Why Does Go Throw a "Cannot Type Switch on Non-Interface Value" Error?

Type Assertion in Go: Resolving "Cannot Type Switch on Non-Interface Value"

In Go, type assertion involves checking if a value belongs to a particular type. When encountering the error "cannot type switch on non-interface value," it means that the value being checked is not an interface.

Consider the following code:

type Stringer interface {
    String() string
}

type Number struct {
    v int
}

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

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

This code attempts to type switch on the value n, which is of type *Number. However, it results in the "cannot type switch on non-interface value" error.

To resolve this, we need to cast n to an interface value before performing the type assertion. This is because type switching can only be performed on interface values. The following corrected code demonstrates this:

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

By casting n to interface{}, we allow the type switch to be performed on the resulting interface value. This allows us to successfully check if n is of type Stringer and print the corresponding message.

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