MainWindow.xaml

<Window x:Class="XamlShortening.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:XamlShortening"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <!--<Grid local:GridDefinitionExtension.Columns="Auto,*,*" local:GridDefinitionExtension.Rows="Auto,*,*">-->
    <Grid local:GridDefinitionExtension.Columns="100,200,30,500" local:GridDefinitionExtension.Rows="*,*">
        <Button Grid.Column="0" Grid.Row="0" Content="Hello"/>
        <Button Grid.Column="0" Grid.Row="1" Content="Hello"/>
        <Button Grid.Column="1" Grid.Row="0" Content="Hello"/>
        <Button Grid.Column="1" Grid.Row="1" Content="Hello"/>
        <Button Grid.Column="2" Grid.Row="0" Content="Hello"/>
        <Button Grid.Column="2" Grid.Row="1" Content="Hello"/>
        <Button Grid.Column="3" Grid.Row="0" Content="Hello"/>
        <Button Grid.Column="3" Grid.Row="1" Content="Hello"/>
    </Grid>
</Window>