how do i get my datagrid to grow with the window and maintain margins?

Question

Everything about my layout will flow with the resizing of the main window. The problem I'm facing is that as you can see, the datagrid goes off the screen. If you maximize the window, the datagrid will resize with the window, but continue to go off the screen. How do I get it to maintain its margin of 20 with it's parent grid?

<Grid>
    <StackPanel Orientation="Vertical">
        <TextBox Height="170" Name="txtSQL" VerticalAlignment="Top" AcceptsReturn="True" TextWrapping="Wrap" Margin="20"/>
        <Button Content="Run!" Height="23" HorizontalAlignment="Left" Name="btnRun" VerticalAlignment="Top" Margin="20,0,0,0" Width="75" Click="btnRun_Click" />

        <Grid>
            <my:DataGrid Name="dgResults" VerticalAlignment="Top" Margin="20" />
        </Grid>
    </StackPanel>
</Grid>

UPDATE: Just to be more specific. The effect I'm trying to accomplish here is this:

When the window first loads, you are presented with a blank datagrid, so it's only about 15 pixels high. When you run the query, it will populate the datagrid by reassigning the itemssource. As of now, when you do that, if the data exceeds the window size, it will go off the bottom of the screen. I need it to only expand to the bottom of the window then enable the scrollbar. I can do this just by wrapping it in a scrollviewer im sure. However, when the window is resized, the datagrid needs to resize with it.

I'm wondering if the setup might have something to do with it. The form is actually a wpf page being displayed in a frame.

UPDATE:

<Page x:Class="Hold_Database___Prototype_1.Views.SQL"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      mc:Ignorable="d" 
      d:DesignHeight="304" d:DesignWidth="732"
    Title="SQL" xmlns:my="http://schemas.microsoft.com/wpf/2008/toolkit" AllowDrop="True">
        <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="23" />
            <RowDefinition Height="*"/>
        </Grid.RowDefinitions>
            <TextBox Height="170" Name="txtSQL" VerticalAlignment="Top" AcceptsReturn="True" TextWrapping="Wrap" Margin="20" Grid.Row="0"/>
            <Button Content="Run!" Height="23" HorizontalAlignment="Left" Name="btnRun" VerticalAlignment="Top" 
                    Margin="20,0,0,0" Width="75" Grid.Row="1" Click="btnRun_Click" />
        <DockPanel Grid.Row="2">
            <my:DataGrid Name="dgResults" Margin="20" />
        </DockPanel>
    </Grid>
</Page>

Answer 1

What is the dock panel for in this example?

Try putting the DataGrid directly in the cell with no stackpanel. If you are setting the button height then set set grid to auto.

Also, why give so much space to the text?

    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition Height="Auto" />
        <RowDefinition Height="*"/>
    </Grid.RowDefinitions>
        <TextBox Height="170" Name="txtSQL" VerticalAlignment="Top" AcceptsReturn="True" TextWrapping="Wrap" Margin="20" Grid.Row="0"/>
        <Button Content="Run!" Height="23" HorizontalAlignment="Left" Name="btnRun" VerticalAlignment="Top" 
                Margin="20,0,0,0" Width="75" Grid.Row="1" Click="btnRun_Click" />
        <my:DataGrid  Grid.Row="2" <my:DataGrid Name="dgResults" Margin="20" /> 

Then also set HorizontalAlignment, VerticleAlignment, HorizontalContentAlignment, and VerticalContentAlignment = stretch

Answer 2

Your Grid is drawn off screen without a scroll bar because you are using a StackPanel. Care must be taken when using StackPanel - it is the most simplistic of all WPF Panel derived classes since its MeasureOverride calls Measure for all of its children with a size of double.PositiveInifity regardless of how much space the panel actually has available. Even a ScrollViewer will not help you with a StackPanel (the ScrollBar will be shown, but you won't be able to move it).

For example, consider a Window 350 in height and width, and a single Button as its content which is 500 in both height and width:

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="350">
    <StackPanel>
        <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
            <Button Content="Hello" Height="500" Width="500" />
        </ScrollViewer>
    </StackPanel>
</Window>

Similar to your example, the Button here is drawn off screen in both the vertical and horizontal directions and a non-functional scroll bar will be present. If we change the panel to one which respects the size of its given area (e.g. DockPanel):

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <DockPanel>
        <ScrollViewer>
            <Button Content="Hello" Height="500" />
        </ScrollViewer>
    </DockPanel>
</Window>

then scroll bars appear which are functional and hence allow the content off screen to be shown by scrolling.

Hope this helps!