Using reflection SetString

I have a struct like this one:

type ProductionInfo struct {
	StructA []Entry
}

type Entry struct {
	Field1 string
	Field2 int
}

I would like to change the value of Field1 using reflection but the reflect object returned always CanSet() = false. What can I do? See playground example.

https://play.golang.org/p/eM_KHC3kQ5

Here is the code:

func SetField(source interface{}, fieldName string, fieldValue string) {
	v := reflect.ValueOf(source)
	tt := reflect.TypeOf(source)

	for k := 0; k < tt.NumField(); k++ {
		fieldValue := reflect.ValueOf(v.Field(k))

		fmt.Println(fieldValue.CanSet())
		if fieldValue.CanSet() {
			fieldValue.SetString(fieldValue.String())
		}
	}
}

func main() {
	source := ProductionInfo{}
	source.StructA = append(source.StructA, Entry{Field1: "A", Field2: 2})

	SetField(source, "Field1", "NEW_VALUE")
}

Multiple errors. Let's iterate over them.

First, you pass a value of ProductionInfo and not a value of Entry whose field you want to modify, so first change it to:

SetField(source.StructA[0], "Field1", "NEW_VALUE")

Next, you are passing a (non-pointer) value. You can't modify the fields of a non-pointer struct with reflection, because that would only modify a copy which would be discarded. In order to avoid this (and further confusion), this is not allowed (CanSet() returns false). So you have to pass a pointer to the struct:

SetField(&source.StructA[0], "Field1", "NEW_VALUE")

Now inside SetField() the reflect.ValueOf(source) will describe the passed pointer. You may use Value.Elem() to navigate to the reflect.Value of the pointed object (the struct value):

v := reflect.ValueOf(source).Elem()

And now it works. Modified code:

func SetField(source interface{}, fieldName string, fieldValue string) {
	v := reflect.ValueOf(source).Elem()

	fmt.Println(v.FieldByName(fieldName).CanSet())

	if v.FieldByName(fieldName).CanSet() {
		v.FieldByName(fieldName).SetString(fieldValue)
	}
}

func main() {
	source := ProductionInfo{}
	source.StructA = append(source.StructA, Entry{Field1: "A", Field2: 2})

	fmt.Println("Before: ", source.StructA[0])
	SetField(&source.StructA[0], "Field1", "NEW_VALUE")
	fmt.Println("After: ", source.StructA[0])
}

Output (try it on the Go Playground):

Before:  {A 2}
true
After:  {NEW_VALUE 2}