Both the Textblock and Texbox must be hidden or visible when you snoth one of the combobox's dire options. both the textblock and texbox must be hidden or visible when you snoth one of the combobox's dire options. example of textblock and texbox to hide and make visible txttanqueel(textbox) and textbLock(txtelectric)
ComboBox x:Name="CbTipocombustibles" SelectionChanged="CbTipoCombustibles_SelectionChanged" Width="200" Margin="0,12,0,0" >
<ComboBoxItem IsSelected="true">
<TextBlock x:Name="textgasolina" x:Uid="gasolina" FontSize="{StaticResource SmallFontSize}"/>
</ComboBoxItem>
<ComboBoxItem >
<TextBlock x:Name="textdiesel" x:Uid="diesel" FontSize="{StaticResource SmallFontSize}"/>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock x:Name="textlectrico" x:Uid="Electrico" FontSize="{StaticResource SmallFontSize}"/>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock x:Name="textgasoelectrico" x:Uid="gasolina_electrico" FontSize="{StaticResource SmallFontSize}"/>
</ComboBoxItem>
<ComboBoxItem>
<TextBlock x:Name="textdielectrico" x:Uid="diesel_electrico" FontSize="{StaticResource SmallFontSize}"/>
</ComboBoxItem>
</ComboBox>
xaml
TextBlock x:Name="txtcombustible" x:Uid="CapacidadCombustible" Margin="0,21,0,0" Grid.Row="26" Style="{StaticResource TextTextStyle}"/>
<TextBox x:Name="txttanquec" Text="" Width="120" Margin="-200,23,0,0" FontSize="{StaticResource SmallFontSize}"/>
<TextBlock x:Name="txtelectrico" x:Uid="Capacidadelectrico" Margin="0,25,0,0" Style="{StaticResource TextTextStyle}" />
<TextBox x:Name="txttanqueel" Text="" Width="120" Margin="-200,27,0,0" FontSize="{StaticResource SmallFontSize}" />