WPF组合框绑定占位符项

Question

I'm trying to add a "fake" item to my source list binded to a combobox.

<Window ...
        DataContext="{Binding Source={x:Static local:Singleton.Instance}}">
    ...
        <ComboBox ItemsSource="{Binding MyList}">

--

public List<Object> MyList{ get; private set; }

I want to have a "Add New" as a combobox item that do not belong to MyList, since I need to have only proper objects inside it. If I try to add it programmatically an exception is raised since the source cannot be edit in such a way.

Answer 1

Add the following to your XAML

IsEditable="True"
Text="Add New"

Caution : If the user selects one of the bound values, they cannot "go back" and select "Add New" as it will no longer appear. Also, now that the control is editable, you will need to validate the content before you do any processing to avoid potential errors introduced by the user entering a bad value.

Answer 2

HappyNomad gives in https://stackoverflow.com/a/4053371/5188233 a xaml-only solution by modifying the Combobox.Template . In this proposal you don't need to manipulate your sourcelist with fake items and keep them clean.

<ComboBox ItemsSource="{Binding MyList}" ... >
    <ComboBox.Template>
        <ControlTemplate TargetType="ComboBox">
            <Grid>
                <ComboBox x:Name="cbItems" 
                    DataContext="{Binding RelativeSource={RelativeSource TemplatedParent}}" 
                    ItemsSource="{Binding ItemsSource, RelativeSource={RelativeSource TemplatedParent}}"
                    SelectedValue ="{Binding SelectedValue, RelativeSource={RelativeSource TemplatedParent}}" 
                    />
                <TextBlock x:Name="tbItem" Text="Add New" IsHitTestVisible="False" Visibility="Hidden"/>
            </Grid>
            <ControlTemplate.Triggers>
                <Trigger SourceName="cbItems" Property="SelectedItem" Value="{x:Null}">
                    <Setter TargetName="tbItem" Property="Visibility" Value="Visible"/>
                </Trigger>
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </ComboBox.Template>
</ComboBox>