WPF DataGrid CellTemplateSelector Item

Question

I have a grid bound to a collection of VMs. When using DataTemplateSelector for my DataGridTemplateColumn I'm getting the whole VM as a data item, how do I narrow it down to a specific property value (otherwise I have to create 'DataTemplateSelector' for each VM or use interfaces, with both are too cumbersome) ?

Saw Bind a property to DataTemplateSelector, but it looks like a nasty workaround.

Answer 1

You can use Expressions Trees in a DataTemplateSelector's derived class, that I called PropertyTemplateSelector. Here its code:

public abstract class PropertyTemplateSelector : DataTemplateSelector
{
    private Delegate getPropertyValue;
    private string propertyName;
    private Type itemType;

    public string PropertyName
    {
        get
        {
            return propertyName;
        }
        set
        {
            propertyName = value;
        }
    }

    public Type ItemType
    {
        get
        {
            return itemType;
        }
        set
        {
            itemType = value;
        }
    }

    public override DataTemplate SelectTemplate(object item, DependencyObject container)
    {
        if (ItemType.IsInstanceOfType(item))
        {
            if (getPropertyValue == null)
            {
                System.Linq.Expressions.ParameterExpression instanceParameter =
                    System.Linq.Expressions.Expression.Parameter(item.GetType(), "p");

                System.Linq.Expressions.MemberExpression currentExpression =
                    System.Linq.Expressions.Expression.PropertyOrField(instanceParameter, PropertyName);

                System.Linq.Expressions.LambdaExpression lambdaExpression =
                    System.Linq.Expressions.Expression.Lambda(currentExpression, instanceParameter);

                getPropertyValue = lambdaExpression.Compile();
            }

            return SelectTemplateImpl(getPropertyValue.DynamicInvoke(item), container);
        }

        return base.SelectTemplate(item, container);
    }

    protected abstract DataTemplate SelectTemplateImpl(object propertyValue, DependencyObject container);
}

You can extend this class with your own logic, just by implementing the SelectTemplateImpl method. As you can see the PropertyTemplateSelector narrows the item object down to a specific property value (which is passed to the SelectTemplateImpl method). For example I created a NameTemplateSelector in this way:

public class NameTemplateSelector : PropertyTemplateSelector
{
    protected override DataTemplate SelectTemplateImpl(object propertyValue, DependencyObject container)
    {
        string name = (string)propertyValue;

        if (name != null && name.StartsWith("A", StringComparison.OrdinalIgnoreCase))
        {
            return (DataTemplate)App.Current.MainWindow.FindResource("VipName");
        }

        return (DataTemplate)App.Current.MainWindow.FindResource("NormalName");
    }
}

The you can easly use these template selectors in your XAML

<Window.Resources>
    <DataTemplate x:Key="NormalName">
        <TextBlock Text="{Binding Mode=OneWay, Path=Name}" Margin="3" />
    </DataTemplate>
    <DataTemplate x:Key="VipName">
        <TextBlock Text="{Binding Mode=OneWay, Path=Name}" Margin="3" Foreground="Red" FontWeight="Bold" />
    </DataTemplate>

    <DataTemplate x:Key="NormalSurname">
        <TextBlock Text="{Binding Mode=OneWay, Path=Surname}" Margin="3" />
    </DataTemplate>
    <DataTemplate x:Key="VipSurname">
        <TextBlock Text="{Binding Mode=OneWay, Path=Surname}" Margin="3" Foreground="Green" FontStyle="Italic" />
    </DataTemplate>

    <local:NameTemplateSelector x:Key="NameTemplateSelector" PropertyName="Name" ItemType="{x:Type local:Person}" />
    <local:SurnameTemplateSelector x:Key="SurnameTemplateSelector" PropertyName="Surname" ItemType="{x:Type local:Person}" />
</Window.Resources>

<Grid>
    <DataGrid ItemsSource="{Binding Path=People, Mode=OneWay}" AutoGenerateColumns="False"
                CanUserAddRows="False">
        <DataGrid.Columns>
            <DataGridTemplateColumn Header="Name"
                                    CellTemplateSelector="{StaticResource NameTemplateSelector}" />
            <DataGridTemplateColumn Header="Surname"
                                    CellTemplateSelector="{StaticResource SurnameTemplateSelector}" />
        </DataGrid.Columns>
    </DataGrid>
</Grid>

I hope it can help you.