We needed to be able to use an entity directly from an action parameter. This research was done to learn how to utilize custom model binders to achieve this goal.

We've decided to solve this problem through code generation instead of model binding because of inconsistency between libraries & lack of features;

• model binding does not seem to support nested properties, which requires custom solution for json serializer as well
• it doesn't work well with swashbuckle, which is another thing to maintain
• microsoft does not recommend to use model binders to lookup for entities

We've used IModelBinder interface to implement a custom solution. It provides a clean base to design a custom model binding for entities.

public class QueryModelBinder<TModel>(IQuery<TModel> _query)
    : IModelBinder
{
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        // ... put find entity logic here using `_query`
    }
}

To map a model binder for a model, we've used IModelBinderProvider.

public class QueryModelBinderProvider : IModelBinderProvider
{
    public IModelBinder? GetBinder(ModelBinderProviderContext context)
    {
        // check if given context.Metadata.ModelType is an entity
        // return query of that type using context.Services
    }
}

• When a value type implements IParseable<T>, .NET automatically makes use of static abstract TryParse method in the interface. So no need to write custom model binders for custom value types any more 🥳
• In case IParseable is not enough, TypeConverter classes is another option to bind a value type
• record types are allowed to have primary constructors, but regular class types with a primary constructor does not get automatically bound as a model
• Returning status code directly from model binders is not recommended

• Model Binding
• Custom Model Binding
• Author Sample