Если вопрос про WPF, то
Способ 1 (для людей) - Переопределяем Template для ComboBox как нам надо, вариантов много, в примере ниже с помощью триггера отображаем TextBlock (SelTextBlock) с текстом "Выберите из списка" если SelectedItem == null:
<Style TargetType="ComboBoxItem">
...
</Style>
<Style x:Key="ComboBoxToggleButtonStyle" TargetType="ToggleButton">
...
</Style>
<Style x:Key="ComboBoxEditableTextBoxStyle" TargetType="TextBox">
...
</Style>
<Style TargetType="ComboBox">
<Setter Property="SnapsToDevicePixels" Value="True" />
<Setter Property="OverridesDefaultStyle" Value="True" />
...
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ComboBox">
<Grid>
<ToggleButton x:Name="btn"
ClickMode="Press"
Focusable="false"
IsChecked="{Binding IsDropDownOpen,
Mode=TwoWay,
RelativeSource={RelativeSource TemplatedParent}}"
Style="{StaticResource ComboBoxToggleButtonStyle}">
<Grid>
<ContentPresenter x:Name="ContentSite"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}" />
<TextBlock x:Name="SelTextBlock"
Text="Выберите из списка"
Visibility="Hidden" />
<TextBox x:Name="PART_EditableTextBox"
IsReadOnly="{TemplateBinding IsReadOnly}"
Style="{StaticResource ComboBoxEditableTextBoxStyle}"
Visibility="Hidden" />
</Grid>
</ToggleButton>
<Popup x:Name="PART_Popup"
AllowsTransparency="True"
Focusable="False"
IsOpen="{Binding IsDropDownOpen,
RelativeSource={RelativeSource TemplatedParent}}"
Placement="Bottom"
PopupAnimation="Fade"
SnapsToDevicePixels="True">
<Border MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}"
Background="White"
BorderBrush="Lavender"
BorderThickness="1,0,1,1"
Padding="0,0,0,4">
<ScrollViewer CanContentScroll="True"
HorizontalScrollBarVisibility="Auto"
VerticalScrollBarVisibility="Auto">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Border>
</Popup>
</Grid>
<ControlTemplate.Triggers>
<Trigger Property="IsEditable" Value="True">
<Setter TargetName="btn" Property="Tag" Value="Editable" />
<Setter TargetName="PART_EditableTextBox" Property="Visibility" Value="Visible" />
<Setter TargetName="ContentSite" Property="Visibility" Value="Hidden" />
<Setter TargetName="SelTextBlock" Property="Visibility" Value="Hidden" />
</Trigger>
<Trigger Property="SelectedItem" Value="{x:Null}">
<Setter TargetName="SelTextBlock" Property="Visibility" Value="Visible" />
</Trigger>
...
</ControlTemplate.Triggers>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
Способ 2 (для извращенцов) - Накладываем TextBlock с текстом "Выберите из списка" поверх ComboBox и с помощью Binding и кастомного конвертера ObjectToVisibilityConverter отображаем его если SelectedItem == null:
...
<Window.Resources>
<local:ObjectToVisibilityConverter x:Key="ObjectToVisibilityConverter" />
</Window.Resources>
...
<Grid>
<TextBlock Margin="4,0,0,0"
HorizontalAlignment="Left"
VerticalAlignment="Center"
Panel.ZIndex="1000"
Text="Выберите из списка"
Visibility="{Binding ElementName=CmbBox,
Path=SelectedItem,
Converter={StaticResource ObjectToVisibilityConverter}}" />
<ComboBox x:Name="CmbBox" Width="200" />
</Grid>
public class ObjectToVisibilityConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
return value == null ? Visibility.Visible : Visibility.Hidden;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
