金慶的專欄

# C#異步方法返回void和Task的區(qū)別

(金慶的專欄 2021.2)

如果異步(async關(guān)鍵字)方法有返回值，返回類型為T時(shí)，返回類型必然是 `Task<T>`。
但是如果沒有返回值，異步方法的返回類型有2種，一個(gè)是返回 Task, 一個(gè)是返回 void：
```
    public async Task CountDownAsync(int count)
    {
        for (int i = count; i >= 0; i--)
        {
            await Task.Delay(1000);
        }
    }

    public async void CountDown(int count)
    {
        for (int i = count; i >= 0; i--)
        {
            await Task.Delay(1000);
        }
    }
```

調(diào)用時(shí)，如果返回 Task, 但返回值被忽略時(shí)，VS 會(huì)用綠色波浪線警告：
```
    CountDownAsync(3);
    ~~~~~~~~~~~~~~~~~
```

信息為：
```
(awaitable) Task AsyncExample.CountDownAsync(int count)

Usage:
 await CountDownAsync(...);

Because this call is not awaited, execution of the current method continues before the call is completed. Consider applying the 'await' operator to the result of the call.
```

中文為：
```
CS4014:由于此調(diào)用不會(huì)等待，因此在此調(diào)用完成之前將會(huì)繼續(xù)執(zhí)行當(dāng)前方法。請(qǐng)考慮將"await"運(yùn)算符應(yīng)用于調(diào)用結(jié)果。
```

添加 await 后就正常了：
```
    await CountDownAsync(3);
```

如果調(diào)用者不是一個(gè)異步方法，因?yàn)橹挥性诋惒椒椒ㄖ胁趴梢允褂?await,
或者并不想在此等待，如想同時(shí)執(zhí)行多個(gè) CountDownAsync(),
就不能應(yīng)用 await 來消除警告。

此時(shí)可以改用 void 返回值的版本：
```
void Test()
{
    ...
    CountDown(3);
    CountDown(3);
    ...
}

async void CountDown(int count)
{
    for (int i = count; i >= 0; i--)
    {
        await Task.Delay(1000);
    }
}
```

> Never call `async Task` methods without also awaiting on the returned Task. If you don’t want to wait for the async behaviour to complete, you should call an `async void` method instead.

摘自：http://www.stevevermeulen.com/index.php/2017/09/using-async-await-in-unity3d-2017/

CountDown() 可以直接調(diào)用 CountDownAsync() 實(shí)現(xiàn)：
```
async void CountDown(int count)
{
    await CountDownAsync(count);
}
```

使用下劃線變量忽略異步方法的返回值也可以消除警告：
```
void Test()
{
    ...
    _ = CountDownAsync(3);
    _ = CountDownAsync(3);
    ...
}
```

但是這樣同時(shí)也會(huì)忽略 CountDownAsync() 中的異常。如以下異常會(huì)被忽略。

```
void Test()
{
    ...
    _ = CountDownAsync(3);
    ...
}

async Task CountDownAsync(int count)
{
    for (int i = count; i >= 0; i--)
    {
        await Task.Delay(1000);
    }
    throw new Exception();
}
```

如果是調(diào)用返回 void 的異步方法，Unity 會(huì)報(bào)錯(cuò)：
```
Exception: Exception of type 'System.Exception' was thrown.
```

## 對(duì) Async 后綴的說明

```
You could say that the Async suffix convention is to communicate to the API user that the method is awaitable. For a method to be awaitable, it must return Task for a void, or Task<T> for a value-returning method, which means only the latter can be suffixed with Async.
```

摘自：https://stackoverflow.com/questions/15951774

grpc 生成的代碼中，異步請(qǐng)求返回了一個(gè) AsyncCall 對(duì)象，AsyncCall 實(shí)現(xiàn)了 GetAwaiter() 接口：
```
      public virtual grpc::AsyncUnaryCall<global::Routeguide.Feature> GetFeatureAsync(global::Routeguide.Point request, ...)
```

可以這樣調(diào)用并等待：
```
    var resp = await client.GetFeatureAsync(req);
```

雖然返回類型不是`Task<>`, 但是可等待，所以添加了 Async 后綴。