How does the Required attribute interact with JSON deserialization in MvvmCross

MvvmCross Data Binding

MvvmCross uses C# properties for data binding on both the View and the ViewModel. These properties typically follow a pattern where a private backing field stores the value, and the property setter updates this field and raises a `PropertyChanged` event to notify the UI of changes[2]. While MvvmCross does not use JSON for data binding anymore (having replaced it with a cleaner syntax called 'Swiss'), it does not directly support attributes like `[Required]` for JSON deserialization validation.

JSON Deserialization in MvvmCross

For JSON deserialization in MvvmCross, you would typically use a JSON serializer like `System.Text.Json` or `Json.NET`. These serializers can enforce required properties during deserialization using attributes like `[JsonRequired]` or the `required` keyword in C# (for `System.Text.Json` in .NET 6 and later)[1]. However, MvvmCross itself does not provide built-in support for these attributes in its data binding framework.

Implementing Required Validation

To enforce required properties during JSON deserialization in an MvvmCross application, you would need to use a JSON serializer that supports such validation. Here’s how you might do it with `System.Text.Json`:

1. Use `System.Text.Json`: Define your model with required properties using the `required` keyword or `[JsonRequired]` attribute.

csharp
   public class MyModel
   {
       public required string RequiredProperty { get; set; }
       public int OptionalProperty { get; set; }
   }
   

2. Deserialize with Validation: When deserializing JSON, use `System.Text.Json` to throw an exception if required properties are missing.

csharp
   var json = "{\"OptionalProperty\": 42}";
   try
   {
       var model = JsonSerializer.Deserialize(json);
   }
   catch (JsonException ex)
   {
       Console.WriteLine("Deserialization failed: " + ex.Message);
   }
   

Custom Validation in MvvmCross

If you need to validate properties in your view model before submitting data, you can implement `INotifyDataErrorInfo` or use validation libraries like Fluent Validation. These approaches allow you to manually validate properties and notify the UI of any errors[10].

In summary, while MvvmCross does not directly support the `Required` attribute for JSON deserialization validation, you can achieve similar functionality by using a JSON serializer that supports required properties and implementing custom validation logic in your view models.