Interface typecasting not working

Question

func GetResult (service interface{}) {
   switch v := service.(type) {
     case services.Account:
       service = service.(services.Account)
     default:
       service = service.(Mock_Account)
    }
   res, err := service.GetAccount()
 }

It says service is type of interface doesn't have any method. There is typecasting is not working any Idea how to call GetAccount method?

Go is statically typed so you can't change the type of `service`, and there is no type casting in go. If you only want to call `GetAccount`, why not accept an interface that only has that method? — JimB, May 31 '18 at 17:28

score 3 · Answer 1 · answered May 31 '18 at 18:13

To annotate your code sample:

func GetResult (service interface{}) {
    // service is of type interface{}
    switch v := service.(type) {
    case services.Account:
        // This does nothing: it assigns the existing value back into service, which is still of type interface{}
        // v exists in scope and is of type services.Account, but it's unused.
        service = service.(services.Account)
    default:
        // Ditto
        service = service.(Mock_Account)
    }
    // service is still of type interface{}, which defines no methods
    res, err := service.GetAccount()
}

As you can see, the variable service here is always of type interface{}. A variable's type cannot change: Go is static typed. However, if you just want to call a method GetAccount on whatever type you get passed in, this can be done without the type assertions and with full type safety by declaring an interface:

type AccountGetter interface {
    GetAccount() (res ResultType, err error)
}

func GetResult(service AccountGetter) {
    res,err := service.GetAccount()
}

You can then call GetResult and pass a services.Account or a Mock_Account or any type that implements a GetAccount method with the appropriate signature.

Interface typecasting not working

1 Answers1