TransformOutbound of route constraint is not called when the route segment is optional

[vjacquet]

Describe the bug

TransformOutbound is not callled when generating url when the constraint in the route template is is optional.

To Reproduce

https://github.com/vjacquet/bugs/blob/3fbc2924345021175b911b651b5afe3524cde361/aspnetcore/RouteConstraint/Startup.cs#L51

The Index page show that the last segment of the 2 urls are different when they shouldn't
Route.cshtml as the route definition @page "{id:hex}"
RouteWithOptionalSegment.cshtml as the route definition @page "{id:hex?}"

The problem is that OptionalRouteConstraint, that wrap the specified route contraint, does not implement IOutboundParameterTransformer, therefore _parameterTransformers in TemplateBinder is not initialized correctly.

One possible fix is to define

class OutboundParameterTransformerOptionalRouteConstraint : OptionalRouteConstraint, IOutboundParameterTransformer
{
    public OutboundParameterTransformerOptionalRouteConstraint(IRouteConstraint innerConstraint) : base(innerConstraint)
    {
    }

    public string TransformOutbound(object value) => ((IOutboundParameterTransformer)InnerConstraint).TransformOutbound(value);
}

and have the DefaultParameterPolicy's InitializeRouteConstraint method return an instance of it when the constraint is optional and the routeConstraint implements IOutboundParameterTransformer.

private IParameterPolicy InitializeRouteConstraint(bool optional, IRouteConstraint routeConstraint)
{
    if (!optional)
        return routeConstraint;
    else if (routeConstraint is IOutboundParameterTransformer)
        return new OutboundParameterTransformerOptionalRouteConstraint(routeConstraint);
    else
        return new OptionalRouteConstraint(routeConstraint);
}

Further technical details

• ASP.NET Core version 3.1

.NET Core SDK (reflecting any global.json):
Version: 3.1.301
Commit: 7feb845744

Runtime Environment:
OS Name: Windows
OS Version: 10.0.18362
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\3.1.301\

Host (useful for support):
Version: 3.1.5
Commit: 65cd789777

.NET Core SDKs installed:
1.1.14 [C:\Program Files\dotnet\sdk]
2.1.401 [C:\Program Files\dotnet\sdk]
2.1.802 [C:\Program Files\dotnet\sdk]
2.2.103 [C:\Program Files\dotnet\sdk]
3.1.200 [C:\Program Files\dotnet\sdk]
3.1.202 [C:\Program Files\dotnet\sdk]
3.1.301 [C:\Program Files\dotnet\sdk]

.NET Core runtimes installed:
Microsoft.AspNetCore.All 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.All 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
Microsoft.AspNetCore.App 2.1.2 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
Microsoft.NETCore.App 1.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 1.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.13 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.1.19 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 2.2.1 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.NETCore.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
Microsoft.WindowsDesktop.App 3.1.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

[mkArtakMSFT]

Thanks for contacting us.
We're moving this issue to the Next sprint planning milestone for future evaluation / consideration. We will evaluate the request when we will planning the work for the next milestone. To learn more about what to expect next and how this issue will be handled you can read more about our triage process here.

[davidfowl]

cc @JamesNK

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.