Below is a conceptual example illustrating how you can **strongly type** your deserialization using generics when you already know exactly which subclass you expect. Afterwards, I’ll explain the syntax and meaning of `where TMarketData : AbstractMarketData`.
## Example
Suppose you have the following abstract base class and subclasses:
```csharp
public abstract class AbstractMarketData
{
public DateTime DateRef { get; set; }
}
public class Repo : AbstractMarketData
{
public string Iosin { get; set; }
// Other Repo-specific properties...
}
public class Dividend : AbstractMarketData
{
public double SpotRef { get; set; }
// Other Dividend-specific properties...
}
public class Volatility : AbstractMarketData
{
public List<double> Strikes { get; set; }
// Other Volatility-specific properties...
}
```
### “RootObject” with Generics
You might receive JSON whose root structure looks something like:
```json
{
"MarketData": {
"Iosin": "XYZ123",
"DateRef": "2025-01-01T00:00:00"
}
}
```
…for a Repo. Or something like:
```json
{
"MarketData": {
"Strikes": [100, 110, 120],
"DateRef": "2025-01-01T00:00:00"
}
}
```
…for a Volatility.
To deserialize cleanly, you can define a generic `RootObject<TMarketData>`:
```csharp
public class RootObject<TMarketData> where TMarketData : AbstractMarketData
{
public TMarketData MarketData { get; set; }
}
```
**What does `where TMarketData : AbstractMarketData` mean?**
- This **generic constraint** ensures that `TMarketData` is always a class that **inherits** from `AbstractMarketData`.
- That means you can’t do something like `RootObject<int>`, because `int` does not derive from `AbstractMarketData`.
- You can only do `RootObject<Repo>`, `RootObject<Dividend>`, `RootObject<Volatility>`, or any other class that extends `AbstractMarketData`.
### Deserializing for a Known Type
Now, let’s say in your application logic, you have an enum that indicates the kind of market data you want to load:
```csharp
public enum DataType
{
Repos,
Dividends,
Volatilities
}
```
In your “getMarketData” method, you already know which `DataType` you want to load, so you can choose the appropriate *concrete* class:
```csharp
public AbstractMarketData GetMarketData(DataType dataType)
{
// Call API, read JSON string
var response = client.GetAsync(uriBuilder.Uri).Result;
var json = response.Content.ReadAsStringAsync().Result;
switch (dataType)
{
case DataType.Repos:
// Deserialize expecting a Repo
var repoRoot = JsonConvert.DeserializeObject<RootObject<Repo>>(json);
return repoRoot.MarketData;
case DataType.Dividends:
// Deserialize expecting a Dividend
var dividendRoot = JsonConvert.DeserializeObject<RootObject<Dividend>>(json);
return dividendRoot.MarketData;
case DataType.Volatilities:
// Deserialize expecting a Volatility
var volRoot = JsonConvert.DeserializeObject<RootObject<Volatility>>(json);
return volRoot.MarketData;
default:
throw new InvalidOperationException($"Unrecognized DataType: {dataType}");
}
}
```
With this pattern:
1. **Strong Typing** – Each case is deserialized into the proper subclass.
2. **Maintainability** – When you add a new subclass (e.g., `FXSwap` or something else), just add a new enum entry **and** add a new `case` block.
3. **Elimination of Casting** – No need to cast `(Repo)data` or `(Volatility)data` in the code. The generic deserialization handles that for you.
---
## Why Use `RootObject<TMarketData>`?
Often, the JSON you get has a wrapper object like:
```json
{
"SomeHeader": { "SomeInfo": "..." },
"MarketData": { ... your actual data ... }
}
```
Having a **generic root** type is advantageous because:
- You don’t have to repeatedly define different root classes for each subtype.
- You keep the “outer” structure (`RootObject<T>`) consistent while the **inner** type of `MarketData` can vary.
If your JSON is simpler and **only** contains the object for the subclass itself, you might not need the `RootObject<TMarketData>` at all, and could do something like:
```csharp
var repo = JsonConvert.DeserializeObject<Repo>(json);
```
But very often the JSON is wrapped with extra metadata, so a `RootObject<T>` is a clean approach.
---
## Summary
- `public class RootObject<TMarketData> where TMarketData : AbstractMarketData`
- This means: “Define a class `RootObject<TMarketData>`, and for the type parameter `TMarketData`, it **must** derive from `AbstractMarketData`.”
- The `where` clause is called a **generic type constraint**.
- You then sanitize each **specific** scenario (Repos, Dividends, Volatilities) by using
```csharp
JsonConvert.DeserializeObject<RootObject<Repo>>(json)
```
and so on.
- This ensures that the final type of the `.MarketData` property lines up exactly with your domain model (e.g., `Repo` vs. `Dividend` vs. `Volatility`).
This is a good approach if, in your higher-level logic, you truly **know** which type of market data you are fetching from each call.