How to set width to 100% in WPF

Is there any way how to tell component in WPF to take 100% of available space?

Like

width: 100%;  

in CSS

I've got this XAML, and I don't know how to force Grid to take 100% width.

<ListBox Name="lstConnections">
  <ListBox.ItemTemplate>
    <DataTemplate>
      <Grid Background="LightPink">
        <Grid.ColumnDefinitions>
          <ColumnDefinition Width="Auto"/>
          <ColumnDefinition Width="Auto"/>
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
          <RowDefinition Height="Auto"/>
          <RowDefinition Height="Auto"/>
        </Grid.RowDefinitions>
        <TextBlock Grid.Row="0" Grid.Column="0" Text="{Binding Path=User}" Margin="4"></TextBlock>
        <TextBlock Grid.Row="0" Grid.Column="1" Text="{Binding Path=Password}" Margin="4"></TextBlock>
        <TextBlock Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2" Text="{Binding Path=Host}" Margin="4"></TextBlock>
      </Grid>
    </DataTemplate>
  </ListBox.ItemTemplate>
</ListBox>

Result looks like

I made it pink so it's obvious how much space does it take. I need to make the pink grid 100% width.

It is the container of the Grid that is imposing on its width. In this case, that's a ListBoxItem, which is left-aligned by default. You can set it to stretch as follows:

<ListBox>
    <!-- other XAML omitted, you just need to add the following bit -->
    <ListBox.ItemContainerStyle>
        <Style TargetType="ListBoxItem">
            <Setter Property="HorizontalAlignment" Value="Stretch"/>
        </Style>
    </ListBox.ItemContainerStyle>
</ListBox>

You could use HorizontalContentAlignment="Stretch" as follows:

<ListBox HorizontalContentAlignment="Stretch"/>