title: error:The ObjectContext instance has been disposed and can no longer be used for operations that require a connection
body: <p>i have a listbox and when i select an item from this listbox called as ListofKBrands1, i take this error message:</p>
  
  <blockquote>
    <p>The ObjectContext instance has been disposed and can no longer be used for operations that require a connection.</p>
  </blockquote>
  
  <p>In code-behind,place of this error:</p>
  
  <pre><code>if (co.Company != null)
  </code></pre>
  
  <p>my code:</p>
  
  <pre><code>private void ListofKBrands1_SelectionChanged(object sender, SelectionChangedEventArgs e)
      {
          RSPDbContext c = new RSPDbContext();
  
          if (ListofKBrands1.SelectedItem != null)
          {
              ListBoxItem item = ListofKBrands1.SelectedItem as ListBoxItem;
              KBrand co = item.Tag as KBrand;
  
              if (ListofKBrands1.SelectedItem != null)
                  txtNewKBrand.Text = co.Name;
              else
                  txtNewKBrand.Text = "";
  
              int count = 0;
              if (co.Company != null)
              {
                  foreach (string a in cbCompany.Items)
                  {
                      if (a == co.Company.Name)
                          cbCompany.SelectedIndex = count;
                      count++;
                  }
              }
              else
                  cbCompany.SelectedIndex = 0;
          }
      }
  </code></pre>
  
  <p>before error:</p>
  
  <p><img src="https://i.stack.imgur.com/Cz3gu.png" alt="enter image description here"></p>
  
  <p>my KBrand.cs:</p>
  
  <pre><code>public class KBrand {
      [Key]
      public int Id { get; set; }
      public String Name { get; set; }
      public virtual Company Company { get; set; }
  
      public override string ToString() {
          return Name;
      }
  }
  </code></pre>
  
  <p>company.cs:</p>
  
  <pre><code>public class Company
  {
      [Key]
      public int Id { get; set; }
      public String Name { get; set; }
  
      public override string ToString() {
          return Name;
      }
  }
  </code></pre>
  
  <p>if company of selected KBrand is null, this error does not appear. but if company of selected KBrand is not null, i take this error.how can i fix this error ? thanks in advance.</p>