C# project cant find IApplicationService

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Windows.Forms;
using System.Numerics;
using Microsoft.Xna;
using Microsoft.Xna.Framework.Audio;
using System.Windows.Threading;
using Microsoft.Xna.Framework;
using System.Windows;

namespace DSP
{
    class XNAFrameworkDispatcherService : IApplicationService
    {
    }
}

but is cant find IApplicationService Why? I have using System.Windows;