Using reflection SetString

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")
}

icza · Accepted Answer · 2017-05-30 07:05:43Z

6

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}

edited May 30, 2017 at 7:05

answered May 30, 2017 at 7:00

icza

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1 Comment

Davide Over a year ago

Thank you for tips Icza. I know there was errors during assegnation because i tried to create "dummy" code for example and I did it in a very short time :) . BTW I got where my erorrs were. Thank you

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