NativeActivity Class

public abstract class NativeActivity : Activity

public sealed class MySequence : NativeActivity
{
    Collection<Activity> children;
    Collection<Variable> variables;
    Variable<int> currentIndex;
    CompletionCallback onChildComplete;

    public MySequence()
        : base()
    {
        this.children = new Collection<Activity>();
        this.variables = new Collection<Variable>();
        this.currentIndex = new Variable<int>();
    }

    public Collection<Activity> Activities
    {
        get
        {
            return this.children;
        }
    }
    public Collection<Variable> Variables
    {
        get
        {
            return this.variables;
        }
    }
    protected override void CacheMetadata(NativeActivityMetadata metadata)
    {
        //call base.CacheMetadata to add the Activities and Variables to this activity's metadata
        base.CacheMetadata(metadata);
        //add the private implementation variable: currentIndex 
        metadata.AddImplementationVariable(this.currentIndex);
    }                   

    protected override void Execute(NativeActivityContext context)
    {
        InternalExecute(context, null);
    }
    void InternalExecute(NativeActivityContext context, ActivityInstance instance)
    {
        //grab the index of the current Activity
        int currentActivityIndex = this.currentIndex.Get(context);
        if (currentActivityIndex == Activities.Count)
        {
            //if the currentActivityIndex is equal to the count of MySequence's Activities
            //MySequence is complete
            return;
        }

        if (this.onChildComplete == null)
        {
            //on completion of the current child, have the runtime call back on this method
            this.onChildComplete = new CompletionCallback(InternalExecute);
        }
        //grab the next Activity in MySequence.Activities and schedule it
        Activity nextChild = Activities[currentActivityIndex];
        context.ScheduleActivity(nextChild, this.onChildComplete);

        //increment the currentIndex
        this.currentIndex.Set(context, ++currentActivityIndex);
    }
}