Not equal list<> in entity framework

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I have 4 tables as following

table 1 Location A

public class CustmLocationA {
        public int Location1_ID { get; set; }
        public string Location1 { get; set; }
        public string Location1_Descrip { get; set; }
        public bool IsActive { get; set; }
}

Table 2 location B

public class CustmLocationB{
       pubblic int Location2_ID { get; set; }
       public string Location2 { get; set; }
       public string Location2_Descrip { get; set; }
       public int Location1_ID { get; set; }
       public bool IsActive { get; set; }
}

Table 3 Location C

public class CustmLocationC {

        public int Location3_ID { get; set; }
        public string Location3 { get; set; }
        public string Location3_Descrip { get; set; }
        public int Location2_ID { get; set; }
        public bool IsActive { get; set; }
        //Location B data
        public string Location2 { get; set; }
        [NotMapped] 
        public bool LocBIsActive { get; set; }
        //location A data
        [NotMapped] 
        public int Location1_ID { get; set; }
        [NotMapped] 
        public string Location1 { get; set; }
        [NotMapped] 
        public bool LocAIsActive { get; set; }
        [NotMapped] 
        public string LocAandB { get; set; }

}

Table 4

public class CustmContact
    {
        
        public int contactID { get; set; }
        public int Location3_ID { get; set; }
        public int UserID { get; set; }
        public bool Notify { get; set; }
        public bool Access { get; set; }
        [NotMapped]
        public string UserName { get; set; }
        [NotMapped]
        public string LocationAll { get; set; }

    }

What I have tried:

I'm trying to select all location C with related tables information in location A and B after that I need to select the selected user locations in table Contact (4th table) after that I need to remove all selected user table from selected locations but I found that cannot be applied to operands of type int and list of <int> in this line (a.Location3_ID != (from z in db.contacts where z.UserID == UserID select z.Location3_ID))


//Get Un-selected location by user ID

public List GetSelectedLocByUID (int UserID = 0)
        {
var data = (from LocC in db.locationsC
           join LocB in db.locationsB on LocC.Location2_ID equals LocB.Location2_ID
           join LocA in db.locationsA on LocB.Location1_ID equals LocA.Location1_ID
           select new CustmLocationC
                        {
                            Location1_ID = LocA.Location1_ID,
                            Location1 = LocA.Location1,
                            LocAIsActive = LocA.IsActive,
                            Location2_ID = LocB.Location2_ID,
                            Location2 = LocB.Location2,
                            LocBIsActive = LocB.IsActive,
                            Location3_ID = LocC.Location3_ID,
                            Location3 = LocC.Location3,
                            Location3_Descrip = LocC.Location3_Descrip,
                            LocAandB = LocA.Location1 + "-" + LocB.Location2,
                            IsActive = LocC.IsActive
 }).Where(a => a.LocAIsActive == true && a.LocBIsActive == true && a.IsActive == true 
                        
&& a.Location3_ID != (from z in db.contacts where z.UserID == UserID select z.Location3_ID)

).OrderBy(a => a.Location1).ToList();

            return (data);
            }