default value for required arguments of custom activities in Workflow Foundation

Question

In writing Custom Activities for a re-hosted Workflow Designer, it gives me an error that there is a value required for the Required Argument if I don't have one, so I either use a null as seen below or "ALL". It does not accept null or "ALL" or anything else for a default argument. Please note that my required argument is of type string.

[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; }

Error Message when Running the Workflow :

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()

Thanks for your help

So you're saying that if you don't use DefaultValue you're getting a compile error? — Mike Perrenoud
No I don't get a compile error. When I run the program the workflow wont run. I will post the error message in a minute. — user1298925
Why are you executing a workflow without validating it first? And isn't the workflow designer warning you that a value is required for those InArguments? — Joao
I did not write the original code, I have to check to see if there is a way to invoke a validate first or could it be that validate is called. By the way, when running the workflow that is exactly the error I get. Required Argument value missing (paraphrasing) — user1298925

Joao Joao · Accepted Answer · 2014-01-21T10:32:34

Neither workflow runtime nor workflow designer will look into DefaultValueAttribute. Searching attributes requires reflection which might be costly on performance. Moreover that's not the purpose of the attribute.

Anyway, you can either initialize the variable with a default value. On constructor, for example:

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

or force a value through accessor. Something like this:

private InArgument<string> text = "All";

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

These are different approaches, use the one that fits into the behavior you want. In both cases the value can always be modified through XAML so a small check on runtime might be good:

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

    Console.WriteLine(text);
}

default value for required arguments of custom activities in Workflow Foundation

1 Answers