C# 4 and .NET 4

activities ❘ 1313
The If activity shown in Figure 44-4 also includes a glyph indicating that the Condition property needs to
be defined. This condition is evaluated when the activity is executed; if it returns True, the Then branch will
execute, otherwise the Else branch will be called.
The Condition property is an expression that evaluates to a Boolean value, so you can include any
expression here that is valid.
Workflow 4 includes an expression engine that uses Visual Basic syntax. This might seem strange to a C#
programmer, as VB is significantly different from C#. Unfortunately, that is the way it is at the moment, so
to use the built-in activities you’ll have to learn enough VB to get by. Just remember to be extremely verbose
and leave off any semicolons and you’ll be OK.
An expression can reference any variables defined in the workflow and also access many static classes
available in the .NET framework. So you could, for example, define an expression based on the
Environment.Is64BitOperatingSystem value, if that was crucial to some part of your workflow.
Naturally, you can define arguments that are passed into the workflow and that can then be evaluated
by an expression inside an If activity. We’ll cover arguments and variables later in the chapter.
invokemethod activity
This is one of the most useful activities in the box, as it allows you to execute code that already exists and
effectively wrap that code within the execution semantics of a workflow.
It’s typical to have a lot of preexisting code, and this activity allows you to call that code directly from
within a workflow.
There are two ways that you can use InvokeMethod to call code; which method you use depends on
whether you wish to call a static method or an instance method. If you’re calling a static method, you need
to define the TargetType and the MethodName parameters. However if you’re calling an instance method,
then the TargetObject and MethodName properties are used, and in this instance the TargetObject could
be created inline, or it could be a variable defined somewhere within the workflow itself. The example code
in the 02_ParallelExecution sample shows both modes of using the InvokeMethod activity.
If you need to pass arguments to the method you’re invoking, you can define these using the Parameters
collection. The order of the parameters in the collection must match the order of the parameters to the
method. In addition, there is a Result property that is set to the return value of the function call. You can
bind this to a variable within the workflow in order to use the value as appropriate.
Parallel activity
The Parallel activity is rather poorly named because, at first sight, you might think that on a
multiprocessor machine this activity would schedule its children in true parallel; however, that isn’t the case
apart from some special circumstances.
After you drop a Parallel activity onto the design surface, you can then drop in other subordinate
activities, as shown in Figure 44-5.
figure 44-5
These child activities can be singular activities, as in Figure 44-5, or they can form a composite activity,
such as a Sequence or another Parallel activity.
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