TypeSpec C#: Add AdditionalProperties Models Option

Aug 5, 2025 by Mei Lin 52 views

Add Option to Turn On AdditionalProperties Based Models in @typespec/http-client-csharp

Introduction

Guys, we've got some exciting updates coming to the @typespec/http-client-csharp library! We're diving deep into improving how our models handle dynamic properties, and this means a new feature that lets you turn on AdditionalProperties based models. This enhancement is all about making our models more flexible and adaptable, especially when dealing with data structures that might have extra, unexpected fields. If you've ever struggled with models that need to accommodate dynamic data, this one's for you. So, let's break down what this new feature entails, why it's a big deal, and how you'll be able to use it.

The Problem: Handling Dynamic Properties

In many real-world scenarios, our models need to interact with APIs that return data structures with properties that aren't explicitly defined in our model schema. This is where the concept of dynamic properties comes into play. Imagine you're working with an API that occasionally adds new fields to its responses. If your model is strictly defined, it might ignore these extra fields, leading to potential data loss or unexpected behavior.

The System.ClientModel library is tackling this challenge head-on, and we're aligning our @typespec/http-client-csharp library to take full advantage of these improvements. The core issue we're addressing is how to elegantly handle these additional properties without sacrificing type safety or model integrity. We need a solution that allows our models to seamlessly incorporate these extra fields while still providing a clear and consistent interface for developers.

The Current Approach and Its Limitations

Previously, handling dynamic properties often involved workarounds like using dictionaries or other generic data structures to store the additional data. While these approaches worked, they weren't ideal. They could lead to more complex code, reduced type safety, and a less intuitive API for developers. We needed a more integrated solution that made working with dynamic properties feel like a natural part of the model definition.

The Need for a More Flexible Solution

To truly address the challenge of dynamic properties, we need a mechanism that allows models to:

Store additional properties without explicitly defining them in the schema.
Access these properties in a type-safe manner.
Serialize and deserialize these properties correctly when interacting with APIs.

This is where the AdditionalProperties feature comes in. It provides a structured way to handle dynamic data, making our models more robust and adaptable to evolving API contracts. By adopting this new approach, we can ensure that our models are not only accurate representations of the data they're designed to handle but also flexible enough to accommodate the unexpected.

The Solution: Introducing AdditionalProperties

To address the dynamic properties challenge, the System.ClientModel library introduces a new feature centered around the AdditionalProperties type. This feature provides a structured way to handle extra properties in our models, making them more flexible and robust. So, what exactly does this entail?

The core idea is to allow models to store additional, unexpected properties in a dedicated structure. This structure, the AdditionalProperties type, acts as a container for these dynamic fields. When a model is deserialized, any properties found in the data that don't match the model's explicit schema are stored in this AdditionalProperties instance. This way, no data is lost, and the model remains a faithful representation of the data it's handling.

Key Components of the Solution

The AdditionalProperties Type: This new type acts as a container for dynamic properties. It provides methods for setting, getting, and managing these properties in a type-safe manner.
The @dynamicModel Decorator: In @typespec/http-client-csharp, we're introducing a new decorator, @dynamicModel, that you can apply to your models. This decorator signals to the code generator that the model should be treated as a dynamic model, capable of handling additional properties.
Serialization and Deserialization Enhancements: The serialization and deserialization logic for dynamic models is being updated to correctly handle the AdditionalProperties. This includes populating the AdditionalProperties during deserialization and including these properties when serializing the model.

How It Works

When you mark a model with the @dynamicModel decorator, several things happen under the hood:

No More serializedAdditionalRawData: The model no longer needs the private serializedAdditionalRawData field, which was previously used to store raw, additional data.
The Patch Property: A new property named Patch of type AdditionalProperties is added to the model. This is where the dynamic properties will be stored.
Deserialization Logic: During deserialization, an instance of AdditionalProperties is created. As the data is parsed, any properties that aren't explicitly defined in the model are added to this AdditionalProperties instance using the ap.Set(...) method.
Serialization Logic: During serialization, the code checks if a property has been patched (i.e., modified or added via the AdditionalProperties). If so, it includes the property in the serialized output. This ensures that dynamic properties are correctly persisted.

By implementing these changes, we're providing a more seamless and intuitive way to work with dynamic data in our models. The AdditionalProperties feature not only simplifies the code but also enhances type safety and data integrity.

