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Hey @bjarnef, thanks for recreating this PR, I'll check back on the historic conversations around this one, and myself (or another core collaborator) will review this soon😸

@bergmania @elit0451 would this makes sense to support now to streamline content type features and as containers are supported for blueprints as well?
#15983

@nul800sebastiaan looks like the base method only has 5 arguments now? :)

MemberTypeContainerService has 7 arguments, max arguments = 5

looks like the base method only has 5 arguments now? :)

It's just the code analyzer complaining that there are too many arguments. This is not something we can avoid, the base must take 7 arguments at this time. So all good.

I've merged this now and marked it as a breaking change (a build will be needed, but that will definitely be needed once you upgrade to v17 anyway).
Now that the APIs are there, it doesn't mean the functionality is available in the backoffice interface.

However, this is now something that can be done programmatically and that means @ronaldbarendse Umbraco Deploy will need to support it and @KevinJump uSync will need to support it.

@nul800sebastiaan Did someone forget to review this PR before merging? 😅 Using the Management API to create a new folder instantly returns the following error:

POST /umbraco/management/api/v1/member-type/folder
{
  "name": "Test"
}

{
  "type": "Error",
  "title": "Not a contained object type. (Parameter 'containedObjectType')",
  "status": 500,
  "detail": "
   at Umbraco.Cms.Core.Models.EntityContainer..ctor(Guid containedObjectType)
   at Umbraco.Cms.Core.Services.EntityTypeContainerService`2.CreateAsync(Nullable`1 key, String name, Nullable`1 parentKey, Guid userKey)
   at Umbraco.Cms.Api.Management.Controllers.FolderManagementControllerBase`1.CreateFolderAsync[TCreatedActionController](CreateFolderRequestModel createFolderRequestModel, Expression`1 createdAction)
   at Umbraco.Cms.Api.Management.Controllers.MemberType.Folder.CreateMemberTypeFolderController.Create(CancellationToken cancellationToken, CreateFolderRequestModel createFolderRequestModel)
   ...",
  "instance": "ArgumentException"
}

Adding { Constants.ObjectTypes.MemberType, Constants.ObjectTypes.MemberTypeContainer } to the EntityContainer.ObjectTypeMap dictionary does fix this and the endpoint then returns a 201 with a newly created GUID.

However, because the IMemberTypeContainerRepository is a no-op implementation, the created item isn't stored anywhere, so there's not really anything Deploy (or uSync) can add support for...

I've partially fixed these issues in #20706, but the backoffice UI still needs to be implemented. If this isn't something we can ship in the final v17 release, I'd suggest reverting this PR and postponing it for v18.