从Powershell ISE调用时无法以异步C＃代码写入控制台

Question

I have a PowerShell script that needs to call C# to do work, some of it asynchronously. While debugging some errors that only occur when the C# code is called from PowerShell, I noticed something I wasn't expecting. In the C# code, if I made a call to Console.WriteLine it wrote to the PowerShell console as expected unless I was using PowerShell ISE and it was called after I did an await in an async function.

Here's a sample script that shows the issue:

$source = @"
using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

public class TestClass
{   
    public static int TestWrapper()
    {
        var task = TestAsync();
        task.Wait();
        return task.Result;
    }

    private static async Task<int> TestAsync()
    {
        Console.WriteLine("Before await");
        await Task.Run(() => { Thread.Sleep(2000); return; });
        Console.WriteLine("After await");
        return 2;
    }
}

"@

Add-Type -TypeDefinition $source
[TestClass]::TestWrapper()

In here, I'm just calling an asynchronous function, waiting for it to complete, and returning the result. Inside the function, I just try to log before and after putting a thread to sleep for a few seconds before returning a value so that I know that the function fully executed.

I would expect to see:

Before await
After await
2

And that's what I see when running as a C# console application or in basic PowerShell, but when running through PowerShell ISE, I see:

Before await
2

Can anyone explain why that would be?

Answer 1

PowerShell ISE use Console.SetOut with HostTextWriter to capture and print Console.WriteLine output on PowerShell ISE host window. And HostTextWriter redirect Console.WriteLine to PowerShell host only if host registered ( HostTextWriter.RegisterHost ) for current thread.

In absence of SynchronizationContext continuation caused by await keyword will be executed in arbitrary thread pool thread, which likely does not have any registered PowerShell host to redirect Console.WriteLine to.