Implementing the Solution in @typespec/http-client-csharp

Alright, let's get into the nitty-gritty of how we're implementing this solution in @typespec/http-client-csharp. This involves several key steps, from introducing the @dynamicModel decorator to modifying the serialization and deserialization logic. So, buckle up, guys, we're about to dive deep into the code!

1. Introducing the `@dynamicModel` Decorator

The first step is to introduce a new decorator, @dynamicModel, that you can use in your TypeSpec models. This decorator acts as a signal to the code generator that a particular model should be treated as a dynamic model, capable of handling additional properties.

Here's how you'll use it in your client.tsp file:

@@dynamicModel(MyModel);

By applying this decorator to MyModel, you're telling the code generator to generate code that supports the AdditionalProperties feature for this model.

2. Model Modifications

For each model marked with the @dynamicModel decorator, we need to make a few modifications to its generated code:

Remove serializedAdditionalRawData: The model no longer needs the private serializedAdditionalRawData field. This field was previously used to store raw, additional data, but with the AdditionalProperties type, we have a more structured way to handle this.
Add Patch Property: We add a new property named Patch of type AdditionalProperties to the model. This property will hold the dynamic properties.
```
public AdditionalProperties Patch { get; set; }
```

3. Deserialization Logic

The deserialization logic needs to be updated to handle the AdditionalProperties type. This involves creating an instance of AdditionalProperties and populating it with any properties found in the data that aren't explicitly defined in the model's schema.

Here's a simplified example of how this might look:

public static MyModel DeserializeMyModel(JsonElement element)
{
    AdditionalProperties ap = new AdditionalProperties();
    // ... existing deserialization logic ...
    foreach (var property in element.EnumerateObject())
    {
        if (/* property is not a known property */)
        {
            ap.Set(property.Name, property.Value.ToString());
        }
    }
    return new MyModel(/* ... known properties ...,*/ ap);
}

In this example, we iterate over the properties in the JSON element. If a property isn't recognized as a known property of the model, we add it to the AdditionalProperties instance using the ap.Set(...) method. This ensures that all dynamic properties are captured during deserialization.

4. Serialization Logic

The serialization logic also needs to be updated to handle the AdditionalProperties. This involves checking if a property has been patched (i.e., modified or added via the AdditionalProperties) and including it in the serialized output.

Here's a simplified example of how this might look:

public void Serialize(Utf8JsonWriter writer)
{
    writer.WriteStartObject();
    // ... existing serialization logic for known properties ...
    if (Patch != null)
    {
        foreach (var property in Patch.GetProperties())
        {
            writer.WritePropertyName(property.Name);
            writer.WriteStringValue(property.Value.ToString());
        }
    }
    writer.WriteEndObject();
}

In this example, we first serialize the known properties of the model. Then, we check if the Patch property is not null. If it's not, we iterate over the properties in the AdditionalProperties instance and include them in the serialized output. This ensures that dynamic properties are correctly serialized.

5. Handling Complex Scenarios

In addition to the basic serialization and deserialization logic, we also need to handle more complex scenarios, such as:

Nested Objects: When dealing with complex child objects, we need to propagate the patch. This means ensuring that the AdditionalProperties are correctly handled in nested objects.
Flattened Properties: Flattened properties also need to be handled correctly to ensure that dynamic properties are serialized and deserialized properly.
Dictionaries: If the model contains dictionaries, we need to ensure that any additional properties in the dictionary are also handled.
Arrays: For arrays, we need to check if items were added via the patch and merge the arrays accordingly.

By addressing these scenarios, we can ensure that the AdditionalProperties feature works seamlessly in a wide range of situations.

Testing the Implementation

Alright, team, we've implemented the AdditionalProperties feature, but we're not done yet! Testing is a crucial part of the development process, and we need to ensure that our new implementation works correctly in all scenarios. This means writing unit tests that verify the serialization and deserialization logic when the @dynamicModel decorator is present on a model.

Why Unit Tests Matter

Unit tests are the foundation of robust software. They allow us to isolate individual components of our code and verify that they behave as expected. In the context of the AdditionalProperties feature, unit tests will help us ensure that:

Dynamic properties are correctly deserialized into the AdditionalProperties instance.
Dynamic properties are correctly serialized when the model is serialized.
The feature works seamlessly with nested objects, flattened properties, dictionaries, and arrays.
There are no unintended side effects on existing model behavior.

