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

Passing Nil Values to Interfaces via Reflection in Go

In Go, interfaces provide a way to represent values of different types as a single entity. However, passing a nil value to an interface as an argument can be challenging, especially via reflection.

Consider the function f() that takes an interface error as an argument:

func f(e error) {
    if e == nil {
        fmt.Println("YEY! NIL")
    } else {
        fmt.Println("NOT NIL :(")
    }
}

Suppose we want to pass a nil value to this function using reflection. Two approaches are attempted:

Approach 1:

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

However, this approach panics because reflect.Zero(nil) is not allowed.

Approach 2:

nilArg := reflect.Zero(reflect.TypeOf(&MyError{}))

This approach fails because it doesn't create a nil interface value for error.

The solution lies in using reflect.TypeOf((*error)(nil)).Elem() to obtain the concrete type for the interface error:

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

This trick works because passing a pointer to a nil value to reflect.TypeOf() allows the reflection system to determine the concrete type. The Elem() method then retrieves the underlying type for the pointer.

By creating nilArg using this expression, we can now pass a true nil value to the function f() via reflection:

rf := reflect.ValueOf(f)
rf.Call([]reflect.Value{nilArg})

This call will correctly execute the if statement in the function, printing "YEY! NIL".

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