Azure-functions-durable-extension: NullReferenceException when Task returns void

[mark-szabo]

A NullReferenceException is thrown when a Durable Function Task returns void like this and is called by a typed CallActivityAsync:

[FunctionName("MyFunctions_RunTaskAsync")]
public async Task RunTaskAsync([ActivityTrigger] string input)
{
    ...
}

await context.CallActivityAsync<bool>("MyFunctions_RunTaskAsync", input);

Function 'MyFunctions (Orchestrator)' failed with an error.Reason: System.NullReferenceException: Object reference not set to an instance of an object.
at DurableTask.Core.TaskOrchestrationContext.ScheduleTaskToWorker[TResult](String name, String version, String taskList, Object[] parameters) in C:\source\durabletask\src\DurableTask.Core\TaskOrchestrationContext.cs:line 80
at DurableTask.Core.TaskOrchestrationContext.ScheduleTask[TResult](String name, String version, Object[] parameters) in C:\source\durabletask\src\DurableTask.Core\TaskOrchestrationContext.cs:line 70
at Microsoft.Azure.WebJobs.Extensions.DurableTask.DurableOrchestrationContext.CallDurableTaskFunctionAsync[TResult](String functionName, FunctionType functionType, Boolean oneWay, String instanceId, String operation, RetryOptions retryOptions, Object input, Nullable`1 scheduledTimeUtc) in d:\a\r1\a\azure-functions-durable-extension\src\WebJobs.Extensions.DurableTask\ContextImplementations\DurableOrchestrationContext.cs:line 607
at MyNamespace.MyFunctions.RunOrchestrator(IDurableOrchestrationContext context)

[mark-szabo]

You should add something like this:

if (result == null) throw new Exception("Task result is null, and cannot be casted to TResult.");

to line 79:
https://github.com/Azure/durabletask/blob/effb469a04480d35935c67bf07c50e902cb3e9be/src/DurableTask.Core/TaskOrchestrationContext.cs#L75-L81

[cgillum]

What happens if you use the non-generic overload of CallActivityAsync?

await context.CallActivityAsync("MyFunctions_RunTaskAsync", input);

[mark-szabo]

@cgillum Yup, that's the correct way to do, I know, but an internal NullReferenceException is not something the SDK should throw. Maybe an InvalidOperationException with some nice error message that you should call the non-generic method or something.