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How do I add more options in a generic Combo Box

I want to have a combobox like this:
But in code there is no way of doing it. I'm using MVVM pattern. So I have a view:
<ComboBox Style="{StaticResource ComboStyle}" ItemsSource="{Binding ResultObjects}" SelectedItem="{Binding SelectedObject,Mode=TwoWay}" />
and ViewModel:
public IEnumerable<DateTime> ResultObjects { get;set; }
public DateTime SelectedObject{ get;set; }
The fact is that the -All- and -custom- are not DateTime. And it can't be in added to this list.
I remember that in MVC we had a "Dropdown helper".
What can I do here in MVVM?

You could bind to an IEnumerable<KeyValuePair<DateTime?, string>> where the key represent the actual value and the value represents a custom string representation of that value:
public IEnumerable<KeyValuePair<DateTime?, string>> ResultObjects { get; set; }
public DateTime? SelectedObject { get; set; }
...
ResultObjects = new List<KeyValuePair<DateTime?, string>>()
{
new KeyValuePair<DateTime?, string>(null, "All"),
new KeyValuePair<DateTime?, string>(new DateTime(2018,04,17), new DateTime(2018,04,17).ToString("yyyy/MM/dd")),
new KeyValuePair<DateTime?, string>(new DateTime(2018,04,17), new DateTime(2018,04,17).ToString("yyyy/MM/dd # HH:mm:ss")),
new KeyValuePair<DateTime?, string>(new DateTime(2018,04,17), "Custom...")
};
...
XAML:
<ComboBox
ItemsSource="{Binding ResultObjects}"
SelectedValue="{Binding SelectedObject}"
DisplayMemberPath="Value"
SelectedValuePath="Key"/>
Obviosuly you cannot return anything but actual DateTime values from an IEnumerable<DateTime> to you should change the type of your source collection if you want to be able to represent other types of values as well.

