Why the height is incorrect?

I have a simple WPF form with next XAML

<Window x:Class="ReikartzDataConverter.MainWindow开发者_如何学JAVA"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="650" Width="800">
    <Grid Width="780" Height="650">
        <Grid.RowDefinitions>
            <RowDefinition Height="50"></RowDefinition>
            <RowDefinition Height="500"></RowDefinition>
            <RowDefinition Height="50"></RowDefinition>
            <RowDefinition Height="50"></RowDefinition>
        </Grid.RowDefinitions>

        <Label Grid.Row="0" Content="Process information" Height="28" HorizontalAlignment="Left" Margin="0,20,0,0" Name="label1" VerticalAlignment="Top" Width="235" />
        <DataGrid Grid.Row="1" Width="780" Height="500" Name="paysTable">            
        </DataGrid>
        <Label Grid.Row="2" Height="28" Name="lblError" VerticalAlignment="Top" Visibility="Hidden" Foreground="OrangeRed" FontWeight="Bold" FontSize="12" />
        <Button Grid.Row="3" Content="Quit" Height="23" Name="button1" VerticalAlignment="Top" Width="75" Click="button1_Click" />
        <Button Grid.Row="3" Content="Start" Height="23" Name="button2" VerticalAlignment="Top" Width="75" Click="button2_Click" />
    </Grid>
</Window>

Why my 2 buttons from Grid.Row="3" are partially located out of the visible part of the window?

My window has the height="650" and my Grid also has the Height="650" I have 4 Rows: 50, 50, 500, 50. So the last row must be located inside the window. Why is not so?

The Window Height of 650 also includes the 'chrome', i.e. the bar at the top of the window with the minimize, maximize buttons. It is a much better approach to create a layout which does not rely on a specific height. In your case, I would make the row that contains your grid auto-sized:

    <Grid.RowDefinitions>
        <RowDefinition Height="50"></RowDefinition>
        <RowDefinition Height="Auto"></RowDefinition>
        <RowDefinition Height="50"></RowDefinition>
        <RowDefinition Height="50"></RowDefinition>
    </Grid.RowDefinitions>

Then you can remove the height / width from your Grid, and all your other UI elements, just let the grid dictate the size of its children.

@ColinE's answer is the right approach in that you should adopt a "fluid" layout in WPF, but if you really want a fixed height for your content and you need the window to be the right size, you can use the SizeToContent property:

<Window x:Class="ReikartzDataConverter.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Width="800"
    SizeToContent="Height">

    <Grid Width="780" Height="650">
        ...
    </Grid>
</Window>

Setting SizeToContent to "Height" will make the window resize vertically so that its contents fit. Don't forget to remove the Height property from the Window declaration.