Harvzor.Optional.SystemTextJson 0.1.0

Harvzor.Optional

NuGet Package	Version
Harvzor.Optional
Harvzor.Optional.SystemTextJson
Harvzor.Optional.NewtonsoftJson
Harvzor.Optional.Swashbuckle

The problem

// The JSON would normally come from some external data source:
string json = "{}";

Foo foo = JsonSerializer.Deserialize<Foo>(json);

// This will print an empty string because C# will hydrate the model with the default value.
// There's no way to check if the model used a default value.
Console.WriteLine(foo.MyProperty); // ""    

public class Foo
{
    public string MyProperty { get; set; }
}

So just use a nullable value? (string?)

In the above example, you could make the string nullable (with string?), but now you're explicitly saying that null is value you want to accept (which you'll have to handle in your code).

What if you want to allow a client to explicitly set null ("{ "myProperty": null }"), and want to handle this in your code, while also knowing if the client didn't send anything?

The solution: Optional<T>

Basic example

You can use Optional<T> to know if a property or variable has been explicitly instantiated:

using Harvzor.Optional;

Foo foo = new Foo
{
    DefinedProperty = "Bar"
};

Console.WriteLine(foo.DefinedProperty.IsDefined); // True
Console.WriteLine(foo.UndefinedProperty.IsDefined); // False

// Now I can check if a value was explicitly instantiated:
if (foo.DefinedProperty.IsDefined) // True
    Console.WriteLine(foo.DefinedProperty.Value); // "Bar" 
    
if (foo.UndefinedProperty.IsDefined) // False
    Console.WriteLine(foo.UndefinedProperty.Value); // Won't print as the value wasn't explicitly instantiated.

Console.WriteLine(foo.UndefinedProperty.Value); // Will print the default value.

public class Foo
{
    public Optional<string> DefinedProperty { get; set; }
    public Optional<string> UndefinedProperty { get; set; }
}

Use it with JSON

This example uses System.Text.Json:

using System.Text.Json;
using Harvzor.Optional;

JsonSerializerOptions jsonSerializerOptions = new JsonSerializerOptions();
jsonSerializerOptions.Converters.Add(new Harvzor.Optional.SystemTextJson.OptionalJsonConverter());
jsonSerializerOptions.TypeInfoResolver = new DefaultJsonTypeInfoResolver
{
    Modifiers = { OptionalTypeInfoResolverModifiers.IgnoreUndefinedOptionals }
};

// The JSON would normally come from some external data source:
string json = "{\"DefinedProperty\":\"Bar\"}";

Foo foo = JsonSerializer.Deserialize<Foo>(json, jsonSerializerOptions)!;

Console.WriteLine(foo.DefinedProperty.IsDefined); // True
Console.WriteLine(foo.UndefinedProperty.IsDefined); // False

// Now I can check if a value was defined before I try using it:
if (foo.DefinedProperty.IsDefined) // True
    Console.WriteLine(foo.DefinedProperty.Value); // "Bar" 
    
if (foo.UndefinedProperty.IsDefined) // False
    Console.WriteLine(foo.UndefinedProperty.Value); // Won't print as the value wasn't defined.

public class Foo
{
    public Optional<string> DefinedProperty { get; set; }
    public Optional<string> UndefinedProperty { get; set; }
}

Use in an API

To use it in your controller models, simply register in your startup:

Harvzor.Optional.SystemTextJson

using Harvzor.Optional.SystemTextJson;

services
    .AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.Converters.Add(new OptionalJsonConverter());
        options.JsonSerializerOptions.TypeInfoResolver = new DefaultJsonTypeInfoResolver
        {
            Modifiers = { OptionalTypeInfoResolverModifiers.IgnoreUndefinedOptionals }
        };
    });

Harvzor.Optional.NewtonsoftJson

using Harvzor.Optional.NewtonsoftJson;

services
    .AddNewtonsoftJson(options =>
    {
        options.SerializerSettings.Converters.Add(new OptionalJsonConverter());
        options.SerializerSettings.ContractResolver = new IgnoreUndefinedOptionalsContractResolver();
    });

Swagger support

Harvzor.Optional.Swashbuckle

Warning This package is experimental.

