A common issue in API client libraries is mismatches between the JSON response and the expected strongly typed objects. I encountered this with TMDbLib and created a validator to catch these mismatches during unit testing.

The Settings

We can set up a JsonSerializerSettings object to:

  • Treat missing properties as errors.
  • Use a custom ContractResolver to enforce that all expected properties must exist in the JSON.
  • Handle and log deserialization errors without throwing immediately.
private static readonly List<ErrorEventArgs> Errors = new List<ErrorEventArgs>();

public static void Main()
{
    JsonSerializerSettings settings = new JsonSerializerSettings
    {
        MissingMemberHandling = MissingMemberHandling.Error,
        ContractResolver = new FailingContractResolver(),
        Error = Error
    };

    JsonSerializer serializer = JsonSerializer.Create(settings);
}

private static void Error(object sender, ErrorEventArgs errorEventArgs)
{
    Errors.Add(errorEventArgs);
    errorEventArgs.ErrorContext.Handled = true;
}

This configuration makes sure that:

  • Missing members in JSON trigger errors.
  • All expected C# properties must appear in the JSON unless explicitly marked ignored.
  • Errors are collected rather than thrown, allowing full diagnostics.

Custom ContractResolver

The contract resolver ensures every C# property must appear in the JSON unless marked with [JsonIgnore]:

public class FailingContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty res = base.CreateProperty(member, memberSerialization);

        if (!res.Ignored)
            res.Required = Required.AllowNull; // Make properties required unless ignored

        return res;
    }
}

Using It

Once configured, the custom serializer can be used to Deserialize any object. If the JSON is missing properties or includes unexpected ones, they will be logged via the Error handler.

This technique is great for catching subtle mismatches during unit testing.

Example Code

You can find the code used in this post here: github.com/LordMike/blog-examples/detect-mismatched-objects-newtonsoft

Example

Seeing It in Action

This validation setup is used in the TMDbLib test suite:

Each test inherits from TestBase, which configures the validator and throws errors at teardown if mismatches were detected. Some exceptions can be suppressed where needed.