如何绑定一个DataTable到一个WPF编辑ComboBox：selectedItem属性显示System.Data.DataRowView

结果
我约束的数据表到一个ComboBox，并确定在itemTemplate.i一个DataTemplate可以看到组合框下拉列表中所需的值，我在则selectedItem看到的是 System.Data.DataRowView 这里是我的codeS：

I bound a datatable to a combobox and defined a dataTemplate in the itemTemplate.i can see desired values in the combobox dropdown list,what i see in the selectedItem is System.Data.DataRowView here are my codes:

 <ComboBox Margin="128,139,123,0" Name="cmbEmail" Height="23" VerticalAlignment="Top" TabIndex="1" ToolTip="enter the email you signed up with here" IsEditable="True" IsSynchronizedWithCurrentItem="True" ItemsSource="{Binding}">

            <ComboBox.ItemTemplate>
            <DataTemplate>
                <StackPanel>
                      <TextBlock Text="{Binding Path=username}"/>
                </StackPanel>

            </DataTemplate>
        </ComboBox.ItemTemplate>

在code的背后，是这样：

The code behind is so :

 if (con != null)
 {
    con.Open();
    //users table has columns id | username | pass
    SQLiteCommand cmd = new SQLiteCommand("select * from users", con);
    SQLiteDataAdapter da = new SQLiteDataAdapter(cmd);
    userdt = new DataTable("users");
    da.Fill(userdt);
    cmbEmail.DataContext = userdt;

 }

我一直在寻找类似SelectedValueTemplate或SelectedItemTemplate做同样类型的数据模板，但我没有发现。

I've been looking for something like SelectedValueTemplate or SelectedItemTemplate to do the same kind of data templating but i found none.

我就想问一下，如果我做错了什么，或者它是一个已知的问题为组合框绑定？结果
如果事情是错误的，我code请点我到正确的方向。结果
 感谢您阅读本

I'll like to ask if i'm doing something wrong or it's a known issue for combobox binding?
if something is wrong in my code please point me to the right direction.
thanks for reading this