How do I implement a custom JSON converter in MvvmCross

Step 1: Choose a JSON Serialization Library

MvvmCross supports both `System.Text.Json` and `Json.Net`. For this example, we'll focus on `System.Text.Json`, which is the default JSON serialization library in .NET Core and .NET 5+.

Step 2: Create a Custom Converter

To create a custom converter, you need to derive from `JsonConverter`, where `T` is the type you want to serialize or deserialize. Here's an example of a custom converter for a `DateTimeOffset` type that uses a specific format:

csharp
using System;
using System.Globalization;
using System.Text.Json;
using System.Text.Json.Serialization;

public class CustomDateTimeOffsetConverter : JsonConverter
{
    public override DateTimeOffset Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options)
    {
        return DateTimeOffset.ParseExact(reader.GetString()!, "MM/dd/yyyy", CultureInfo.InvariantCulture);
    }

    public override void Write(
        Utf8JsonWriter writer,
        DateTimeOffset dateTimeValue,
        JsonSerializerOptions options)
    {
        writer.WriteStringValue(dateTimeValue.ToString("MM/dd/yyyy", CultureInfo.InvariantCulture));
    }
}

Step 3: Register the Custom Converter

To use the custom converter, you need to register it with the `JsonSerializerOptions`. Here's how you can do it:

csharp
var serializeOptions = new JsonSerializerOptions
{
    WriteIndented = true
};

serializeOptions.Converters.Add(new CustomDateTimeOffsetConverter());

// Example usage
var exampleObject = new ExampleObject
{
    Date = new DateTimeOffset(2023, 1, 1, 0, 0, 0, TimeSpan.Zero)
};

var jsonText = JsonSerializer.Serialize(exampleObject, serializeOptions);

Step 4: Using MvvmCross Json Plugin

If you prefer to use the MvvmCross Json plugin, which wraps `Json.Net`, you can resolve the `IMvxJsonConverter` from the IoC container and use it for serialization and deserialization:

csharp
var serializer = Mvx.IoCProvider.Resolve();
var exampleObject = new ExampleObject
{
    Date = new DateTime(2023, 1, 1)
};

var jsonText = serializer.SerializeObject(exampleObject);
var deserialized = serializer.DeserializeObject(jsonText);

However, to implement a custom converter with `Json.Net`, you would need to create a converter class similar to the `System.Text.Json` example but derive from `JsonConverter` and override its methods.

Step 5: Handling Complex Types

For complex types or polymorphic deserialization, you might need to use the factory pattern by deriving from `JsonConverterFactory`. This allows you to dynamically create converters for different types at runtime.

Conclusion

Implementing a custom JSON converter in MvvmCross involves choosing the appropriate JSON serialization library, creating a custom converter class, and registering it with the serializer options. Whether you use `System.Text.Json` or the MvvmCross Json plugin, the process involves similar steps but with different underlying libraries.