введен не числовой элемент." );}
        }

        private void EquallyButton_Click(object sender, RoutedEventArgs e)
        {
            b = Convert.ToDouble(ResultTextBox.Text);
            switch (opertaiontype)
            {
                case " Plus" :
                c = a + b;
                break;
                case " Minus" :
                c = a - b;
                break;
                case " Multiplication" :
                c = a * b;
                break;
                case " Dividide" :
                c = a / b;
                break;
            }
            ResultTextBox.Text = c.ToString();
        }