hlubek · July 18, 2023 17:03 · Jul 18, 2023
diff --git a/main.go b/main.go
@@ -0,0 +1,95 @@
+package main
+
+import (
+	"encoding/json"
+	"fmt"
+	"reflect"
+)
+
+type Something interface{}
+
+type Something1 struct {
+	Aaa, Bbb string
+}
+
+type Something2 struct {
+	Ccc, Ddd string
+}
+
+var _ Something = Something1{}
+var _ Something = Something2{}
+
+// We need to register all known message types here to be able to unmarshal them to the correct interface type.
+var knownImplementations = []Something{
+	//	Something1{},
+	Something2{},
+}
+
+type Container struct {
+	Value Something `json:"value"`
+}
+
+func (c *Container) UnmarshalJSON(bytes []byte) error {
+	var data struct {
+		Type  string
+		Value json.RawMessage
+	}
+	if err := json.Unmarshal(bytes, &data); err != nil {
+		return err
+	}
+
+	for _, knownImplementation := range knownImplementations {
+		knownType := reflect.TypeOf(knownImplementation)
+		if knownType.String() == data.Type {
+			// Create a new pointer to a value of the concrete message type
+			target := reflect.New(knownType)
+			// Unmarshal the data to an interface to the concrete value (which will act as a pointer, don't ask why)
+			if err := json.Unmarshal(data.Value, target.Interface()); err != nil {
+				return err
+			}
+			// Now we get the element value of the target and convert it to the interface type (this is to get rid of a pointer type instead of a plain struct value)
+			c.Value = target.Elem().Interface().(Something)
+			return nil
+		}
+	}
+
+	return fmt.Errorf("value type not known: %s", data.Type)
+}
+
+func (c Container) MarshalJSON() ([]byte, error) {
+	// Marshal to type and actual data to handle unmarshaling to specific interface type
+	return json.Marshal(struct {
+		Type  string
+		Value any
+	}{
+		Type:  reflect.TypeOf(c.Value).String(),
+		Value: c.Value,
+	})
+}
+
+func main() {
+	c := Container{
+		Value: Something1{
+			Aaa: "aaa",
+			Bbb: "bbb",
+		},
+	}
+
+	data, err := json.Marshal(c)
+	if err != nil {
+		panic(err)
+	}
+
+	var unmarshaled Container
+	err = json.Unmarshal(data, &unmarshaled)
+	if err != nil {
+		panic(err)
+	}
+
+	switch v := unmarshaled.Value.(type) {
+	case Something1:
+		println(v.Aaa)
+	default:
+		panic(fmt.Sprintf("unexpected value type: %T", v))
+	}
+}
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