The way I'd handle this is to define an ObservableCollection<object>.
When wpf comes across an object presented to the ui it first looks to see if it's got a template defined for the thing. If it hasn't, it will use ToString on the object. You can rely on that for simple cases and override .ToString() on any object you want to use.
If you want more sophisticated display than just a string then you can define a datatemplate which targets your objects based on their datatype.
One trick which can be handy.
You can even inherit from one base object and define a template for that, then more specific ones for sub types. A sub type inheriting from your base object will be dealt with by your "default".
Eg.
I have a map editor. The user is selecting from different terrains he's going to draw. I want to display different stuff for these. I have a BaseTerrainVM and then I inherit from that for river, contour, woods etc.
Here's a subset of the markup I use to template the items in a listbox:
<ListBox.Resources>
<DataTemplate DataType="{x:Type local:BaseTerrainVM}">
<Grid>
<TextBlock Text="{Binding DisplayType}" HorizontalAlignment="Left"
VerticalAlignment="Center"/>
<TextBlock Text="{Binding ID}"
HorizontalAlignment="Right"
VerticalAlignment="Center"/>
</Grid>
</DataTemplate>
<DataTemplate DataType="{x:Type local:ContourVM}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*"/>
<ColumnDefinition Width="3*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<TextBlock Text="{Binding DisplayType}"
Grid.Column="0"
VerticalAlignment="Center"/>
<StackPanel Orientation="Horizontal"
Grid.Column="1"
TextElement.FontFamily="Century Gothic"
TextElement.FontSize="{DynamicResource LargeFont}"
TextElement.FontWeight="Normal"
>
<TextBox MinWidth="50"
Text="{Binding Height}"
GotKeyboardFocus="TextBox_GotKeyboardFocus"
>
<i:Interaction.Behaviors>
<ui:TextBoxDecimalRangeBehaviour MaxDecimals="0"
MaxInteger="3"
Minimum="{StaticResource Zero}"
Maximum="{StaticResource TwoFiveFive}" />
<ui:SelectAllTextBoxBehavior/>
</i:Interaction.Behaviors>
</TextBox>
<TextBlock Text="units"
Margin="2,0,0,0"
ToolTip="Elevation is represented by a number 0-255 which is multiplied by a factor to give metres"
/>
</StackPanel>
<TextBlock Text="{Binding ID}"
Grid.Column="2"
VerticalAlignment="Center"
/>
</Grid>
</DataTemplate>
<DataTemplate DataType="{x:Type local:RiverVM}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="2*"/>
<ColumnDefinition Width="3*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<TextBlock Text="{Binding DisplayType}"
Grid.Column="0"
VerticalAlignment="Center"/>
<StackPanel Orientation="Horizontal"
Grid.Column="1"
TextElement.FontFamily="Century Gothic"
TextElement.FontSize="{DynamicResource LargeFont}"
TextElement.FontWeight="Normal"
>
<Button
Command="{Binding PressureFromStartCommand}"
Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}"
Height="20"
ToolTip="Widen from Start of stroke"
>
<Path Data="{StaticResource FlowRight}"
Stretch="Uniform"
Fill="LightBlue"
Stroke="DodgerBlue"
StrokeThickness="1"
/>
</Button>
<Button
Command="{Binding PressureConstantCommand}"
Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}"
Height="20"
ToolTip="Constant width for river"
>
<Rectangle
Width="18"
Height="6"
Fill="LightBlue"
Stroke="DodgerBlue"
StrokeThickness="1"
/>
</Button>
<Button
Command="{Binding PressureFromEndCommand}"
Style="{StaticResource {x:Static ToolBar.ButtonStyleKey}}"
Height="20"
ToolTip="Widen from End of stroke"
>
<Path Data="{StaticResource FlowRight}"
Stretch="Uniform"
Fill="LightBlue"
Stroke="DodgerBlue"
StrokeThickness="1"
RenderTransformOrigin="0.5,0.5"
>
<Path.RenderTransform>
<ScaleTransform ScaleX="-1" ScaleY="1" />
</Path.RenderTransform>
</Path>
</Button>
</StackPanel>
<TextBlock Text="{Binding ID}"
Grid.Column="2"
VerticalAlignment="Center"
/>
</Grid>
</DataTemplate>
<DataTemplate DataType="{x:Type local:BoundaryVM}">
<Grid>
( This may amuse Muds. )
My Terrains that presents these is actually a composite collection. In my case this is because I translate all the water on the map into one object so there's no border between one road and the next or a lake and the river flowing into it. I need to present the two representations of water but switch between them.

You need to use CompositeCollection, that can handle more than one collection/s and other objects.
Try something like this
<ComboBox>
<ComboBox.ItemsSource>
<CompositeCollection>
<ComboBoxItem Content="--All--" />
<CollectionContainer Collection="{Binding Source=ResultObjects}" />
<ComboBoxItem Content="--Custom--" />
</CompositeCollection>
</ComboBox.ItemsSource>
</ComboBox>

How to set a secondary control MaxWidth property to the same as another in a different container

