How to Safely Unmarshal JSON into an Interface{} and Handle Type Assertion?

Unmarshaling into an Interface{} and Performing Type Assertion

Unmarshaling JSON into an interface{} allows for handling a diverse range of data types. However, directly asserting the type of the unmarshaled interface{} poses challenges.

In the given scenario, the interface{} is unmarshaled from a received message. Attempting to perform a type switch on this interface{} as seen in the code snippet produces unexpected results, with the type being declared as map[string]interface{}.

To resolve this issue, it's important to understand the default types that the JSON package unmarshals into, as listed in its documentation:

• bool
• float64
• string
• []interface{}
• map[string]interface{}
• nil

Since the unmarshaling is performed into an interface{}, the resulting type will be limited to this set. Therefore, the package is unaware of custom structs like Something1 and Something2.

Solution Options:

1. Direct Unmarshaling:

To avoid intermediate interface{} handling, JSON data can be directly unmarshaled into the desired struct type. For instance:

var job Something1
json.Unmarshal([]byte(msg), &job)

2. Convert from Generic Interface:

If working with a generic interface{} is necessary, the data can be manually unpacked from the map[string]interface{}. Here's an example:

var input interface{}
json.Unmarshal([]byte(msg), &input)

if smth1, ok := input.(map[string]interface{}); ok {
  job := Something1{
    Thing:        smth1["thing"].(string),
    OtherThing:   smth1["other_thing"].(int64),
  }
}

3. Wrapper Struct:

For cases where handling various data types is common, a wrapper struct with a custom UnmarshalJSON method can simplify the process. This method can attempt to unmarshal the data into different structs and set the Data field accordingly.

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