How Can I Pass a Nil Interface Value Using Reflection in Go?

Passing Nil Value as Interface through Reflection in Go

In Go, you can define functions with interface arguments. While it's straightforward to pass a non-nil interface value, the question arises: how do you pass a nil value via reflection so that it passes an == nil check?

Consider the following function:

func f(e error) {
    if e == nil {
        fmt.Println("YEY! NIL") // how to get here?
    } else {
        fmt.Println("NOT NIL :(")
    }
}

Approach 1: Using reflect.Zero

func main() {
    rf := reflect.ValueOf(f)

    nilArg := reflect.Zero(reflect.TypeOf((error)(nil))) // panic: reflect: Zero(nil)
}

This approach fails as reflect.Zero(reflect.TypeOf((error)(nil))) panics because you cannot create a zero value of a nil type.

Approach 2: Using Typeof(&type)

type MyError struct{}
func (e MyError) Error() string {
    return ""
}

func main() {
    rf := reflect.ValueOf(f)
    nilArg := reflect.Zero(reflect.TypeOf(&MyError{})) // NOT NIL :(
}

This approach also fails due to the Go FAQ item on nil errors, which states that Interface types can be nil or point to a specific type. Passing nil specifically is not normal. If you try to call a method on a nil interface, it will panic.

Solution: Use TypeOf((*error)(nil)).Elem()

To properly pass a nil value via reflection, use the expression reflect.TypeOf((*error)(nil)).Elem() to obtain the reflect.Type for the error interface. This expression creates a non-interface value and uses reflection methods to obtain the desired reflect.Type.

Create nilArg as follows:

nilArg := reflect.Zero(reflect.TypeOf((*error)(nil)).Elem())

With this approach, calling f with nilArg will print "YEY! NIL."

See the playground example for a working implementation.

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