在ASP .NET Core 2.1中实现Cronjob的解决方法如下：

1. 首先，需要安装以下NuGet包：

Microsoft.Extensions.Hosting
Quartz.Extensions.Hosting

2. 在Startup.cs文件中，添加以下代码：

using Microsoft.Extensions.Hosting;
using Quartz;
using Quartz.Impl;

public class Startup
{
    // ...

    public void ConfigureServices(IServiceCollection services)
    {
        // ...

        services.AddSingleton();
        services.AddSingleton();
        services.AddSingleton();
        services.AddHostedService();

        // ...
    }
}

3. 创建一个Job类，示例代码如下：

using System;
using System.Threading.Tasks;
using Quartz;

public class SampleJob : IJob
{
    public async Task Execute(IJobExecutionContext context)
    {
        // 执行定时任务的逻辑代码
        await Task.Run(() =>
        {
            Console.WriteLine("Job executed at: " + DateTime.Now);
        });
    }
}

4. 创建一个JobFactory类，其作用是在每次触发定时任务时创建一个新的Job实例。示例代码如下：

using Quartz;
using Quartz.Spi;

public class JobFactory : IJobFactory
{
    private readonly IServiceProvider _serviceProvider;

    public JobFactory(IServiceProvider serviceProvider)
    {
        _serviceProvider = serviceProvider;
    }

    public IJob NewJob(TriggerFiredBundle bundle, IScheduler scheduler)
    {
        var jobDetail = bundle.JobDetail;
        return (IJob)_serviceProvider.GetService(jobDetail.JobType);
    }

    public void ReturnJob(IJob job) { }
}

5. 创建一个JobScheduler类，用于配置和启动Quartz定时任务。示例代码如下：

using System;
using System.Threading.Tasks;
using Quartz;

public class JobScheduler
{
    private readonly ISchedulerFactory _schedulerFactory;
    private readonly IScheduler _scheduler;

    public JobScheduler(ISchedulerFactory schedulerFactory)
    {
        _schedulerFactory = schedulerFactory;
        _scheduler = _schedulerFactory.GetScheduler().GetAwaiter().GetResult();
    }

    public async Task StartAsync()
    {
        await _scheduler.Start();

        var job = JobBuilder.Create()
            .WithIdentity("sample-job", "sample-group")
            .Build();

        var trigger = TriggerBuilder.Create()
            .WithIdentity("sample-trigger", "sample-group")
            .StartNow()
            .WithCronSchedule("0/5 * * * * ?") // 设置Cron表达式，每5秒执行一次
            .Build();

        await _scheduler.ScheduleJob(job, trigger);
    }

    public async Task StopAsync()
    {
        await _scheduler.Shutdown();
    }
}

6. 创建一个QuartzHostedService类，用于在应用程序启动时启动定时任务。示例代码如下：

using System.Threading;
using System.Threading.Tasks;
using Microsoft.Extensions.Hosting;

public class QuartzHostedService : IHostedService
{
    private readonly JobScheduler _jobScheduler;

    public QuartzHostedService(JobScheduler jobScheduler)
    {
        _jobScheduler = jobScheduler;
    }

    public Task StartAsync(CancellationToken cancellationToken)
    {
        return _jobScheduler.StartAsync();
    }

    public Task StopAsync(CancellationToken cancellationToken)
    {
        return _jobScheduler.StopAsync();
    }
}

7. 在Program.cs文件中，添加以下代码：

using Microsoft.Extensions.Hosting;

public class Program
{
    public static void Main(string[] args)
    {
        CreateHostBuilder(args).Build().Run();
    }

    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            .ConfigureWebHostDefaults(webBuilder =>
            {
                webBuilder.UseStartup();
            });
}

以上代码实现了在ASP .NET Core 2.1中使用Quartz实现Cronjob。在示例代码中，定时任务每5秒执行一次，可以根据需求修改Cron表达式。