Why Does My JSON Deserialization Result in an Interface Assertion Failure?

Interface Assertion Failure in JSON Deserialization

In this situation, the issue arises when attempting to assert an interface into a specific struct type after deserializing JSON data. The error message indicates that the expected object type is a map[string]interface{}, but the actual object is a custom struct of type Data.

Explanation

Interface assertions allow the conversion of an interface value to a specific type. However, it's crucial to ensure that the underlying value of the interface actually matches the target type. In this case, the interface data contains a complex object with nested fields, while main.Data is a simple struct. Therefore, the assertion into type Data is invalid.

Solution

To resolve this issue, the approach is to either ensure that the interface data matches the target struct or to dynamically check the data type before performing the assertion.

Direct De-serialization

For direct de-serialization, you can use the following approach:

var result Data

err := json.Unmarshal(data, &result)
if err != nil {
    // Handle error
}

This method directly de-serializes the JSON data into the Data struct, eliminating the need for interface assertions.

Interface Check and Assertion

Alternatively, if you need to perform an interface assertion, you should first ascertain that the underlying value is of the correct type:

result, ok := anInterface.(Data)
if !ok {
    // Handle type mismatch error
}

This check ensures that only valid type conversions are performed, preventing runtime errors.

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