WPF Application using MVVM Toolkit will not compile with StartupEventHandler - wpf

Question

I have a WPF application that does not compile when I attempt to add any event handler to the App class.
Below is all the code and the exception I am getting. The application uses the MVVM toolkit - so that may be a factor.
If someone could tell me what I may be missing or doing incorrectly, it would be greatly appreciated.
App.xaml code:
<Application x:Class="MyClient.App"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:vm="clr-namespace:Sample.ViewModel"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d" Startup="Application_Startup">
<Application.Resources>
<!--Global View Model Locator-->
<vm:ViewModelLocator x:Key="Locator" d:IsDataSource="True" />
<!-- Resources scoped at the Application level should be defined here. -->
<Style x:Key="TextBlockStyleFooter" TargetType="TextBlock">
<Setter Property="HorizontalAlignment" Value="Center"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="Foreground" Value="White"/>
<Setter Property="Margin" Value="1"/>
</Style>
<Style x:Key="TextBlockStyleClock" TargetType="TextBlock">
<Setter Property="FontFamily" Value="Arial"/>
<Setter Property="Foreground" Value="White"/>
<!--<Setter Property="Margin" Value="0, -1,"/>-->
<Setter Property="TextAlignment" Value="Center"/>
</Style>
<Style x:Key="BorderStyle1" TargetType="Border">
</Style>
</Application.Resources>
App.xaml.cs Code:
using System;
using System.Windows;
using System.Windows.Threading;
using GalaSoft.MvvmLight.Threading;
namespace Sample
{
/// <summary>
/// Interaction logic for App.xaml
/// </summary>
public partial class App : Application
{
static App()
{
DispatcherHelper.Initialize();
}
private void Application_Startup(object sender, StartupEventArgs e)
{
}
}
}
When I try to compile this, I get the following exception:
Error 1 'MyClient.App' does not contain a definition for 'Application_Startup' and no extension method 'Application_Startup' accepting a first argument of type 'EdgePokerClient.App' could be found (are you missing a using directive or an assembly reference?) C:\projects.git\MyClient\src\MyClient\App.xaml 7 73 MyClient

Answer 1

The issue here is that your XAML refers to MyClient.App whereas your code-behind file has the partial class in the Sample namespace. To the compiler, these are two separate classes. So the event handler you've defined in the one class (Sample.App) is not present in the generated class MyClient.App.
You just need to fix either the namespace in your code behind file or the x:Name attribute in the XAML file.
I'd also be careful with the static constructor on App. I'm not sure if the code generator will add a public parameterless constructor or not, but if it doesn't, having only a static constructor will effectively mean that App can't be instantiated.

Answer 2

I'd like to post this comment for other people getting this same error like i did.
I had this same error, however my x:class namespace wasn't different than the code behind namespace. Some tutorials says that after you put the Startup="Application_Startup" on your XAML, your code behind should be updated automatically. it didn't happen leading me to this error.
My Fix - Update code behind with
private void Application_Startup(object sender, StartupEventArgs e){}
before adding
Startup="Application_Startup"
to the xaml.
hope this come to be useful to somebody.