类型或命名空间名称'找不到

我现在是c#新手，我正在跟随一本书学习c#。我有以下代码:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using EssentailToos.Models;
using Ninject;
namespace EssentailToos.Infrastructure
{
    public class NinjectDependencyResolver : IDependencyResolver 
    {
        private IKernel kernel;
        public NinjectDependencyResolver(IKernel kernelParam)
        {
            kernel = kernelParam;
            AddBindings();
        }
        public object GetService(Type serviceType)
        {
            return kernel.TryGet(serviceType);
        }
        public IEnumerable<object> GetServices(Type serviceType)
        {
            return kernel.GetAll(serviceType);
        }
        private void AddBindings()
        {
            kernel.Bind<IValueCalculator>().To<LinqValueCalculator>();
        }
    }
}

当我试图运行这个，我得到错误:The type or namespace name 'IDependencyResolver' could not be found

我是c#的新手，所以有人能解释为什么我得到这个错误信息吗?