Error Handling

In the previous post Adding If / Else Logic We may have noticed a potential bug in our simple application. What happens if we don't pass a UserName to the workflow? In this article, we will add some error handling capabilities to our workflow by using the Try/Catch, Catch<T> and Throw built-in activities.

Notice that if we pass an empty string ("") the application should function correctly. The only way to get a null name passed to the workflow is to create it without supplying the in argument for UserName

1. First open HelloWorkFlow project from last article  Adding If / Else Logic .
2. Create a test to observe what happens when we do not pass the name argument to the workflow. To do this, using Solution Explorer, open SayHelloFixture.cs (C#) or SayHelloFixture.vb (Visual Basic) located under the HelloWorkflow.Tests project and add the following test.
   
   C#
   [TestMethod]

   public void ShouldHandleNullUserName()

   {

     // Invoking with no arguments

     WorkflowInvoker.Invoke(new SayHello());

   }
   Visual Basic
   <TestMethod()>

   Public Sub ShouldHandleNullUserName()

   
   

     ' Invoking with no arguments

     WorkflowInvoker.Invoke(New SayHello())

   
   

   End Sub
3. Now run the test. Right-click over test method name and select Run Tests.
   

   

   Run the test
4.  The result is that the test fails with a NullReferenceException because you have an expression that uses UserName.Length in the If activity and UserName was null.
   

   

   ShouldHandleNullUserName test failing

   Test Result
   HelloWorkflow.Tests.SayHelloFixture.ShouldHandleNullUserName threw exception: System.NullReferenceException: Object reference not set to an instance of an object.

To handle the error, We could validate the name argument prior to using it or we could simply catch the exception and deal with it. So, we will be catching the exception.

Add caption

1. Open SayHello.xaml in the designer and drag a TryCatch activity from Toolbox and drop it at the top of the sequence.
   

   

   The TryCatch activity in the Toolbox

   

   Adding a TryCatch activity

    Try/Catch Activity
   Workflows can use the TryCatch activity to handle exceptions that are raised during the execution of a workflow. These exceptions can be handled or they can be re-thrown using the Throw activity. Activities in the Finally section are executed when either the Try section or the Catches section completes.
   
   
2. Now you need to move the If activity inside the Try block.
   a.       Collapse the If activity () to make the diagram smaller 
   b.       Drag-and-drop the if activity into the Try block. 
   
   

   

   Moving the If activity
3. Now you need to catch a NullReferenceException. To do this, follow these steps:
   
   a.       Click on Add new catch.
   

   b.       Select System.NullReferenceExeption from the combo box and press Enter.
   

   

   Selecting NullReferenceExeption

   
   

4. In the Catch section of the activity we need to decide how to handle the error. In this case, we are going to replace the exception with an ArgumentNullException and throw it. This lets the caller know that the exception was their fault for not supplying the UserName argument. Drag a Throw activity from the Toolbox and drop it into the Catches area.
5. Set the Throw activity expression. To do this, select the Throw activity and in the Properties windows type the following expression in the Exception box.
   
   Visual Basic
   New ArgumentNullException("UserName")
   
   

   

   Adding a Throw activity
6. Now we need to fix the ShouldHandleNullUserName test so that it expects this exception. To do this, open SayHelloFixture.cs and add an ExpectedException annotation to the test method as shown in the following code.
   
   C#
   [TestMethod]

   [ExpectedException(typeof(ArgumentNullException))]

   public void ShouldHandleNullUserName()

   {

    // Invoking with no arguments

    WorkflowInvoker.Invoke(new SayHello());

   }
   C#
   [TestMethod]

   [ExpectedException(typeof(ArgumentNullException))]

   public void ShouldHandleNullUserName()

   {

    // Invoking with no arguments

    WorkflowInvoker.Invoke(new SayHello());

   }
7. Press CTRL+SHIFT+B to build the solution.
8. From the menu select Test / Run / All Tests In Solution or press CTRL+R,A
9. Verify that all the tests pass.
10.