Why Am I Getting "Cannot Type Switch on Non-Interface Value" in Go?

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

While attempting to implement type assertion in Go, you may encounter the error message: "cannot type switch on non-interface value." This error arises when attempting to perform a type switch on a value that is not an interface type.

Consider the following code snippet:

package main

import "fmt"
import "strconv"

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")
    }
}

When running this code, you will encounter the "cannot type switch on non-interface value" error. To resolve this, we need to cast the variable n to an interface{} type before performing the type switch.

switch v := interface{}(n).(type)

This cast converts the concrete type of n (*Number) to the interface type interface{}. Interfaces in Go act as a contract, allowing any value to be stored and accessed through common methods. By casting n to interface{}, we can then perform the type switch on the resulting interface value.

When we run the modified code, it will successfully print "Stringer: 1", as expected. This demonstrates that type assertion in Go requires the value to be an interface type, and casting to interface{} enables type switches on non-interface values.

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