Using reflection SetString - string

Question

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")
}

Answer 1

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}