How to set SelecteItem of ComboBox in WPF .Net4.0

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WPF

.NET4

Maybe you know that ComboBox has SelectedItem.

This ComboBox have this itemsSource(1, 2, 3, 4).

And I want to set anything(1 or 2).

If I use SelectedIndex, it is easy. [SelectedIndex = 1;] <= well done.

But I want to Bind this value using SelectedItem.

I already test like this.

<ComboBox x:Name="cbOperatingType" ItemsSource="{Binding mUserGroupList}" SelectedItem="{Binding mDGSelectedItem.m_Operator_Type, Mode=TwoWay}" />

Binding ItemsSource is well done. But Binding SelectedItem is not operating.

When Program is run, ComboBox's TextBlock is empty

mUserGroupList & mDGSelectedItem.m_Operator_Type is also property(have get, set)

Please tell me answer for this problem.
Please suggest solution for this problem.