How to Dynamically Invoke Methods on `interface{}` in Go, Regardless of Receiver Type?

Dynamic Invocation of Methods on Interface{} Regardless of Receiver Type

In this article, we explore an issue faced when attempting to dynamically call methods on an interface{} in Go. Our goal is to overcome this challenge, enabling efficient method invocation regardless of the underlying data type or receiver type.

The Problem

When dealing with interface{} in Go, we encountered a limitation in dynamic method invocation: if the data stored within the interface{} was a pointer, we faced difficulties in accessing its address. Consequently, methods with pointer receivers could not be dynamically invoked.

The Solution

To resolve this issue, we leverage a technique that encompasses four cases:

1. Interface{} data is a value, receiver is a value: In this case, no modifications are necessary.
2. Interface{} data is a pointer, receiver is a value: We create a new pointer to the interface{} data and retrieve the referenced value.
3. Interface{} data is a value, receiver is a pointer: We create a new pointer to the interface{} data.
4. Interface{} data is a pointer, receiver is a pointer: We utilize the existing pointer.

After determining the appropriate data type, we perform an additional check for existence of the method on both the value and the pointer. By doing so, we ensure method invocation regardless of whether the method is declared as a value or pointer receiver.

Proof of Concept

The following code demonstrates the implementation of our solution:

package main

import (
    "fmt"
    "reflect"
)

type Test struct {
    Start string
}

// value receiver
func (t Test) Finish() string {
    return t.Start + "finish"
}

// pointer receiver
func (t *Test) Another() string {
    return t.Start + "another"
}

func CallMethod(i interface{}, methodName string) interface{} {
    var ptr reflect.Value
    var value reflect.Value
    var finalMethod reflect.Value

    value = reflect.ValueOf(i)

    // if we start with a pointer, we need to get value pointed to
    // if we start with a value, we need to get a pointer to that value
    if value.Type().Kind() == reflect.Ptr {
        ptr = value
        value = ptr.Elem()
    } else {
        ptr = reflect.New(reflect.TypeOf(i))
        temp := ptr.Elem()
        temp.Set(value)
    }

    // check for method on value
    method := value.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }
    // check for method on pointer
    method = ptr.MethodByName(methodName)
    if method.IsValid() {
        finalMethod = method
    }

    if (finalMethod.IsValid()) {
        return finalMethod.Call([]reflect.Value{})[0].Interface()
    }

    // return or panic, method not found of either type
    return ""
}

func main() {
    i := Test{Start: "start"}
    j := Test{Start: "start2"}

    fmt.Println(CallMethod(i, "Finish"))
    fmt.Println(CallMethod(&i, "Finish"))
    fmt.Println(CallMethod(i, "Another"))
    fmt.Println(CallMethod(&i, "Another"))
    fmt.Println(CallMethod(j, "Finish"))
    fmt.Println(CallMethod(&j, "Finish"))
    fmt.Println(CallMethod(j, "Another"))
    fmt.Println(CallMethod(&j, "Another"))
}

By employing this technique, we can dynamically invoke methods on interface{} regardless of the receiver type, facilitating robust and adaptable code in Go.

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