Problem with Quering Nullable and Not Nullable Fields

I have two tables with same fields one is Nullable and one is not Nullable. I want to check Whether
Ids in Nullable fields exists in table with Non Nullable fields.

I cannot change the Types of these two fields. It is neccessary.

Example:

TableName - MasterUser [UserId int NOT Nullable]
TableName - User [UserId int Nullable]

My Query will be like is like this.

from masteruser in ctx.MasterUser
where GetUserIds(profileId).contains(masteruser.UserId)
select masteruser

List<int> GetUserIds(int profileId)
{
List<int> lstUserIds = (from user in ctx.User
where user.profileId) == profileId)
select user.UserId).ToList();
return lstUserIds;

}

Problem 2:
In the GetUserIds method, Can I make the return type as Nullable int List. If I change List<int> to List<int?> then I am getting error in ToList()
How can I return the Nullabnle int list from this method?

Thanks

Anandraj.A.