Go语言switch语句报语法错误，如何从通道获取字符串变量？

你遇到的syntax error: unexpected :=, expecting :错误，是因为Go的语法规则不允许在switch的case条件表达式中直接使用短变量声明:=。case条件只能是可直接求值的表达式，不能包含变量声明操作。

修正方案一：将变量声明移到switch外部 #

先在switch块外声明变量，在case中使用赋值操作=接收通道值：

package main

import (
    "fmt"
)

func main() {
    writeMsgs := make(chan string)
    go sendMsgs(writeMsgs)
    for {
        var msg string
        switch {
        case msg = <-writeMsgs:
            fmt.Println(msg)
        }
    }
}

func sendMsgs(writeMsgs chan string) {
    for i := 0; i < 5; i++ {
        writeMsgs <- "Test"
    }
    close(writeMsgs)
}

修正方案二：使用带初始化语句的switch #

在switch开头添加初始化语句，声明并接收通道值，然后基于固定条件进入case：

package main

import (
    "fmt"
)

func main() {
    writeMsgs := make(chan string)
    go sendMsgs(writeMsgs)
    for {
        switch msg := <-writeMsgs; true {
        case true:
            fmt.Println(msg)
        }
    }
}

func sendMsgs(writeMsgs chan string) {
    for i := 0; i < 5; i++ {
        writeMsgs <- "Test"
    }
    close(writeMsgs)
}

额外优化：处理通道关闭的情况 #

上述代码在通道关闭后会持续接收零值并打印，建议添加ok判断退出循环：

package main

import (
    "fmt"
)

func main() {
    writeMsgs := make(chan string)
    go sendMsgs(writeMsgs)
    for {
        var msg string
        var ok bool
        switch {
        case msg, ok = <-writeMsgs:
            if !ok {
                fmt.Println("通道已关闭，退出循环")
                return
            }
            fmt.Println(msg)
        }
    }
}

func sendMsgs(writeMsgs chan string) {
    for i := 0; i < 5; i++ {
        writeMsgs <- "Test"
    }
    close(writeMsgs)
}

内容的提问来源于stack exchange，提问作者Josh Jameson