Content property is set more than once - xaml

Question

I'm trying to add a title to my xaml page, in the small space above the 4x4 grid, but when i try to add the label, it says content is set more than once.
here is my code:
<Window x:Class="WpfApplicationAssignmentgood.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">
<Grid Name="mainGrid" Margin="0,56,0,0">
<Grid.ColumnDefinitions>
<ColumnDefinition />
<ColumnDefinition />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
</Grid>
<Label Content="Hello" HorizontalAlignment="Center", VerticalAlignment="Center" FontFamily="Arial Black" FontSize="24" />
</Window>

Answer 1

Your Window can only contain one child. In your case, your Window contains a Grid, and a Label.
Ensure that your Window only has one child, you can achieve this by placing your Label inside a Grid. Something like this:
<Window ... >
<Grid>
<Grid ...>
...
</Grid>
<Label Content="Hello" ... />
</Grid>
</Window>