title: ComboBox SelectedItem property won't change
body: <p>Hi I've got a problem with a comboBox.<br/>
  At the beginning the <code>documentTypeComboBox.SelectedItem</code> value is null, then the <code>GetDataRow()</code> method return DataRow which is not null (100% sure that it return not null value for my test)<br/>
  After all the value of the SelectedItem property is still null. No idea what is wrong. I'm 100% sure that the returned DataRow is assigned as comboBox DataSource row</p>
  
  <pre><code>    public void SetDefaultData()
      {
          documentTypeComboBox.SelectedItem = null;
  
          dataWystawieniaDate.Value = _Faktura.DataWystawienia;
          dataSprzedazyDate.Value = _Faktura.DataSprzedazy;
  
          warehouseComboBox.SelectedItem = Helpers.GetDataRow(_Dictionary.Magazines, _Faktura.Magnum);
          paymentMethodComboBox.SelectedItem = Helpers.GetDataRow(_Dictionary.SpPaymentMethodsData, _Faktura.IdSposobuPlatnosci);
          priceComboBox.SelectedItem = Helpers.GetDataRow(_Dictionary.PricesData, _Faktura.RodzajCeny);
          discountComboBox.SelectedItem = Helpers.GetDataRow(_Dictionary.RabatyData, _Faktura.RodzajRabatu);
          documentTypeComboBox.SelectedItem = Helpers.GetDataRow(_Dictionary.SpDocumentsCodes, _Faktura.Kod, documentTypeComboBox.DisplayMember);
  
          documentNumberTextBox.Text = _Faktura.Numer;
          kwotaBruttoTextBox.Text = _Faktura.PlnWartoscBrutto.ToString();
      }
  
  
      public static DataRow GetDataRow(DataTable dt, string value, string fieldName)
      {
          foreach (DataRow dr in dt.Rows)
          {
              if (dr[fieldName].ToString() == value)
                  return dr;
          }
          return null;
      }
  </code></pre>