# 10.4 重温Channel（通道）

一旦掌握了 `select` 关键字，Go channels 可以以几种独特的方式做更多事要比您在[第9章](https://github.com/hantmac/Mastering_Go_ZH_CN/tree/master/eBook/chapter9/09.0.md)（并发-Goroutines,Channel和Pipeline）学到的。这节将揭晓 Go channels 的这些使用方法。

要记住 channel 类型的零值是 `nil`，并且如果您发送一个消息给以关闭的 channel，程序将崩溃。然而，如果您尝试从已关闭的 channel 读取的话，会得到 channel 类型的零值。因此，关闭 channel 后，您不能再往里写，但您能一直读。

为了能关闭 channel, channel 不必是只接受。另外，一个 `nil` channel 总是阻塞的。channels 的这个特性非常有用，当您想要禁用 `select` 表达式的一个分支时，可以分配一个 `nil` 值给一个 channel 变量。

最后，如果您要关闭一个 `nil` channel，程序就会崩溃。最好的说明是下面这个 `closeNilChannel.go` 程序：

```go
package main

func main() {
    var c chan string
    close(c)
}
```

执行 `closeNilChannel.go` 产生如下输出：

```
$go run closeNilChannel.go
panic: close of nil channel

goroutine 1 [running]:
main.main()
    /Users/mtsouk/closeNilChannel.go:5 +0x2a
exit status 2
```


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