What is the System.Text.Json source generator?

Question

Accepted Answer

**System.Text.Json source generator** — compile-time JSON serialization.

**Definition:**
```csharp
[JsonSerializable(typeof(User))]
[JsonSerializable(typeof(List<Order>))]
internal partial class AppJsonContext : JsonSerializerContext { }
```

**Usage:**
```csharp
// Serialize
var json = JsonSerializer.Serialize(
    user, AppJsonContext.Default.User);

// Deserialize
var user = JsonSerializer.Deserialize(
    json, AppJsonContext.Default.User);
```

**vs reflection-based:**
- No runtime reflection.
- 2-5x faster serialization.
- Smaller trimmed/AOT publishes.
- Compile-time type checking.

**Options:**
```csharp
var options = new JsonSerializerOptions
{
    PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
};
var json = JsonSerializer.Serialize(
    user, AppJsonContext.Default.User);
```

C#: What is the System.Text.Json source generator?