Most of the time when you are using Visual Studio and you don’t’ want to open Blend for just setting a very Gradient Background! The below code can surely help you.

Creat gradient background in c#:
LinearGradientBrush linear = new LinearGradientBrush();
linear.StartPoint = new Point(0.5, 0);
linear.EndPoint = new Point(0.5, 1);
GradientStop g1 = new GradientStop();
g1.Color = _backgroundColor1;
g1.Offset = 0;
GradientStop g2 = new GradientStop();
g2.Color = _backgroundColor2;
g1.Offset = 1;
linear.GradientStops.Add(g2);
linear.GradientStops.Add(g1);
Border border = new Border();
border.Background = linear;
Creating gradient background in xaml:
<UserControl x:Class="GradientBackground.Page"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Width="600" Height="400">
<Grid x:Name="LayoutRoot" Background="White">
<Rectangle Stroke="Black" Height="180" HorizontalAlignment="Left" Margin="10,10,0,0" VerticalAlignment="Top" Width="285">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0.5,1" StartPoint="0.5,0">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="White" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
<Rectangle Stroke="Black" Height="190" HorizontalAlignment="Left" Margin="10,0,0,10" VerticalAlignment="Bottom" Width="285">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0,0" StartPoint="0,1">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="White" Offset="0.1"/>
<GradientStop Color="Black" Offset="0.2"/>
<GradientStop Color="White" Offset="0.3"/>
<GradientStop Color="Black" Offset="0.4"/>
<GradientStop Color="White" Offset="0.5"/>
<GradientStop Color="Black" Offset="0.6"/>
<GradientStop Color="White" Offset="0.7"/>
<GradientStop Color="Black" Offset="0.8"/>
<GradientStop Color="White" Offset="0.9"/>
<GradientStop Color="Black" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
<Rectangle Stroke="Black" Height="180" HorizontalAlignment="Right" Margin="0,10,10,0" VerticalAlignment="Top" Width="285">
<Rectangle.Fill>
<LinearGradientBrush EndPoint="0,0" StartPoint="1,1">
<GradientStop Color="Black" Offset="0"/>
<GradientStop Color="Red" Offset="1"/>
</LinearGradientBrush>
</Rectangle.Fill>
</Rectangle>
<Rectangle Stroke="Black" Height="190" HorizontalAlignment="Right" Margin="0,0,10,10" VerticalAlignment="Bottom" Width="285">
<Rectangle.Fill>
<RadialGradientBrush>
<GradientStop Color="White" Offset="0"/>
<GradientStop Color="Black" Offset="1"/>
</RadialGradientBrush>
</Rectangle.Fill>
</Rectangle>
</Grid>
</UserControl>