Use Resources.Add to add static resouce from code

   
  


<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="GradientBrushResourceDemo" Height="300" Width="300">
    <Window.Resources>
        <LinearGradientBrush x:Key="brushGradient"
                             StartPoint="0, 0"
                             EndPoint="1, 1">
            <LinearGradientBrush.GradientStops>
                <GradientStop Offset="0" Color="Black" />
                <GradientStop Offset="0.5" Color="Green" />
                <GradientStop Offset="1" Color="Gold" />
            </LinearGradientBrush.GradientStops>
        </LinearGradientBrush>
    </Window.Resources>
    <StackPanel>
        <TextBlock Margin="{StaticResource thicknessMargin}"
               Foreground="{StaticResource brushGradient}">
            Gradient text
        </TextBlock>
        <TextBlock Margin="{StaticResource thicknessMargin}"
                   Foreground="{StaticResource brushGradient}">
            Of black, green, and gold
        </TextBlock>
        <TextBlock Margin="{StaticResource thicknessMargin}"
                   Foreground="{StaticResource brushGradient}">
            Makes an app pretty,
        </TextBlock>
        <TextBlock Margin="{StaticResource thicknessMargin}"
                   Foreground="{StaticResource brushGradient}">
            Makes an app bold.
        </TextBlock>
    </StackPanel>
</Window>
//File:Window.xaml.cs
using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;

namespace WpfApplication1
{
    public partial class Window1 : Window
    {
        public Window1()
        {
            Resources.Add("thicknessMargin", new Thickness(24, 12, 24, 23));

            InitializeComponent();
        }
    }
}