Your problem was resolved.
To force check validation rules you can use (you can use comboBox name rather than "this" keyword):
To force check validation rules you can use (you can use comboBox name rather than "this" keyword):
this.TryUpdateSource( );<ComboBox x:Name="comboBox">
<ComboBox.Items>
<ComboBoxItem Content="1" />
<ComboBoxItem Content="2" />
<ComboBoxItem Content="3" />
</ComboBox.Items>
<ComboBox.SelectedItem>
<Binding Path="MyComboFiled">
<Binding.ValidationRules>
<ValidationRules:RequiredValidationRule/>
</Binding.ValidationRules>
</Binding>
</ComboBox.SelectedItem>
</ComboBox>