What to Test

Here are some key areas we need to cover with our unit tests:

Basic Deserialization: Test that dynamic properties are correctly deserialized into the AdditionalProperties instance when the model is deserialized from a JSON string.
Basic Serialization: Test that dynamic properties are correctly serialized when the model is serialized to a JSON string.
Nested Objects: Test that the AdditionalProperties feature works correctly with nested objects. This includes scenarios where the nested objects also have dynamic properties.
Flattened Properties: Test that the feature works correctly with flattened properties. This ensures that dynamic properties in flattened objects are handled properly.
Dictionaries: Test that the feature works correctly with dictionaries. This includes scenarios where the dictionary values are complex objects with dynamic properties.
Arrays: Test that the feature works correctly with arrays. This includes scenarios where items are added to the array via the AdditionalProperties and need to be merged correctly during serialization.
Edge Cases: Test various edge cases, such as models with no dynamic properties, models with only dynamic properties, and models with a mix of known and dynamic properties.

Example Test Scenario

Let's consider an example test scenario. Suppose we have a model called MyDynamicModel that's marked with the @dynamicModel decorator. We want to test that dynamic properties are correctly deserialized into the AdditionalProperties instance.

Here's how the test might look:

[Fact]
public void Deserialize_DynamicProperties_Success()
{
    string json = @"{
        "knownProperty": "knownValue",
        "dynamicProperty1": "dynamicValue1",
        "dynamicProperty2": "dynamicValue2"
    }";

    MyDynamicModel model = JsonSerializer.Deserialize<MyDynamicModel>(json);

    Assert.NotNull(model.Patch);
    Assert.Equal("dynamicValue1", model.Patch.Get("dynamicProperty1"));
    Assert.Equal("dynamicValue2", model.Patch.Get("dynamicProperty2"));
    Assert.Equal("knownValue", model.KnownProperty);
}

In this test, we deserialize a JSON string into a MyDynamicModel instance. We then assert that the AdditionalProperties instance is not null and that the dynamic properties are correctly stored in the AdditionalProperties instance. We also assert that the known property is deserialized correctly.

By writing comprehensive unit tests like this, we can ensure that our AdditionalProperties implementation is robust and reliable. So, let's roll up our sleeves and get testing, guys!

Conclusion

Alright, folks, we've reached the end of our deep dive into the AdditionalProperties feature in @typespec/http-client-csharp. We've covered a lot of ground, from understanding the problem of handling dynamic properties to implementing a solution and testing it thoroughly. So, what have we accomplished?

We've successfully introduced a new mechanism for handling dynamic properties in our models. By leveraging the AdditionalProperties type and the @dynamicModel decorator, we've made it easier than ever to work with APIs that return data structures with extra, unexpected fields. This means our models are more flexible, robust, and adaptable to evolving API contracts.

Key Takeaways

Let's recap the key takeaways from our journey:

The Challenge of Dynamic Properties: We started by understanding the challenge of handling dynamic properties in our models. We recognized that APIs often return data structures with fields that aren't explicitly defined in our schemas, and we needed a way to handle these extra fields gracefully.
The AdditionalProperties Solution: We introduced the AdditionalProperties type as a structured way to handle dynamic properties. This type acts as a container for extra properties, allowing us to store and access them in a type-safe manner.
The @dynamicModel Decorator: We introduced the @dynamicModel decorator in @typespec/http-client-csharp. This decorator signals to the code generator that a particular model should be treated as a dynamic model, capable of handling additional properties.
Implementation Details: We delved into the implementation details, including modifying the model structure, updating the deserialization logic, and enhancing the serialization logic.
Testing: We emphasized the importance of testing and discussed how to write unit tests to verify the AdditionalProperties implementation.

Looking Ahead

This is just the beginning, guys! The AdditionalProperties feature is a significant step forward in making our models more flexible and adaptable, but there's always room for improvement. As we continue to evolve @typespec/http-client-csharp, we'll be looking for ways to further enhance this feature and address any new challenges that arise.

So, stay tuned for more updates, and as always, we appreciate your feedback and contributions. Together, we can make @typespec/http-client-csharp the best tool for generating C# HTTP clients!