Q:

Calculator Total failing

I am coding a calculator using C#.
namespace Calculator
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        public void Total(string num)   
        {
            lblTotal.Content = Convert.ToString(lblTotal.Content) + num;
        }
        private void one_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(1) );
        }
        private void two_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(2) );
        }
        private void three_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(3) );
        }
        private void four_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(4) );
        }
        private void five_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(5) );
        }
        private void six_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(6) );
        }
        private void seven_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(7) );
        }
        private void eight_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(8) );
        }
        private void nine_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(9) );
        }
        private void ten_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(10) );
        }
        private void zero_Click(object sender, RoutedEventArgs e)
        {
            Total( Convert.ToString(0) );
        }

        private void add_Click(object sender, RoutedEventArgs e)
        {
            Total("+");
        }

        private void subtract_Click(object sender, RoutedEventArgs e)
        {
            Total("-");
        }

        private void multiply_Click(object sender, RoutedEventArgs e)
        {
            Total("*");
        }

        private void divide_Click(object sender, RoutedEventArgs e)
        {
            Total("/");
        }

        private void total_Click(object sender, RoutedEventArgs e)
        {
            int calculate;
            string content;

            content = Convert.ToString(lblTotal.Content);
            calculate = Convert.ToInt32(content);

        }

    }

}
So when a user clicks the total button, it should open the lblTotal.Content and get the math it needs to calculate and set the lblTotal.Content to the answer.
I get this error:
System.FormatException was unhandled
  _HResult=-2146233033
  _message=Input string was not in a correct format.
  HResult=-2146233033
  IsTransient=false
  Message=Input string was not in a correct format.
  Source=mscorlib
  StackTrace:
       at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
       at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
       at System.String.System.IConvertible.ToInt32(IFormatProvider provider)
       at System.Convert.ToInt32(Object value)
       at Calculator.MainWindow.total_Click(Object sender, RoutedEventArgs e) in c:\Users\Administrator\Documents\Visual Studio 2013\Projects\Calculator\Calculator\MainWindow.xaml.cs:line 100
       at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
       at System.Windows.UIElement.RaiseEvent(RoutedEventArgs e)
       at System.Windows.Controls.Primitives.ButtonBase.OnClick()
       at System.Windows.Controls.Button.OnClick()
       at System.Windows.Controls.Primitives.ButtonBase.OnMouseLeftButtonUp(MouseButtonEventArgs e)
       at System.Windows.UIElement.OnMouseLeftButtonUpThunk(Object sender, MouseButtonEventArgs e)
       at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
       at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
       at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       at System.Windows.UIElement.ReRaiseEventAs(DependencyObject sender, RoutedEventArgs args, RoutedEvent newEvent)
       at System.Windows.UIElement.OnMouseUpThunk(Object sender, MouseButtonEventArgs e)
       at System.Windows.Input.MouseButtonEventArgs.InvokeEventHandler(Delegate genericHandler, Object genericTarget)
       at System.Windows.RoutedEventArgs.InvokeHandler(Delegate handler, Object target)
       at System.Windows.RoutedEventHandlerInfo.InvokeHandler(Object target, RoutedEventArgs routedEventArgs)
       at System.Windows.EventRoute.InvokeHandlersImpl(Object source, RoutedEventArgs args, Boolean reRaised)
       at System.Windows.UIElement.RaiseEventImpl(DependencyObject sender, RoutedEventArgs args)
       at System.Windows.UIElement.RaiseTrustedEvent(RoutedEventArgs args)
       at System.Windows.UIElement.RaiseEvent(RoutedEventArgs args, Boolean trusted)
       at System.Windows.Input.InputManager.ProcessStagingArea()
       at System.Windows.Input.InputManager.ProcessInput(InputEventArgs input)
       at System.Windows.Input.InputProviderSite.ReportInput(InputReport inputReport)
       at System.Windows.Interop.HwndMouseInputProvider.ReportInput(IntPtr hwnd, InputMode mode, Int32 timestamp, RawMouseActions actions, Int32 x, Int32 y, Int32 wheel)
       at System.Windows.Interop.HwndMouseInputProvider.FilterMessage(IntPtr hwnd, WindowMessage msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at System.Windows.Interop.HwndSource.InputFilterMessage(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndWrapper.WndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam, Boolean& handled)
       at MS.Win32.HwndSubclass.DispatcherCallbackOperation(Object o)
       at System.Windows.Threading.ExceptionWrapper.InternalRealCall(Delegate callback, Object args, Int32 numArgs)
       at MS.Internal.Threading.ExceptionFilterHelper.TryCatchWhen(Object source, Delegate method, Object args, Int32 numArgs, Delegate catchHandler)
       at System.Windows.Threading.Dispatcher.LegacyInvokeImpl(DispatcherPriority priority, TimeSpan timeout, Delegate method, Object args, Int32 numArgs)
       at MS.Win32.HwndSubclass.SubclassWndProc(IntPtr hwnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at MS.Win32.UnsafeNativeMethods.DispatchMessage(MSG& msg)
       at System.Windows.Threading.Dispatcher.PushFrameImpl(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.PushFrame(DispatcherFrame frame)
       at System.Windows.Threading.Dispatcher.Run()
       at System.Windows.Application.RunDispatcher(Object ignore)
       at System.Windows.Application.RunInternal(Window window)
       at System.Windows.Application.Run(Window window)
       at System.Windows.Application.Run()
       at Calculator.App.Main() in c:\Users\Administrator\Documents\Visual Studio 2013\Projects\Calculator\Calculator\obj\Debug\App.g.cs:line 0
       at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
       at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

I hope there is a fix. As soon as I click the total button, the program crashes and then I get the error above.

A:

You are trying to use Convert.ToInt32 to convert 2+5 to 7.  That is not what Convert.ToInt32 does.  Convert.ToInt32 and int.Parse take a string representing a number (12345 or -1245) and convert it to an integer.
You should probably look at data types before doing anything else.  
You will need to parse the expression yourself, or use an expression parser.  see I need a fast runtime expression parser or How would you create an expression parser in c#?
For an example of how to make a calculator in WPF without parsing expressions, see the MSDN Sample calculator.

