window resizing on expanding a control

Question

derived from the solution of my original problem here

i have now the following xaml:

<Window x:Class="MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="300" Width="450" SizeToContent="WidthAndHeight">
    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition Height="*" />
            <RowDefinition Height="auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
            <ColumnDefinition Width="auto" />
        </Grid.ColumnDefinitions>
        <TabControl Grid.Column="0" MinWidth="200" Grid.Row="0" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" />       
        <Expander Grid.Column="1" Grid.Row="0" ExpandDirection="Right">
            <StackPanel x:Name="cont">
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
                <Label>testtesttesttesttest</Label>
            </StackPanel>
        </Expander>
    </Grid>
</Window>

i want to extend the window width everytime i expand the expander control to the right. when i run the above quoted xaml and expand the expander control after the build, everything works like expected, but as soon as i manually resize the window, the content of the expander control will expand into the existing area to the left.

how can i change this behaviour, so that the window will extend its width to the right side and the content will end up in that new area?

Answer 1

This isn't something that is built into WPF, so you'd have to write some custom code to handle it. The SizeToContent works until the end-user resizes the window, then the Window size is fixed.

You can use something like this to accomplish what you want:

public partial class MainWindow  {
    public MainWindow() {
        InitializeComponent();
    }

    protected override void OnSourceInitialized(EventArgs e) {
        base.OnSourceInitialized(e);

        IntPtr handle = new WindowInteropHelper(this).Handle;
        HwndSource.FromHwnd(handle).AddHook(new HwndSourceHook(this.WindowProc));
    }

    private const int WM_SIZING = 0x0214;
    private const int WM_EXITSIZEMOVE = 0x0232;
    private IntPtr WindowProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled) {
        switch (msg) {
            case WM_SIZING:
                this.firstColumn.ClearValue(ColumnDefinition.MinWidthProperty);
                break;
            case WM_EXITSIZEMOVE:
                this.firstColumn.MinWidth = this.firstColumn.ActualWidth;
                this.SizeToContent = System.Windows.SizeToContent.WidthAndHeight;
                break;
        }

        return IntPtr.Zero;
    }

}

Then you'd need to give your first ColumnDefinition the name "firstColumn", like so:

<ColumnDefinition x:Name="firstColumn" Width="*" />

So effectively, it uses SizeToContent like you have. If you resize the window, then it ensures the first column's minimum size keeps the size fixed and toggles the SizeToContent back on.

EDIT:

Noticed you used the VB.NET tag, so here is the VB.NET version:

Public Partial Class MainWindow
    Public Sub New()
        InitializeComponent()
    End Sub

    Protected Overrides Sub OnSourceInitialized(e As EventArgs)
        MyBase.OnSourceInitialized(e)

        Dim handle As IntPtr = New WindowInteropHelper(Me).Handle
        HwndSource.FromHwnd(handle).AddHook(New HwndSourceHook(AddressOf Me.WindowProc))
    End Sub

    Private Const WM_SIZING As Integer = &H214
    Private Const WM_EXITSIZEMOVE As Integer = &H232
    Private Function WindowProc(hwnd As IntPtr, msg As Integer, wParam As IntPtr, lParam As IntPtr, ByRef handled As Boolean) As IntPtr
        Select Case msg
            Case WM_SIZING
                Me.firstColumn.ClearValue(ColumnDefinition.MinWidthProperty)
                Exit Select
            Case WM_EXITSIZEMOVE
                Me.firstColumn.MinWidth = Me.firstColumn.ActualWidth
                Me.SizeToContent = System.Windows.SizeToContent.WidthAndHeight
                Exit Select
        End Select

        Return IntPtr.Zero
    End Function

End Class