工作流基礎中自定義活動的必需參數的默認值

Question

在爲重新託管的工作流設計器編寫自定義活動時，如果我沒有必要參數，它會給我一個錯誤，即必需參數需要一個值，所以我使用如下所示的null或「ALL」。它不接受null或「ALL」或任何其他的默認參數。請注意我需要的參數是字符串類型。運行工作流時

[RequiredArgument] 
[DefaultValue(null)] 
[Description(@"The status of the job to perform")] 
public InArgument<string> JobStatus { get; set; } 

[RequiredArgument] 
[DefaultValue("All")] 
[Description(@"The status of the job to perform")] 
public InArgument<string> JobStatus { get; set; } 

錯誤消息：

Workflow Console: Starting Workflow...

Could not start workflow.

Message: Failed to start workflow DynamicActivity.

Exception message: The following errors were encountered while processing the workflow tree: 'DynamicActivity': The private implementation of activity '1: DynamicActivity' has the following validation error: Value for a required activity argument 'JobStatus' was not supplied.

Stack trace: at System.Activities.Validation.ActivityValidationServices.ThrowIfViolationsExist(IList`1 validationErrors) at System.Activities.Hosting.WorkflowInstance.ValidateWorkflow(WorkflowInstanceExtensionManager extensionManager) at System.Activities.Hosting.WorkflowInstance.RegisterExtensionManager(WorkflowInstanceExtensionManager extensionManager) at System.Activities.WorkflowApplication.EnsureInitialized() at System.Activities.WorkflowApplication.Enqueue(InstanceOperation operation, Boolean push) at System.Activities.WorkflowApplication.WaitForTurn(InstanceOperation operation, TimeSpan timeout) at System.Activities.WorkflowApplication.InternalRun(TimeSpan timeout, Boolean isUserRun) at System.Activities.WorkflowApplication.Run()

感謝您的幫助

Answer 1

無論是工作流運行時也不流設計將考慮DefaultValueAttribute。搜索屬性需要反射，這可能會導致性能成本上升。而且這不是屬性的目的。

無論如何，您可以使用默認值初始化變量。在構造函數上，例如：

public class MyCodeActivity : CodeActivity 
{ 
    public MyCodeActivity() 
    { 
     JobStatus = "All"; 
    } 
} 

或通過訪問器強制值。像這樣：

private InArgument<string> text = "All"; 

[RequiredArgument] 
public InArgument<string> Text 
{ 
    get { return text ?? "All"; } 
    set { text = value; } 
} 

這些是不同的方法，使用適合你想要的行爲的方法。在這兩種情況下的價值總是能夠通過XAML進行修改，以便在運行時的一個小檢查可能是件好事：

protected override void Execute(CodeActivityContext context) 
{ 
    string text = context.GetValue(this.Text); 
    if (text == null) 
    { 
     text = "All"; 
    } 

    Console.WriteLine(text); 
}