I have this StackPanel in one grid on my form:
<StackPanel x:Name="panelWeelyBibleReading" Grid.Row="2" Grid.Column="0" Margin="2">
<Label>Bible Reading for Week:</Label>
<TextBox x:Name="textWeeklyBibleReading">PSALMS 60-68</TextBox>
</StackPanel>
<StackPanel Grid.Row="3" Grid.Column="0" Margin="2">
<Label>Opening Song:</Label>
<ComboBox x:Name="comboSongOpen" ItemsSource="{StaticResource SongTitles}">
<ComboBox.ItemTemplate>
<DataTemplate DataType="local:SongTitleItem">
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Number, StringFormat='000 - '}" />
<TextBlock Text="{Binding Title}" />
</StackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
<ComboBox.MaxWidth>
<Binding Path="ActualWidth"
ElementName="textWeeklyBibleReading"/>
</ComboBox.MaxWidth>
</ComboBox>
</StackPanel>
Further down the form in a different grid I have this :
<StackPanel Grid.Row="3" Grid.Column="0" Margin="2">
<Label>Closing Song:</Label>
<ComboBox x:Name="comboSongEnd" ItemsSource="{StaticResource SongTitles}">
<ComboBox.ItemTemplate>
<DataTemplate DataType="local:SongTitleItem">
<StackPanel Orientation="Horizontal">
<TextBlock Text="{Binding Number, StringFormat='000 - '}" />
<TextBlock Text="{Binding Title}" />
</StackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
<ComboBox.MaxWidth>
<Binding Path="ActualWidth"
ElementName="panelWeeklyBibleReading.textWeeklyBibleReading"/>
</ComboBox.MaxWidth>
</ComboBox>
</StackPanel>
Initially I tried using the same "textWeeklyBibleReading" for the MaxWidth in the second combo. But it doesn't work. No warnings. Just doesn't work. So I thought about applying a name to the previous StackPanel in teh hopes I could use it as some kind of access point. But it still doesn't work.
So my problem is that I need to set the MaxWidth on the second combo, which is in a different grid, but using the value that was calculated on the first grid combo. Am I making sense?
I would show the complete form markup if it helps.

You should edit to:
<ComboBox.MaxWidth>
<Binding Path="Width"
ElementName="textWeeklyBibleReading"/>
</ComboBox.MaxWidth>
<Binding Path=Width, ElementName=textWeeklyBibleReading/>
cause there is no property such as ActualWidth, instead it should be Width. And there is no need to write container name panelWeeklyBibleReading, just write ElementName that you need.
Update:
<ComboBox.ItemTemplate>
<DataTemplate DataType="local:SongTitleItem">
<StackPanel Orientation="Horizontal" MaxWidth="{Binding Path=Width,
ElementName=textWeeklyBibleReading}">
<TextBlock Text="{Binding Number, StringFormat='000 - '}" />
<TextBlock Text="{Binding Title}" />
</StackPanel>
</DataTemplate>
</ComboBox.ItemTemplate>
You should set MaxWidth of StackPanel of ComboBox cause outer StackPanel``(<StackPanel Grid.Row="3" Grid.Column="0" Margin="2">) which contains your ComboBox automatically sets Width to the outer StackPanel to your ComboBox. So I suggest to bind Width of your ComboBox to its ItemTemplate. Cause ItemTemplate has Data/template, then it is better to bind Width of ComboBox to StackPanel located into your DataTemplate.

