Creating a ModelBinder for MongoDB ObjectId on Asp.Net Core

If I remove [FromBody], player is set to a default value, not to the values I post.

Reading data from the body is opt-in (unless you're using [ApiController]). When you remove [FromBody] from your Player parameter, the model-binding process will look to populate properties of Player using the route, query-string and form-values, by default. In your example, there are no such properties in these locations and so none of Player's properties get set.

If I put [FromBody] before the Player action parameter, player is set to null.

With the presence of the [FromBody] attribute, the model-binding process attempts to read from the body according to the Content-Type provided with the request. If this is application/json, the body will be parsed as JSON and mapped to your Player's properties. In your example, the JSON-parsing process fails as it doesn't know how to convert from a string to an ObjectId. When this happens, ModelState.IsValid within your controller will return false and your Player parameter will be null.

For this code to work, I mean for the model binder to run, I inherited the controller from Controller and removed the [FromBody] attribute from the Player parameter.

When you remove [FromBody], the [ModelBinder(...)] attribute you've set on your Id property is respected and so your code runs. However, with the presence of [FromBody], this attribute effectively is ignored. There's a lot going on behind-the-scenes here, but essentially it boils down to the fact that you've already opted-in to model-binding from the body as JSON and that's where model-binding stops in this scenario.

I mentioned above that it's the JSON-parsing process that's failing here due to not understanding how to process ObjectId. As this JSON-parsing is handled by Newtonsoft.Json (aka JSON.NET), a possible solution is to create a custom JsonConverter. This is covered well here on Stack Overflow, so I won't go into the details of how it works. Here's a complete example (error-handling omitted for brevity and laziness):

public class ObjectIdJsonConverter : JsonConverter
{
    public override bool CanConvert(Type objectType) =>
        objectType == typeof(ObjectId);

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) =>
        ObjectId.Parse(reader.Value as string);

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) =>
        writer.WriteValue(((ObjectId)value).ToString());
}

To make use of this, just replace your existing [ModelBinder(...)] attribute with a [JsonConverter(...)] attribute, like this:

[BsonId]
[JsonConverter(typeof(ObjectIdJsonConverter))]    
public ObjectId Id { get; set; }

Alternatively, you can register ObjectIdJsonConverter globally so that it applies to all ObjectId properties, using something like this in Startup.ConfigureServices:

services.AddMvc()
        .AddJsonOptions(options =>
            options.SerializerSettings.Converters.Add(new ObjectIdJsonConverter());
        );

You were mistaken at ModelBinder. Correct code:

public class ObjectIdModelBinder : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        var result = bindingContext.ValueProvider.GetValue(bindingContext.FieldName);

        bindingContext.Result = ModelBindingResult.Success(new ObjectId(result.FirstValue));

        return Task.CompletedTask;
    }
}