Hehe. Are you using here-string with @" and "@ ?

Because the last "@ needs to have no indentation at all

And you should call .Trim() at the end.

Like so:

    $xaml=
    @"
    <Window xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Width="900" Height="720">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition />
            <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>

        <Grid.RowDefinitions>
            <RowDefinition Height="*"/>
            <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>

        <ListView x:Name="itemList" Grid.ColumnSpan="2">
            <ListView.View>
                <GridView>
                    <GridViewColumn Header="Item" DisplayMemberBinding="{Binding Name}" />
                    <GridViewColumn Header="Item" DisplayMemberBinding="{Binding Name.Length}" />

                    <GridViewColumn Header="Color" >
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <Rectangle Fill="{Binding Name}" Width="25" Height="25"/>
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                    <GridViewColumn Header="Selected" >
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <CheckBox IsChecked="{Binding RelativeSource={RelativeSource AncestorType=ListViewItem}, Path=IsSelected}" />
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                </GridView>
            </ListView.View>
        </ListView>

        <WrapPanel Grid.ColumnSpan="2" Grid.Row="1" HorizontalAlignment="Right">
            <Button Name="button_OK" Content="OK" Width="75" Margin="5"/>
            <Button Name="button_Cancel" Content="Cancel" Width="75" Margin="5"/>
        </WrapPanel>

        <StackPanel Grid.Column="2" HorizontalAlignment="Left">
            <Button Name="button_SelectAll" Content="Select all" Width="75" Margin="5"/>
            <Button Name="button_UnselectAll" Content="Unselect all" Width="75" Margin="5"/>
        </StackPanel>

    </Grid>
</Window>
"@
    $xaml.Trim()