Hi

I noticed a similar question the other day: https://our.umbraco.com/forum/using-umbraco-and-getting-started/107936-how-to-get-member-by-udi-in-umbraco-9

In Umbraco 9 there is no longer a MemberShipHelper, but now a IMemberManager which one can inject in controllers or views: https://our.umbraco.com/documentation/reference/querying/IMemberManager/

and resolving the id from Udi or Guid by injecting IIdKeyMap service.

Find member by id

var memberById = await _memberManager.FindByIdAsync("1234");
IPublishedContent member = _memberManager.AsPublishedMember(memberById);

Find member by udi

var memberUdiAttempt = _idKeyMap.GetIdForUdi(nodeUdi);
if (memberUdiAttempt.Success)
{
   var memberId = memberUdiAttempt.Result;
   var member = await _memberManager.FindByIdAsync(memberId.ToString());
}

Find member by key

var memberKeyAttempt = _idKeyMap.GetIdForKey(nodeKey);
if (memberKeyAttempt.Success)
{
   var memberId = memberKeyAttempt.Result;
   var member = await _memberManager.FindByIdAsync(memberId.ToString());
}

I have submitted a PR here to update the documentation: https://github.com/umbraco/UmbracoDocs/pull/3752

If using a member picker this should already be handled in the PVC: https://github.com/umbraco/Umbraco-CMS/blob/v9/contrib/src/Umbraco.Core/PropertyEditors/ValueConverters/MemberPickerValueConverter.cs

And the same goes for MNTP: https://github.com/umbraco/Umbraco-CMS/blob/v9/contrib/src/Umbraco.Core/PropertyEditors/ValueConverters/MultiNodeTreePickerValueConverter.cs