C#•3d ago

✅ Binding Nested values in RequestBody to Model

Consider the following model I use inside the MVC structure:

public class Collection
{
    /// <summary>
    /// The UUID of this Collection
    /// </summary>
    public required string Id { get; set; }
    /// <summary>
    /// The external Url representing the entries of this collection
    /// </summary>
    public required string Url { get; set; }
    /// <summary>
    /// The date the Collection was created at.
    /// </summary>
    public required DateOnly CreatedAt { get; set; }
}

public class Collection
{
    /// <summary>
    /// The UUID of this Collection
    /// </summary>
    public required string Id { get; set; }
    /// <summary>
    /// The external Url representing the entries of this collection
    /// </summary>
    public required string Url { get; set; }
    /// <summary>
    /// The date the Collection was created at.
    /// </summary>
    public required DateOnly CreatedAt { get; set; }
}

However, when responding to requests, this is instead formatted as application/hal+json, which would look like this:

{
  "_links": {
    "self": {
      "href": "string"
    },
    "external": {
      "href": "string"
    }
  },
  "createdAt": "2024-11-18"
}

{
  "_links": {
    "self": {
      "href": "string"
    },
    "external": {
      "href": "string"
    }
  },
  "createdAt": "2024-11-18"
}

As you can see, the property Url has been moved into _links:external:href. Now I would like to receive a Collection in a [HttpPut] request, but I'm having trouble figuring out how to properly parse the incoming application/hal+json to my Model. I've read online that Model Binding is probably the solution to this, but the vast majority of examples and explanations I could find were centered around Routes / Query parameters, not RequestBody formatting. The part I did find mentioned ValueProviders, but I couldn't find out how to obtain the JSON data from the body and store it as keys there. Any suggestions are appreciated, as I'm rather new to MVC architecture and C# in general.

1 Reply

Luca | LeCarbonatorOP•3d ago

It looks like Model Binding is the way to go, but getting the request body's text is more difficult than I thought it enforces the read to be async, but IModelBuilder throws an error even though it's async

'CollectionBinder' does not implement interface member 'IModelBinder.BindModelAsync(ModelBindingContext)'. 'CollectionBinder.BindModelAsync(ModelBindingContext)' cannot implement 'IModelBinder.BindModelAsync(ModelBindingContext)' because it does not have the matching return type of 'Task'.

'CollectionBinder' does not implement interface member 'IModelBinder.BindModelAsync(ModelBindingContext)'. 'CollectionBinder.BindModelAsync(ModelBindingContext)' cannot implement 'IModelBinder.BindModelAsync(ModelBindingContext)' because it does not have the matching return type of 'Task'.

Is there another way to obtain it other than bindingContext.HttpContext.Request.BodyReader? I seem to have find a way to do it in the model binder:

public class FooBinder : IModelBinder
{
    public async Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType != typeof(Foo))
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        var readResult = await bindingContext.HttpContext.Request.BodyReader.ReadAsync();
        var content = Encoding.UTF8.GetString(readResult.Buffer);

        if (content == null)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        var jo = JObject.Parse(content);

        // Set all properties with a [JsonProperty] attribute attached
        Foo? output = jo.ToObject<Foo>();

        if (output == null)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        output.BarProp = (string)jo.SelectToken("some.nested.property.from.json")!;

        bindingContext.Result = ModelBindingResult.Success(output);
        return;
    }
}

public class FooBinder : IModelBinder
{
    public async Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext.ModelType != typeof(Foo))
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        var readResult = await bindingContext.HttpContext.Request.BodyReader.ReadAsync();
        var content = Encoding.UTF8.GetString(readResult.Buffer);

        if (content == null)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        var jo = JObject.Parse(content);

        // Set all properties with a [JsonProperty] attribute attached
        Foo? output = jo.ToObject<Foo>();

        if (output == null)
        {
            bindingContext.Result = ModelBindingResult.Failed();
            return;
        }

        output.BarProp = (string)jo.SelectToken("some.nested.property.from.json")!;

        bindingContext.Result = ModelBindingResult.Success(output);
        return;
    }
}

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✅ Binding Nested values in RequestBody to Model