调用异步方法时，StreamWriter使用同步调用

我发现了混乱的根源。

StreamWriter 默认情况下，同步刷新，除非您专门进行异步刷新。

Stream.Flush 在关闭（当然是冲洗）时被调用。

这将调用Write而不是WriteAsync在messageWriter流中：

using (var sw = new StreamWriter(messageWriter, Encoding.UTF8))
    await sw.WriteAsync("Hi");

在StreamWriter将缓冲“Hi”的消息，并且其将被释放到底层流上Flush关闭时同步地调用StreamWriter。

因此，异步调用toStreamWriter.WriteAsync不会对基础流产生任何影响（因为有缓冲），并且在调用StreamWriter.Close时StreamWriter.Flush会被同步调用，因此Flush将对基础流进行调用。

此代码还Write同步调用：

using (var sw = new StreamWriter(messageWriter, Encoding.UTF8))
{
    sw.AutoFlush = true; // will call Write here to send the UTF8 BOM
    await sw.WriteAsync("Hi"); // Write the actual data 
}

显然这也是：

using (var sw = new StreamWriter(messageWriter, Encoding.UTF8))
{
    await sw.WriteAsync("Hi"); 
    sw.Flush(); 
}

这将称为WriteAsync：

using (var sw = new StreamWriter(messageWriter, Encoding.UTF8))
{
    await sw.WriteAsync("Hi");
    await sw.FlushAsync()
}

我创建了一个简单的类来跟踪方法的调用方式：

public class MSWrite : MemoryStream
{
    public override bool CanWrite { get { return true; } }

    public override void Write(byte[] buffer, int offset, int count)
    {
        Console.WriteLine("Write");
    }

    public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
    {
        Console.WriteLine("WriteAsync");
        return Task.Run(() => { });
    }
}

而这个测试：

Console.WriteLine("Simple");
using(var ms = new MSWrite())
using (StreamWriter sw = new StreamWriter(ms, Encoding.UTF8))
{
    await sw.WriteAsync("Hi");
}
Console.WriteLine();

Console.WriteLine("AutoFlush");
using (var ms = new MSWrite())
using (StreamWriter sw = new StreamWriter(ms, Encoding.UTF8))
{
    sw.AutoFlush = true;
    await sw.WriteAsync("Hi");
}
Console.WriteLine();

Console.WriteLine("Flush async");
using (var ms = new MSWrite())
using (StreamWriter sw = new StreamWriter(ms, Encoding.UTF8))
{
    await sw.WriteAsync("Hi");
    await sw.FlushAsync();
}

产量：

Simple
Write
Write

AutoFlush
Write
WriteAsync

Flush async
WriteAsync
WriteAsync

第一个是UTF8 BOM，第二个是实际数据。