Создан класс Contract. ComboBox'ы функционируют в данном коммите.

eCompanies/App.xaml.cs

@@ -19,7 +19,7 @@ namespace eCompanies
         {
             using (var db = new CompaniesDBContext())
             {
-                db.Companies.Add(new Companies { Name = "Unknown company", ContractStatus = (int) Status.NotReachedYet });
+                db.Companies.Add(c);
                 var count = db.SaveChanges();
             }
         }

eCompanies/Companies.cs

@@ -3,14 +3,16 @@ using System.Collections.Generic;
 
 namespace eCompanies
 {
-    public enum Status { NotReachedYet, Reached, Terminated }
     public partial class Companies
     {
         public Companies()
         {
             Users = new HashSet<Users>();
+            contract = new Contract(this);
         }
 
+
+        public Contract contract { get; set; }
         public int CompanyId { get; set; }
         public string Name { get; set; }
         public int ContractStatus { get; set; }  

eCompanies/CompaniesDBContext.cs

@@ -34,8 +34,10 @@ namespace eCompanies
 
                 entity.Property(e => e.ContractStatus)
                     .IsRequired();
-
+                
                 entity.Property(e => e.Name).IsRequired();
+
+                entity.Ignore("contract");
             });
 
             modelBuilder.Entity<Users>(entity =>

eCompanies/Contract.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace eCompanies
+{
+    public enum Status { NotReachedYet, Reached, Terminated }
+    public class Contract
+    {
+        private Companies c { get; set; }
+        public Contract (Companies c)
+        {
+            this.c = c;
+        }
+        public string ContractStatusString
+        {
+            get
+            {
+                switch (c.CompanyId)
+                {
+                    case 1: return "Заключен";
+                    case 2: return "Расторгнут";
+                    default: return "Еще не заключен";
+                }
+            }
+            set
+            {
+
+            }
+        }
+
+        public Status ContractStatusId
+        {
+            get { return (Status)c.ContractStatus; }
+            set { c.ContractStatus = (int)value; }
+        }
+    }
+}

eCompanies/MainWindow.xaml

@@ -5,6 +5,7 @@
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:local="clr-namespace:eCompanies"
         xmlns:core="clr-namespace:System;assembly=mscorlib"
+        xmlns:col="clr-namespace:System.Collections;assembly=mscorlib"
         mc:Ignorable="d"
         Title="MainWindow" Height="450" Width="800">
     <Window.Resources>
@@ -18,8 +19,11 @@
         <DataGrid x:Name="companyGrid" HorizontalAlignment="Left" Height="354" Margin="10,55,0,0" VerticalAlignment="Top" Width="772" ItemsSource="{Binding}" AutoGenerateColumns="False" MouseDoubleClick="companyGrid_MouseDoubleClick">
             <DataGrid.Columns>
                 <DataGridTextColumn Header="ID"  Binding="{Binding CompanyId}" IsReadOnly="True"/>
-                <DataGridTextColumn Header="Название компании" Binding="{Binding Name}"/>
-                <DataGridComboBoxColumn x:Name="companyGridBox" Header="Состояние контракта" SelectedItemBinding="{Binding ContractStatus}" ItemsSource="{Binding Source={StaticResource Status}}"/>
+                <DataGridTextColumn Header="Название компании" Binding="{Binding Name}" Width="*"/>
+                <DataGridComboBoxColumn x:Name="ContractComboBox"  Header="Состояние контракта" 
+                                        SelectedValueBinding="{Binding Path=contract.ContractStatusId}" 
+                                        ItemsSource="{Binding Source={StaticResource Status}}">
+                </DataGridComboBoxColumn>
             </DataGrid.Columns>
         </DataGrid>
     </Grid>

eCompanies/MainWindow.xaml.cs

@@ -35,10 +35,15 @@ namespace eCompanies
 
         private void companyGrid_MouseDoubleClick(object sender, MouseButtonEventArgs e)
         {
-            Companies company = (Companies)companyGrid.CurrentItem;
-            if (usersWindowInstance != null) usersWindowInstance.Close();
-            usersWindowInstance = new UsersWindow(company);
-            usersWindowInstance.Show();
+            try
+            {
+                Companies company = (Companies)companyGrid.CurrentItem;
+                if (usersWindowInstance != null) usersWindowInstance.Close();
+                usersWindowInstance = new UsersWindow(company);
+                usersWindowInstance.Show();
+            }
+            catch { }
         }
+       
     }
 }

eCompanies/eCompanies.csproj

@@ -139,6 +139,7 @@
       <SubType>Designer</SubType>
     </ApplicationDefinition>
     <Compile Include="CompaniesDBContext.cs" />
+    <Compile Include="Contract.cs" />
     <Compile Include="Users.cs" />
     <Compile Include="UsersWindow.xaml.cs">
       <DependentUpon>UsersWindow.xaml</DependentUpon>