Pass multiple parameters to a task

Question

I wish to pass two BlockingCollection<>s to a task. I tried to put them in an object array and pass them but it doesn't work. Can anyone help me with this? The code where i am trying to pass the values is written below:

var lineHolders = new[]
{
     new BlockingCollection<string>(linesCapacity),
     new BlockingCollection<string>(linesCapacity),
     new BlockingCollection<string>(linesCapacity),
     new BlockingCollection<string>(linesCapacity)
};

var chunksHolder = new[]
{
     new BlockingCollection<List<BsonDocument>>(chunksCapacity),
     new BlockingCollection<List<BsonDocument>>(chunksCapacity)
};

for (var processors = 0; processors < 16; processors++)
{
      var myLineHolder = lineHolders[processors%lineHolders.Length];
      var myChunkHolder = chunksHolder[processors%chunksHolder.Length];
      processorTaskArray[processors] = Task.Factory.StartNew((arg) =>
      {
          var lines = (BlockingCollection<string>) arg[0];  // compiler generates error here
          var chunks = (BlockingCollection<List<BsonDocument>>) arg[1]; // compiler generates error here

          // perform my work...


      },
      new object []
      {
          myLineHolder, 
          myChunkHolder
      });
}

the lines BlockingCollection for a particular task is dependent on the loop index processor. Can't do a lineHolders[0]. — displayName, Jul 11 '14 at 00:50
introduce a new int variable inside the loop to capture the iteration indexer then use it inside the task and then mylineholder and mychunckholder can be processed inside task. — terrybozzio, Jul 11 '14 at 00:56
The compiler error is: `Cannot apply indexing to an expression of type 'object'`. — displayName, Jul 11 '14 at 01:02
@terrybozzio : It worked too. You may post it as your answer. It won't work though when i have to pass two (or more) objects independent of the loop index. Still, it is a smarter workaround for this case. — displayName, Jul 11 '14 at 13:40

Marcin Waligora · Accepted Answer · 2014-07-11T07:05:25.747

You're using the following overload of StartNew:

public Task StartNew(
    Action<Object> action,
    Object state
)

Since it's just an object you can't apply indexing on it. Cast it and it will work fine.

for (var processors = 0; processors < 16; processors++)
        {
            var myLineHolder = lineHolders[processors % lineHolders.Length];
            var myChunkHolder = chunksHolder[processors % chunksHolder.Length];
            processorTaskArray[processors] = Task.Factory.StartNew((arg) =>
            {
                var properArg = (object[]) arg;
                var lines = (BlockingCollection<string>) properArg[0]; // compiler generates error here
                var chunks = (BlockingCollection<List<BsonDocument>>) properArg[1]; // compiler generates error here

                // perform my work...

            },
            new object[]
              {
                  myLineHolder, 
                  myChunkHolder
              });
        }

Do you need to capture local variables inside a loop or just the loop variable itself? My understanding is if you referenced processors in the StartNew you'd get a reference to the changing loop variable and results would be unexpected. But what about the locals inside the loop - if you capture a reference to myLineHolder when processors = 0 is that a reference to the same variable next time around the loop? Surely not? — David Waterworth, Oct 07 '15 at 21:02

Pass multiple parameters to a task

1 Answers1