How do I read a file asynchronously at application start-up?

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C#

Async

FileIO

I have a strange problem with FileStream.ReadAsync(). I call a function that performs an asynchronous read from a stream from my main application OnStartup(). When I read asynchronously, the application doesn't ever display its main window. If I simply change the read to a synchronous version, the application starts up.

This is my OnStartup() override:

protected override async void OnStartup( StartupEventArgs e )
{
	base.OnStartup( e );
	Application.Current.ShutdownMode = ShutdownMode.OnExplicitShutdown;
	LogsEmailer logsEmailer = new LogsEmailer();
	string testString = await logsEmailer.TestReadFileAsync( Path.Combine( logsFolder, "TestFile.txt" ) );
	StartupUri = new Uri( "./View/MainWindow.xaml", UriKind.Relative );
}

The application never starts with this function:

public async Task<string> TestReadFileAsync( string filePath )
{
	using( FileStream sourceStream = File.OpenRead( filePath ) )
	{
		byte[] copyBuffer = new byte[ 4096 ];
		bool doneCopying = false;
		while( !doneCopying )
		{
			int numSourceBytes = await sourceStream.ReadAsync( copyBuffer, 0, copyBuffer.Length );
			if( numSourceBytes == 0 )
				doneCopying = true;
		}
	}
	return "test string";
}

But it does start with this:

public async Task<string> TestReadFileAsync( string filePath )
{
	using( FileStream sourceStream = File.OpenRead( filePath ) )
	{
		byte[] copyBuffer = new byte[ 4096 ];
		bool doneCopying = false;
		while( !doneCopying )
		{
			int numSourceBytes = sourceStream.Read( copyBuffer, 0, copyBuffer.Length );
			if( numSourceBytes == 0 )
				doneCopying = true;
		}
	}
	return "test string";
}

Obviously, I do get a warning with the second piece of code that says it will complete synchronously. And when I say that the second version works, I mean only that it allows the application to start; I can't use it because the operation I am trying to perform takes too long to be done synchronously in OnStartup().

I guess I am missing something silly here. Can anyone tell me what?

What I have tried:

I I mentioned above, I have tried removing the async from the stream-read and the application starts okay.

I have just discovered that if I change my OnStartup() override to be as follows then the application starts up okay.

protected override async void OnStartup( StartupEventArgs e )
{
	base.OnStartup( e );
	Application.Current.ShutdownMode = ShutdownMode.OnExplicitShutdown;
	StartupUri = new Uri( "./View/MainWindow.xaml", UriKind.Relative );
	LogsEmailer logsEmailer = new LogsEmailer();
	string testString = await logEmailer.TestReadFileAsync( Path.Combine( logsFolder, "TestFile.txt" ) );
}