How to Correctly Use Multiple Cases in Go's Type Switch to Avoid Method Errors?

Type Switch with Multiple Cases

In Go, a type switch statement can be used to dynamically select a corresponding case based on the type of a value. When multiple types are specified in a single case, an error may be raised if the type of the value does not match any of the listed types.

Consider the following code snippet:

package main

import (
    "fmt"
)

type A struct {
    a int
}

func(this *A) test(){
    fmt.Println(this)
}

type B struct {
    A
}

func main() {
    var foo interface{}
    foo = A{}
    switch a := foo.(type){
        case B, A:
            a.test()
    }
}

When this code is run, it produces the following error:

a.test undefined (type interface {} is interface with no methods)

This error indicates that the type switch did not take effect because the variable a has the type interface{}, which does not have the test() method.

The Go language specification explains that in a type switch statement, when multiple types are specified in a case, the variable declared in that case will have the type of the expression in the type switch guard (in this case, foo). Since foo is of type interface{}, a also becomes an interface{} type.

To resolve this issue and ensure that the test() method can be called, you can explicitly assert that foo has the test() method before performing the type switch, like so:

package main

import (
    "fmt"
)

type A struct {
    a int
}

func (this *A) test() {
    fmt.Println(this)
}

type B struct {
    A
}

type tester interface {
    test()
}

func main() {
    var foo interface{}
    foo = &B{}
    if a, ok := foo.(tester); ok {
        fmt.Println("foo has test() method")
        a.test()
    }
}

By asserting that foo has the test() method, you can retrieve a value of the appropriate type and call the test() method successfully.

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