为什么需要工作流？在之前博文.Net依赖注入技术学习：基本模型中，有提到这个世界的基本构型是纵向分层和横向组合，而工作流模型在纵向上比源码级别提升了一个层次，它的基本操作单元是步骤；在横向上通过一些规则，可以使步骤灵活组合。实现了更高层次抽象的简洁性和表达力的平衡。

本文介绍了.Net体系比较优秀的开源工作流WorkflowCore，github地址：https://github.com/danielgerlag/workflow-core。

下面定义了2个步骤：HelloWorld和GoodbyeWorld

    public class HelloWorld : StepBody
    {
        public override ExecutionResult Run(IStepExecutionContext context)
        {
            Console.WriteLine("Hello world");
            return ExecutionResult.Next();
        }
    }

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    public class GoodbyeWorld : StepBody
    {

        private ILogger _logger;

        public GoodbyeWorld(ILoggerFactory loggerFactory)
        {
            _logger = loggerFactory.CreateLogger<GoodbyeWorld>();
        }

        public override ExecutionResult Run(IStepExecutionContext context)
        {
            Console.WriteLine("Goodbye world");
            _logger.LogInformation("Hi there!");
            return ExecutionResult.Next();
        }
    }

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然后定义一个工作流：

    public class HelloWorldWorkflow : IWorkflow
    {
        public void Build(IWorkflowBuilder<object> builder)
        {
            builder                
                .StartWith<HelloWorld>()                
                .Then<GoodbyeWorld>();
        }

        public string Id => "HelloWorld";
            
        public int Version => 1;
                 
    }

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最后启动工作流：

    public class Program
    {
        
        public static void Main(string[] args)
        {
            IServiceProvider serviceProvider = ConfigureServices();

            //start the workflow host
            var host = serviceProvider.GetService<IWorkflowHost>();
            host.RegisterWorkflow<HelloWorldWorkflow>();        
            host.Start();            

            host.StartWorkflow("HelloWorld");
            
            Console.ReadLine();
            host.Stop();
        }
                
        private static IServiceProvider ConfigureServices()
        {
            //setup dependency injection
            IServiceCollection services = new ServiceCollection();
            services.AddLogging();
            services.AddWorkflow();
            //services.AddWorkflow(x => x.UseMongoDB(@"mongodb://localhost:27017", "workflow"));
            services.AddTransient<GoodbyeWorld>();
            
            var serviceProvider = services.BuildServiceProvider();

            return serviceProvider;
        }


    }

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