IHttpContextAccessor HttpContext始终为null

当我尝试从IHttpContextAccessor字段获取httpcontext时，我总是遇到问题。

有我的startup.cs

    public void ConfigureServices(IServiceCollection services)
    {
        // Add framework services.
        services.AddMvc();
        services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    }

    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
        .....

        // This is always null
        var httpContext = app.ApplicationServices.GetService<IHttpContextAccessor>().HttpContext;

        .....
    }