Unable to unmarshal data in go

I got this error:
cannot unmarshal string into Go struct field Attributes.length of type float64
but I’m trying to unmarshal the field with the lowercase letter which is indeed a number.
Why the error?

Notice the tag: json:"length", which should get me the lower case field, right?

Here is the playground, if you want to reproduce it https://go.dev/play/p/v7EWz5OGou4

import (
    "encoding/json"
    "fmt"
)

const problematicData = `{
    "length": 3350,
    "Length": "3350"
}`

type Attributes struct {
    Length float64 `json:"length"`
}

func main() {
    var attributes Attributes
    err := json.Unmarshal([]byte(problematicData), &attributes)
    fmt.Println("error:", err)
}

>Solution :

Golang’s Unmarshal function is case in-sensitive by default. See https://pkg.go.dev/encoding/json#Unmarshal

preferring an exact match but also accepting a case-insensitive match

In order to get this working in your case, you’ll need to implement a custom unmarshaller.

The following will accept the length key from the json. Playground: https://go.dev/play/p/OAUdMBc0Szi

package main

import (
    "encoding/json"
    "fmt"
)

const problematicData = `{
    "length": 3350,
    "Length": "3350"
}`

type Attributes struct {
    Length float64 `json:"length"`
}

func (b *Attributes) UnmarshalJSON(data []byte) error {
    var v map[string]any
    if err := json.Unmarshal(data, &v); err != nil {
        return err
    }
    for key, v := range v {
        if key != "length" {
            continue
        }
        if floatValue, ok := v.(float64); ok {
            b.Length = floatValue
        }
    }
    return nil
}

func main() {
    var attributes Attributes
    err := json.Unmarshal([]byte(problematicData), &attributes)
    fmt.Println("error:", err, attributes)
}