Formatting of text in a textblock in windows phone 7

I am have a text which is has bold, underline and italic html characters. For example
<b> hello<b> how are <i>you</i>. I am <u>fine</u>
I have to show it in formatted form in a textblock on WP7. I have a listbox like this
<ListBox x:Name="LBayaDetail" Loaded="LBayaDetail_Loaded" Margin="6,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<ListBox.ItemContainerStyle>
<Style TargetType="ListBoxItem">
<Setter Property="HorizontalContentAlignment" Value="Stretch" />
</Style>
</ListBox.ItemContainerStyle>
<ListBox.ItemTemplate>
<DataTemplate>
<Grid x:Name="ayaContent" Margin="0,6,0,0" Hold="ayaContent_Hold" Tap="ayaContent_Tap" Loaded="ayaContent_Loaded" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="Auto"/>
<RowDefinition Height="6"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="80" />
<ColumnDefinition Width="6" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid Background="#FFC5AC88" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<TextBlock x:Name="ayaIndex" Text="{Binding aya}" FontSize="36" Margin="0" FontWeight="Bold" HorizontalAlignment="Center" />
<StackPanel VerticalAlignment="Bottom" HorizontalAlignment="Center">
<Image Source="{Binding BookmarkImage}" HorizontalAlignment="Center" Width="48" Height="48" Margin="0,0,0,12" />
<Image Source="{Binding NoteImage}" HorizontalAlignment="Center" Width="48" Height="48" Margin="0,0,0,12" />
<Image Source="{Binding TagImage}" HorizontalAlignment="Center" Width="48" Height="48" Margin="0,0,0,12" />
</StackPanel>
</Grid>
<Grid Grid.Row="1" Background="#FFC5AC88" x:Name="Media" Tap="Media_Tap" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<Image Source="/Images/Media-Play(1).png" Width="30" Height="30" HorizontalAlignment="Center" Margin="12,0,0,0" VerticalAlignment="Top" />
</Grid>
<!--ini pak dimana tempat untuk ayat dan translasi-->
<Grid Grid.Column="2" Background="#FFAC9574" Margin="6,0,0,0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<TextBlock x:Name="aya" TextWrapping="Wrap" Text="{Binding text}" HorizontalAlignment="Right" FontFamily="/Fonts/me_quran2.ttf#me_quran2" FontSize="{Binding FontSizeAya}" Foreground="Black" Margin="24,0,12,-12" TextAlignment="Right" Visibility="{Binding visibility1}" />
</Grid>
<Grid Grid.Column="2" Grid.Row="1" Margin="6,0,0,0" Background="#FFAC9574" HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
<StackPanel>
<TextBlock Visibility="{Binding visibility2}" x:Name="translation" Text="{Binding translation}" TextWrapping="Wrap" HorizontalAlignment="Right" FontFamily="/Fonts/ARIALUNI.TTF#Arial Unicode MS" FontSize="{Binding FontSizeTranslation}" Foreground="#FF5D2A07" Margin="12,6,6,0" />
<TextBlock Visibility="{Binding visibility3}" x:Name="translation2" Text="{Binding translation2}" TextWrapping="Wrap" HorizontalAlignment="Right" FontFamily="/Fonts/ARIALUNI.TTF#Arial Unicode MS" FontSize="{Binding FontSizeTranslation}" Foreground="DarkGreen" Margin="12,20,6,0" />
</StackPanel>
</Grid>
<!-- -->
</Grid>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>
I fetch from database like
App.Listxyz = (Application.Current as App).db.SelectList<Aya>(strSelect);
And assign it to Listbox like this
LBayaDetail.ItemsSource = App.ListAyaBySurah;
And shows the text as it is and do not format it which is obvious. I searched for it and I was able to format individual textblock by using "RUN" but I am unable to do it in listbox.
I also tried to use HTMLTextBlock but it also doesn't format the text and shows it like this
Hi
How
Are
You
Any help will be much appreciated that how do I format a textblock with different text decorations.
Thanks

You should place a grid and a stackpanel inside the listbox. Something like the following
<Grid>
<StackPanel Grid.Column="1">
<TextBlock Padding="0,5,0,2" TextWrapping="Wrap">
<Run Text="{Binding test}" FontWeight="Bold" /> <Run Text="{Binding test2}" />
<LineBreak/>
<Run Text="{Binding test3}" />
</TextBlock>
</StackPanel>
</Grid>

One way to do that is:
public class FormattedText
{
public string Text { get; set; }
public bool IsBold { get; set; }
public bool IsItalic { get; set; }
public bool IsUnderlined { get; set; }
}
Have a method that converts the HTML you stored in your db to a list of the class you have above
example:
From this:
<b> hello<b> how are <i>you</i>. I am <u>fine</u>
to this:
First element:
Text = "hello"
IsBold = true
skip what not needed since bool default value is false
Second element
Text =" how are"
skip what not needed since bool default value is false
Third item
Text ="you"
IsItalic=true;
skip what not needed since bool default value is false
and so on....
and then have another method that from that list creates a list of Runs to be added to your TextBlock or
maybe create a custom TextBlock witch take the List<FormattedText> from DataContext and process it by adding the Run elements to self

How to freeze a column in Grid and make it always shown in client area of a WPF user control

