参考 IdentityServer 的帮助文档 Protecting APIs ，在一个 .NERT 5.0 项目的 Startup 中添加了下面的代码

services.AddAuthentication("token")

    // JWT tokens
    .AddJwtBearer("token", options =>
    {
        options.Authority = Constants.Authority;
        options.Audience = "resource1";

        options.TokenValidationParameters.ValidTypes = new[] { "at+jwt" };

        // if token does not contain a dot, it is a reference token
        options.ForwardDefaultSelector = Selector.ForwardReferenceToken("introspection");
    })

    // reference tokens
    .AddOAuth2Introspection("introspection", options =>
    {
        options.Authority = Constants.Authority;

        options.ClientId = "resource1";
        options.ClientSecret = "secret";
    });

提示下面的错误

The name 'Selector' does not exist in the current context

请问需要安装哪个 nuget 包？