Swashbuckle SwaggerGen doesn't know how to handle Optional<T> and will attempt to generate complicated objects to express all the properties, for a simple class like:

public class Foo
{
    public Optional<int> OptionalInt { get; set; }
}

This ends up being generated like:

Instead we want SwaggerGen to treat Optional<T> as the generic type T. To handle doing this, add this:

using Harvzor.Optional.Swashbuckle;

services
    .AddSwaggerGen(options =>
    {
        // The assembly you pass in should include your controllers and perhaps even your DTOs.
        options.FixOptionalMappings(Assembly.GetExecutingAssembly());
    });

This results in the correct OpenAPI spec:

This will:

• ensure that all basic types like Optional<string> are mapped to the string type in the OpenAPI schema
• try to treat complex objects such as Optional<MyType> as the underlying generic type MyType

This doesn't work in all cases though, for example, with Optional<Version>, we want it to be treated as a string type and not as a Version, so this must be added:

using Harvzor.Optional.Swashbuckle;

// Add your custom mappings first:
options.MapType<Optional<Version>>(() => new OpenApiSchema()
{
    Type = "string"
});

options.FixOptionalMappings(Assembly.GetExecutingAssembly());

Alternatively, if you don't want to call FixOptionalMappings(params Assembly[] assemblies) which automagically finds any references to Optional<T> in your assembly, you can just directly feed it Optional<T> types that you know are used in your controllers:

using Harvzor.Optional.Swashbuckle;

options
    .FixOptionalMappingForType<Optional<Foo>>()
    .FixOptionalMappingForType<Optional<Bar>>()
    .FixOptionalMappingForType<Optional<int>>();

Known caveats

• FixOptionalMappings(params Assembly[] assemblies) does not work with minimal APIs as it searches for Optional<T> references on parameters and properties of any classes that implement controller methods, and then maps those Optional<T> types to their generic type T
• Optional<T> doesn't work with query parameters
  • You can't have the following:

    [HttpGet]
    public string Get([FromQuery] Optional<string> foo)
    {
        return foo;
    }
    

  • This is because of the following issue: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/2226

Improvements

This package could be improved if these issues are ever resolved:

• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/1810
• https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/2359

Manual Swagger support

If you're using Swashbuckle.AspNetCore.SwaggerGen but don't want to use Harvzor.Optional.Swashbuckle, you can also manually tell it how your types should look. Here are some basic types mapped:

services.AddSwaggerGen(options =>
{ 
    options.MapType<Optional<string>>(() => new OpenApiSchema
    {
        Type = "string"
    });
    
    options.MapType<Optional<int>>(() => new OpenApiSchema
    {
        Type = "integer",
        Format = "int32"
    });
    
    options.MapType<Optional<float>>(() => new OpenApiSchema
    {
        Type = "number",
        Format = "float"
    });
    
    options.MapType<Optional<double>>(() => new OpenApiSchema
    {
        Type = "number",
        Format = "double"
    });
    
    options.MapType<Optional<bool>>(() => new OpenApiSchema
    {
        Type = "boolean"
    });
    
    options.MapType<Optional<DateTime>>(() => new OpenApiSchema
    {
        Type = "string",
        Format = "date-time"
    });
    
    // IEnumerables:
    options.MapType<Optional<IEnumerable<int>>>(() => new OpenApiSchema
    {
        Type = "array",
        Items = new OpenApiSchema
        {
            Type = "integer",
            Format = "int32"
        }
    });
});

You can see what basic types are available here: https://swagger.io/docs/specification/data-models/data-types/

However, handling custom objects such as Optional<MyObject> is quite complicated and not recommended, however, here's how to do it anyway:

// Rewrite the mapping so it's an object reference:
options.MapType<Optional<MyObject>>(() => new OpenApiSchema
{
    Type = "object",
    Format = format,
    Reference = new OpenApiReference
    {
        Id = nameof(MyObject),
        Type = ReferenceType.Schema,
    }
});

// Now add `MyObject` to the schema repisitory so the mapping actually points somewhere:
options.DocumentFilter<GenerateSchemaFor<MyObject>>();

private class GenerateSchemaFor<T> : IDocumentFilter where T : class
{
    public void Apply(OpenApiDocument swaggerDoc, DocumentFilterContext context)
    {
          context.SchemaGenerator.GenerateSchema(typeof(T), context.SchemaRepository);
    }
}

Use case: JSON Merge PATCH

... need docs ...

Releasing

GitLab CI

1. Create a new release with a semver release name

Manual release

In case the CI doesn't work:

1. Get an API key from https://www.nuget.org/account/apikeys

docker-compose build --build-arg version="{version}" push-nuget
docker-compose run --rm push-nuget --api-key {key}

Further reading

• https://stackoverflow.com/questions/63418549/custom-json-serializer-for-optional-property-with-system-text-json
• https://stackoverflow.com/questions/12522000/optionally-serialize-a-property-based-on-its-runtime-value
• Optional in Swagger definition and how to handle generic types: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/2359
  • ISchemaGenerator: https://github.com/domaindrivendev/Swashbuckle.AspNetCore/issues/2333#issuecomment-1035695675