Why Do I Get the 'Cannot Type Switch on Non-Interface Value' Error in Golang?

Error: "Cannot Type Switch on Non-Interface Value" in Golang

When working with type assertions, you may encounter the error "cannot type switch on non-interface value." This occurs when attempting to perform a type switch on a value that is not an interface.

In Golang, type switching allows you to conditionally execute code based on the type of a variable. However, it requires the variable to be of an interface type. An interface represents a contract that defines a set of methods that an implementing type must implement.

In the provided code snippet, a type named Stringer is defined with a single String() method. The Number type also implements the String() method, making it a concrete implementation of the Stringer interface.

type Stringer interface {
    String() string
}

type Number struct {
    v int
}

func (number *Number) String() string {
    return strconv.Itoa(number.v)
}

However, the error occurs in the main function, where a type switch is attempted on the n variable, which is of type *Number rather than Stringer.

func main() {
    n := &Number{1}
    switch v := n.(type) {
    case Stringer:
        fmt.Println("Stringer:", v)
    default:
        fmt.Println("Unknown")
    }
}

To resolve this issue, you need to cast n to interface{} before performing the type assertion. This is because interface{} can represent any arbitrary value.

func main() {
    n := &Number{1}
    switch v := interface{}(n).(type) {
    case Stringer:
        fmt.Println("Stringer:", v)
    default:
        fmt.Println("Unknown")
    }
}

By casting n to interface{}, you essentially allow the type switch to consider the possibility that n might be of type Stringer. Once the type switch is entered, the actual type of n can be determined and the appropriate case branch executed.

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