Unable to get activity instance from work item

Question

I'm using Tridion 2013 SP1. I'm getting below error while finishing an automatic activity from a decision activity.

Here is my code:

var session = new Session();
WorkItem  m_WorkItem = session.GetObject(workitemid) as WorkItem;
ActivityInstance currentactivity = m_WorkItem.Activity as ActivityInstance;
IEnumerable<ActivityDefinition> ieActivities = currentactivity.ActivityDefinition.ProcessDefinition.ActivityDefinitions;
ActivityDefinition activityInstance = ieActivities.ToList().Where(c => c.Title == "Publish EN to Testing and Staging").First();
DecisionActivityFinish finish = new DecisionActivityFinish(activityInstance, session.User, "Publish EN to Testing and Staging", session);
currentactivity.Finish(finish);

Error on the event viewer is like this,

Unable to get activity instance from work item. caused by: WorkItem.ActivityInstance

and description:

<?xml version="1.0"?>
<tcm:Error xmlns:tcm="http://www.tridion.com/ContentManager/5.0" ErrorCode="D" Category="18" Source="Kernel" Severity="1">
    <tcm:Line Cause="false" MessageID="16138"><![CDATA[Unable to get ActivityInstance of Workflow work item (tcm:3-125-131200).]]><tcm:Token>RESID_4485</tcm:Token>
        <tcm:Token>ActivityInstance</tcm:Token>
        <tcm:Token>RESID_15212</tcm:Token>
        <tcm:Token>tcm:3-125-131200</tcm:Token>
    </tcm:Line>
    <tcm:Line ErrorCode="D" Cause="true"><![CDATA[Type mismatch]]></tcm:Line>
    <tcm:Details>
        <tcm:CallStack>
            <tcm:Location>WorkItem.ActivityInstance</tcm:Location>
            <tcm:Location>WorkItem.ActivityInstance</tcm:Location>
        </tcm:CallStack>
    </tcm:Details>
</tcm:Error>

Answer 1

I think, it's going wrong while casting

ActivityInstance currentactivity = m_WorkItem.Activity as ActivityInstance;

Try changing to

ActivityInstance currentactivity = (ActivityInstance)m_WorkItem.Activity;

Update:

For Automatic Activities, you should use Core Services.

Example:

WorkItemData mWorkItemData = (WorkItemData)mClient.Read(workItemId, mReadOptions);
ActivityInstanceData mCurrentActivity = (ActivityInstanceData)mClient.Read(mWorkItemData.Activity.IdRef, mReadOptions);
TridionActivityDefinitionData mTridionActivityDefinition = (TridionActivityDefinitionData)mClient.Read(mCurrentActivity.ActivityDefinition.IdRef, mReadOptions);
ProcessDefinitionData mProcessDefinition = (ProcessDefinitionData)mClient.Read(mTridionActivityDefinition.ProcessDefinition.IdRef, mReadOptions);
IList<ActivityDefinitionData> mActivityDefinitionList = new List<ActivityDefinitionData>(mProcessDefinition.ActivityDefinitions);

ActivityDefinitionData mNextActivity = (ActivityDefinitionData)mActivityDefinitionList.Where(c => c.Title == "Publish EN to Testing and Staging").FirstOrDefault();

if (mNextActivity != null)
{
    DecisionActivityFinishData mDecisionActivityFinishData = new DecisionActivityFinishData
    {
        NextActivity = new LinkToActivityDefinitionData { IdRef = mNextActivity.Id }
    };
    mClient.FinishActivity(mCurrentActivity.Id, mDecisionActivityFinishData, mReadOptions);
}

Note: I have not tested above code, just wrote for you.

Answer 2

Here a sample using an External Activity source code.

ProcessDefinitionData ProcessDefinition = (ProcessDefinitionData)CoreServiceClient.Read(ProcessInstance.ProcessDefinition.IdRef, ReadOptions);

ActivityDefinitionData NextActivity = (ActivityDefinitionData)CoreServiceClient.Read(ProcessDefinition.ActivityDefinitions.Where(w => w.Title.IndexOf("Publish EN to Testing and Staging") != -1).ElementAt(0).Id, ReadOptions);

CoreServiceClient.FinishActivity(ActivityInstance.Id, new DecisionActivityFinishData() {
        Message = "Finish Message",
        NextAssignee = GetNextAssignee(), 
        NextActivity = new LinkToActivityDefinitionData() { IdRef = NextActivity.Id }
    }, 
new ReadOptions());

Notice that I don't need to write code to get the current Activity Instance since it is already available in the ActivityInstance property as well as for the CoreServiceClient object.

As answered above, you shouldn't use the TOM .NET API to manipulate Workflow Objects unless you are doing it in an Event System. If your intention is to run that code in an Automatic Activity (C# or External) you must use Core Services.