ManualResetEvent.WaitOne() throws NullReferenceException: Object reference not set to an instance of an object

Question

I have a weird random NPE error when dealing with ManualResetEvent.WaitOne(). Here's my code.

I have a method that creates ManualResetEvent object and then it passes it down to the Windows Workflow Foundation (WWF) workflow instance as one of the dependency parameter (manualResetEvent) and then I go into manualResetEvent.WaitOne() API.

ManualResetEvent manResetEvt = new ManualResetEvent(false);

Dictionary<String, Object> wfArgs = new Dictionary<string, object>();
wfArgs["manualResetEvent"] = manResetEvt;

WorkflowInstance instance = workflowRuntime.CreateWorkflow(typeof(MyWWFProcess), wfArgs);

instance.Start();
manResetEvt.WaitOne();

When the job is done within WWF, I simply call manualResetEvent.set().

if (this.manualResetEvent != null)
{
    this.manualResetEvent.Set();
}

All these compile well and while running, I see that the flow gets into the WWF as expected and the caller does wait on WaitOne() call too.

The moment the WWF invokes manualResetEvent.Set() to notify the caller, I see an NPE exception with the caller NOT WWF.

System.NullReferenceException: Object reference not set to an instance of an object.

I really don't know where this exception arises from. When I debug this code in VS IDE, all works well but only when with the application in the Release mode, I see this exception.

What am I doing wrong here?

Answer 1

I found the answer to my question. Based on James Thorpe suggestion, I printed the stack-trace from the exception and it turns out the application is sending a NULL data point when invoking this method and WaitOne() is not causing the NPE. When I ran the unit-test from VS IDE debugger, I passed in a good value hence no exception found. After fixing the caller method, all is working well. Thanks everyone who pitched in ideas to resolve this issue.

Answer 2

You need to add the object to the dictionary

Dictionary<String, Object> wfArgs = new Dictionary<string, object>();
wfArgs.Add("manualResetEvent", manResetEvt);