Consider adding an API that enables invoking the default parameter binding logic of RequestDelegateFactory explicitly

[DamianEdwards]

We should consider adding an API that enables the ability to invoke the parameter binding logic inherent in RequestDelegateFactory without having to create a RequestDelegate. This would be useful in situations where the default logic isn't invoked implicitly, e.g. when a complex parameter implements BindAsync, but invoking the behavior in an observable fashion would still add utility.

Such an API exists in the MinimalApis.Extensions library and is used by many of the included utility classes (example).

Strawman API proposal:

public static class RequestDelegateFactory
{
+    public static BindResult<TValue> BindParameterAsync<TValue>(HttpContext httpContext, ParameterInfo parameter);
}

+public class BindResult<TValue>
+{
+    public TValue? Value { get; }
+    public int? StatusCode { get; }
+    public void Deconstruct(out TValue? value, out int statusCode);
+}

Example use:

app.MapPost("/widgets", (Param<CreateWidget> widget, WidgetDb db) =>
{
    if (widget.AutoResponseStatusCode != 200)
    {
        // There was an issue binding, implement custom handling logic here
        return Results.Problem(statusCode: widget.AutoResponseStatusCode);
    }
    var newWidget = db.Insert(widget);
    return Results.Created($"/widgets/{newWidget.Id}", newWidget);
});

public class CreateWidget
{
    public string? Name { get; set; }
}

public class Param<TValue>
{
    public Param(TValue? value)
    {
        Value = value;
    }

    public TValue? Value { get; }
    public int AutoResponseStatusCode { get; init; } = 200;

    public static async ValueTask<Param<TValue?>> BindAsync(HttpContext context, ParameterInfo parameter)
    {
        var (defaultValue, statusCode) = await RequestDelegateFactory.BindParameterAsync(context, parameter);
        if (statusCode is not 200)
        {
            // The default logic would have implicitly returned due to an issue
            return new Param(null) { AutoResponseStatusCode = statusCode };
        }
        return new Param(defaultValue);
    }
}