Task.WaitAll Method (Task, Int32)
Waits for all of the provided Task objects to complete execution within a specified number of milliseconds.
Assembly: mscorlib (in mscorlib.dll)
One or more of the Task objects in tasks has been disposed.
The tasks argument is a null reference (Nothing in Visual Basic).
An exception was thrown during the execution of at least one of the Task instances.
millisecondsTimeout is a negative number other than -1, which represents an infinite time-out.
The tasks argument contains a null element.