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Chapter 1: Workﬂ ow Programming Principles

Developing Custom Composite Activities

A composite activity or logical program statement, just like a C# composite program statement such as

“ { }, ” “ for, ” “ while, ” and so on, is a logical program statement that contains other logical program

statements or activities. In general, there are two types of composite activities. The first type includes flow

control constructs such as “ { }, ” “ for, ” and “ while. ” The second type includes those composite activities

that inherit from the flow control constructs but do not change the flow control logic implemented in the

flow control constructs from which they inherit. The main purpose behind implementing this second type

of composite activity is to assemble a custom activity from other composite or leaf activities. This enables

you to reuse the functionality implemented in other composite or leaf activities when you ’ re building

your own custom activity.

Both types of composite activity directly or indirectly inherit from a base activity named

CompositeActivity, which in turn inherits from the Activity base class and exposes the members shown

in Listing 1 - 6 .

Listing 1 - 6: The CompositeActivity activity

public class CompositeActivity : Activity

{

// Methods

protected internal override void Initialize(

IServiceProvider provider);

protected internal override void Uninitialize(

IServiceProvider provider);

// Properties

public ActivityCollection Activities { get; }

protected internal bool CanModifyActivities { get; set; }

public ReadOnlyCollection < Activity > EnabledActivities { get; }

}

Note that the CompositeActivity activity does not override the Execute method. It is the responsibility of

the flow control constructs that inherit from the CompositeActivity activity to override this method to

include the logic that determines in which order the child activities of the composite activity should be

executed. You ’ ll see examples of this later in this chapter.

CompositeActivity does override the Initialize method where it invokes the Initialize method of all its

containing child activities. CompositeActivity also overrides the Uninitialize method where it invokes

the Uninitialize methods of all its containing child activities.

Note that the CompositeActivity activity exposes a collection property named Activities, which is of the

ActivityCollection type. This is where the CompositeActivity activity maintains its child activities. The

CompositeActivity activity also exposes a read - only collection property named EnabledActivities, which

is of the ReadOnlyCollection < Activity > type. This collection contains references to all enabled activities

of the composite activity. An enabled activity is an activity whose Enabled property is set to true.

Disabling an activity or logical program statement is like commenting out a C# program statement.

A disabled activity does not participate in the execution of the composite activity. It is like dead code.

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