колобок повесился :))))))))))

Bank/BankDataModels/HelperInterfaces/IId.cs

@@ -6,7 +6,9 @@ using System.Threading.Tasks;
 
 namespace BankDataModels.HelperInterfaces
 {
-    internal class Id
+    public interface IId
     {
+        int Id { get; }
     }
 }
+

Bank/BankDataModels/ICostModel.cs

@@ -3,10 +3,16 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using BankDataModels.HelperInterfaces;
+using BankDataModels.ProxyModels;
 
 namespace BankDataModels
 {
-    internal class ICostModel
+    public interface ICostModel : IId
     {
+        int EmployeeId { get; }
+        string NameOfCost { get; }
+        double Price { get; }
     }
 }
+

Bank/BankDataModels/IEmployeeModel.cs

@@ -3,10 +3,10 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
-
 namespace BankDataModels
 {
-    internal class IEmployeeModel
+    public interface IEmployeeModel : IClientModel
     {
+        string Post { get; }
     }
 }

Bank/BankDataModels/IOperationModel.cs

@@ -3,10 +3,15 @@ using System.Collections.Generic;
 using System.Linq;
 using System.Text;
 using System.Threading.Tasks;
+using BankDataModels.HelperInterfaces;
 
 namespace BankDataModels
 {
-    internal class IOperationModel
+    public interface IOperationModel : IId
     {
+        int EmployeeId { get; }
+        string Model { get; }
+        string Mark { get; }
+        double Price { get; }
     }
 }