I searched in stackoverflow, and there is a property FrozenColumnCount for DataGrid, but Grid control doesn't have that property.
We have a Grid with 2 columns. Now, we need to freeze the last column in a grid, and make it always shown in the client area of the user control. otherwise, if the data (such as text) in first column is too long, customer has to use mouse to drag the horizon scroll bar to show the 2nd column.
We want customer can always see the data in 2nd column, so I wonder if we could freeze the specified column.
Update 1: I paste my code for the tree view control.
<HierarchicalDataTemplate x:Uid="HierarchicalDataTemplate_1" x:Key="MyPaletteMyTestTreeCell"
ItemsSource="{Binding Converter={StaticResource MyTestDataAccessor}}"
ItemTemplateSelector="{StaticResource MyTestDataAccessor}">
<ContentControl x:Uid="ContentControl_1" MouseDoubleClick="Item_MouseDoubleClick" ContextMenu="{StaticResource TreeListViewItemContextMenu}" MouseRightButtonDown="MRClick" Focusable="False">
<Grid x:Uid="Grid_2" Background="Transparent">
<Grid.ColumnDefinitions>
<ColumnDefinition x:Uid="ColumnDefinition_3" />
<ColumnDefinition x:Uid="ColumnDefinition_2" Width="Auto" />
</Grid.ColumnDefinitions>
<Grid x:Uid="Grid_3" Grid.Column="0">
<StackPanel x:Uid="StackPanel_3" HorizontalAlignment="Left" Orientation="Horizontal">
<pc:ThemedImage x:Uid="Image_4"
LightSource="{Binding CategoryId, Converter={StaticResource LightMyTestIconConverter}, Mode=OneWay}"
DarkSource="{Binding CategoryId, Converter={StaticResource DarkMyTestIconConverter}, Mode=OneWay}"
Width="16" Height="16"
Margin="0,1,0,1"
VerticalAlignment="Center"/>
<TextBlock x:Uid="TextBlock_13" Text="{Binding Name}" VerticalAlignment="Center" Margin="3,0,0,1" TextTrimming="CharacterEllipsis" TextWrapping="NoWrap"/>
</StackPanel>
</Grid>
<Grid x:Uid="Grid_4" Grid.Column="1">
<CheckBox x:Uid="CheckBox_3" HorizontalAlignment="Right" Click="CheckBox_Click" Grid.Column="1" Style="{StaticResource MyPaletteMyTestVisibilityStyle}" ToolTip="On/Off" Focusable="False">
<CheckBox.IsChecked>
<Binding x:Uid="Binding_1" Converter="{StaticResource MyTestDataAccessor}" Path="Visibility" Mode="OneWay">
<Binding.ConverterParameter>
<FrameworkElement x:Uid="FrameworkElement_1" DataContext="{TemplateBinding DataContext}" Tag="Visibility"/>
</Binding.ConverterParameter>
</Binding>
</CheckBox.IsChecked>
<CheckBox.IsEnabled>
<Binding x:Uid="Binding_2" Converter="{StaticResource MyTestDataAccessor}" Path="Visibility" Mode="OneWay">
<Binding.ConverterParameter>
<FrameworkElement x:Uid="FrameworkElement_2" DataContext="{TemplateBinding DataContext}" Tag="Enabled"/>
</Binding.ConverterParameter>
</Binding>
</CheckBox.IsEnabled>
</CheckBox>
</Grid>
</Grid>
</ContentControl>
</HierarchicalDataTemplate>
SOLUTION:
I found the cause. I have specified a horizon scroll bar for it.
So, when I removed it and use the code above, it can work as expected.
thanks a lot.

Clearly, you're not thinking about this correctly. You just need to put the column that you want to 'freeze' into a column of an outer Grid. Try something this:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition Width="Auto" />
</Grid.ColumnDefinitions>
<Grid Grid.Column="0">
<!-- Your other controls -->
</Grid>
<Grid Grid.Column="1" Name="FrozenColumn">
<!-- Your frozen controls -->
</Grid>
</Grid>

How to know why my WPF Window is so big?

