How to Unmarshal XML with Attributes and Values in Go?

Unmarshaling XML Elements with Attributes and Values in Go

XML elements often contain both attributes and values. To successfully unmarshal such elements into Golang structs, it's essential to understand the role of the XMLName and ",chardata" annotations.

Defining the Struct without XMLName

Consider the provided XML:

<code class="xml"><thing prop="1">
  1.23
</thing>
<thing prop="2">
  4.56
</thing></code>

The corresponding struct without XMLName field could be:

<code class="go">type ThingElem struct {
    Prop  int   `xml:"prop,attr"`
    Value float64 `xml:",chardata"`
}</code>

Prop is annotated with xml:"prop,attr" to indicate that it is an attribute of the thing element. Value is annotated with xml:",chardata" to specify that it should hold the contents of the element as a string.

Understanding XMLName

XMLName can be used to explicitly define the XML tag name of a struct. In our case, the XML tag name is inferred since it matches the struct name (ThingElem). So, XMLName is not required in this scenario.

Using a Wrapper Struct

If the XML structure is more complex or could be ambiguous, you can use a wrapper struct to provide additional context. For instance, if the XML had multiple thing elements within a root element:

<code class="xml"><root>
  <thing prop="1">
    1.23
  </thing>
  <thing prop="2">
    4.56
  </thing>
</root></code>

You would need a wrapper struct:

<code class="go">type ThingWrapper struct {
    T ThingElem `xml:"thing"`
}</code>

Here, T is the embedded struct that represents the thing element.

Unmarshaling Considerations

For the provided XML data, you need to account for the whitespace in the element values. Since XML doesn't preserve whitespace by default, the values should be trimmed or the xml:",innerxml" annotation could be used.

The resulting struct can be unmarshaled as follows:

<code class="go">package main

import (
    "encoding/xml"
    "fmt"
    "strings"
)

type Root struct {
    Things []Thing `xml:"thing"`
}

type Thing struct {
    Prop  int     `xml:"prop,attr"`
    Value float64 `xml:",chardata"`
}

func main() {
    data := `
<root>
<thing prop="1"> 1.23 </thing>
<thing prop="2"> 4.56 </thing>
</root>
`
    thing := &Root{}
    err := xml.Unmarshal([]byte(strings.TrimSpace(data)), thing)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(thing)
}</code>

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