Introduce a more ergonomic way of enabling case-insensitive enum parsing in Minimal API endpoints

[aradalvand]

Basically the same as #45590.
Currently, the only way to make this work would be to create another type that acts as a wrapper around the enum, just to implement a TryParse method that does case-insensitive parsing for the underlying enum; as @captainsafia described in the aforementioned issue:

You might want to consider implementing your own TryParse implementation on a wrapper for your MyEnum type that is case-insensitive. See more info at https://learn.microsoft.com/en-us/aspnet/core/fundamentals/minimal-apis/parameter-binding?view=aspnetcore-7.0#tryparse

But it goes without saying that this is incredibly ugly.

So, either:

• When binding enum parameters in minimal API handlers, use the Enum.TryParse overload that does a case-insensitive comparison. This way the behavior will also be consistent with that of MVC controllers. Or provide a global opt-in option that lets the developer decide.
• Provide a simpler way to customize parameter binding on an individual-parameter-basis for minimal API handlers without having to define a TryParse or BindAsync method on the type of the parameter. This is a direly needed feature in general; as you don't always have access to the type of a parameter in such a way that you can declare a static TryParse or BindAsync method on them (as is effectively the case for enums, or for types that come from third-party libraries), so it should be possible to customize binding for individual parameters in a granular fashion, as opposed to having to modify the types themselves.

The second option is of course preferable.

[captainsafia]

@aradalvand Thanks for filing this issue!

I see there's been some conversation in the related issue around customizing parameter binding in minimal APIs. I think this particular aspects needs some design and, to be frank, some more details about the types of granular modifications that need to be made.

I'll park this under Needs Design for now given the requirement to flesh this out. I'd be curious to see if you have thoughts on how to tackle this separate from what has been proposed in #35489.

[ghost]

We've moved this issue to the Backlog milestone. This means that it is not going to be worked on for the coming release. We will reassess the backlog following the current release and consider this item at that time. To learn more about our issue management process and to have better expectation regarding different types of issues you can read our Triage Process.

[adamijak]

You can use this GitHub gist.

With example usage:

app.MapGet("/", ([FromQuery] EnumBinding<MyEnum> param) => 
{
   MyEnum enumValue = param;
});

[captainsafia]

@adamijak Thanks for sharing the code sample with folks! This is a good reminder that I should post some updates on #35489 based on some recent things I learned about how we might take advantage of keyed services in our parameter binding layer.

[jrebagliatti]

To add to @adamijak's workaround for those that want a proper Swagger documentation, you can add an schema filter like this one:

    public class EnumSchemaFilter : ISchemaFilter
    {
        public void Apply(OpenApiSchema schema, SchemaFilterContext context)
        {
            var type = context.Type;

            if (type == null)
            {
                return;
            }

            if (type.IsGenericType && type.GetGenericTypeDefinition() == typeof(EnumBinding<>))
            {
                var enumType = type.GetGenericArguments().FirstOrDefault(x => x.IsEnum);

                if (enumType == null)
                {
                    return;
                }

                var names = Enum.GetNames(enumType);

                if (names == null)
                {
                    return;
                }
                
                schema.Type = "string";
                schema.Enum = names.OfType<string>().Select(x => new OpenApiString(x)).ToList<IOpenApiAny>();
            }
        }
    }

[adamijak]

@jrebagliatti @captainsafia I updated the gist. Now it accepts boolean and DateTime types as-well.