I have the following Window created in WPF. This is how it's shown:
I have no idea why the screen is shown so big and I don't know how to debug why is it getting so wide.
This is the code related to the problem:
<Window x:Class="Picis.CpCustomize.CustomizeControls.Dialogs.EditIntegerWindow"
MinWidth="350"
SizeToContent="Height"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
ResizeMode="CanResize"
ShowInTaskbar="False"
WindowStartupLocation="CenterScreen"
WindowState="Normal"
Loaded="OnWindowLoaded">
<!-- Main frame -->
<Grid>
<!-- Layout -->
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="Auto"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<TextBlock VerticalAlignment="Center" MinWidth="100" TextAlignment="Right">
<TextBlock.Text>
<Binding Converter="{StaticResource Localizer}" ConverterParameter="General.Value" />
</TextBlock.Text>
</TextBlock>
<TextBox x:Name="valueTextBox" Grid.Column="1" Margin="5" Text="{Binding Path=Value}"/>
<TextBlock VerticalAlignment="Center" Grid.Row="1" Grid.Column="0" HorizontalAlignment="Right" Visibility="{Binding Path=ShowComments, Converter={StaticResource VisiConv}, ConverterParameter=Collapse}" MinWidth="100" TextAlignment="Right">
<TextBlock.Text>
<Binding Converter="{StaticResource Localizer}" ConverterParameter="EditParamDlg.Comment" />
</TextBlock.Text>
</TextBlock>
<TextBox x:Name="commentTextBox" Grid.Row="1" Grid.Column="1" Margin="5" Visibility="{Binding Path=ShowComments, Converter={StaticResource VisiConv}, ConverterParameter=Collapse}" Text="{Binding Path=Comment}"/>
<CheckBox Grid.Row="2" Grid.Column="1" Margin="5" x:Name="isDeletedCheckBox" Visibility="{Binding Path=ShowIsDeleted, Converter={StaticResource VisiConv}, ConverterParameter=Collapse}"
IsChecked="{Binding Path=IsDeleted}">
<CheckBox.Content>
<Binding Converter="{StaticResource Localizer}" ConverterParameter="EditParamDlg.IsDeleted" />
</CheckBox.Content>
</CheckBox>
<UniformGrid Grid.Row="3" Grid.Column="1" Margin="5" HorizontalAlignment="Right" Columns="2">
<Button x:Name="okButton" Click="OnOk" IsDefault="True">
<Button.Content>
<Binding Converter="{StaticResource Localizer}" ConverterParameter="General.Ok" />
</Button.Content>
</Button>
<Button x:Name="cancelButton" Click="OnCancel" Margin="5,0,0,0" IsCancel="True">
<Button.Content>
<Binding Converter="{StaticResource Localizer}" ConverterParameter="General.Cancel" />
</Button.Content>
</Button>
</UniformGrid>
</Grid>
</Window>
My first question is how to debug this issue, the second one is what is happening in this specific scenario.
Thanks in advance.

This question is a duplicate of: Why are my WPF window sizes defaulting to be huge
And according to: Window Size when SizeToContent is not specified the default size when not specified is 60% of the width and 60% of the height of your primary screen.

You explicitly set the window to stretch in width:
SizeToContent="Height"
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
This says, "make the window as wide and tall as possible, but then resize to the content of the height"
If you remove that, and set Width and Height to "Auto" (or leave out), you'll likely get what you want. Try just removing all three of those lines (which will leave out alignment, can cause default Width/Height of "Auto" to be used.)

<Window HorizontalAlignment="Stretch"
that is the problem, I guess.

To me it seems this one:
HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
It stratches the window...

The alignments on a window should not really do anything as there is no container to reference, but you only tell the window to contract to the content in height, i would change that behavior to both dimensions:
SizeToContent="WidthAndHeight"
How to debug this sort of thing? Maybe train your brain to be able to parse XAML and do layout on the fly... i know, not very helpful.

Maybe this is filling existing space (like it is supposed to).
<ColumnDefinition Width="*"/>