Int null checker on a partial view

I have a partial view which does not have any parameters passed to it and is called by multiple apps.

I however want to change the requirements and pass a nullable int to it and a null checker, but it fails, I am a bit confused, how do I go about this ?

My confusion lies with this line >> Id ?? null ? user.UserId() ;

heres my code

[HttpGet]
public PartialViewResult GetUserById (int ? Id)
{
     var user = new GetAuthenticatedUser() ;  /// This is a custom inhouse code that returns the User ID and properties

    Id ?? null ? user.UserId() ;  // I am trying to test if the Id is null then use the user account, if not user the query string Id value
     var userId = _user.GetUserBasedOnId(Id)
 
}

// In a nutshell what I am trying to achieve is this

if (Id == null )
{
         var userId = _user.GetUserBasedOnId(user.UserId)
}
else
{
         var userId = _user.GetUserBasedOnId(Id)
}

If I tried

Id = Id ?? null ? user.UserId() ;

I get the error

CS0019 Operator ?? cannot be applied to operants of type int and

>Solution :

Declare the int as nullable by changing

public PartialViewResult GetUserById (int ? Id)

to

public PartialViewResult GetUserById (int? Id)