Task Parallelism: Passing Values into a Tasks

One road block that I stubble upon when starting to work with Task Parallelism is passing values into my Tasks. Here is a quick guide to help get you head around this issue.

 

We can easily create a single task that takes in non-changing values within a function and run it getting expected results:

int i = 0;
int j = 1;
int k = 2;

Task task = Task.Factory.StartNew(() =>
    {
        Console.WriteLine(string.Format("i='{1}'{0}j='{2}'{0}k='{3}'{0}{0}", Environment.NewLine, i, j, k));
    });

task.Wait();
Console.ReadLine();

Results:

i=’0′
j=’1′
k=’2′

 

Once the the values start changing then the Tasks will run against the latest values rather than the original value when the Task was created. The values are not fixed within the scope of the Task itself but rather in the scope of the parent function, so the variables do not preserve the value from when the Task was created:

int i = 0;
int j = 1;
int k = 2;

Task task = Task.Factory.StartNew(() =>
    {
        Console.WriteLine(string.Format("i='{1}'{0}j='{2}'{0}k='{3}'{0}{0}", Environment.NewLine, i, j, k));
    });

i = -1;
j = -1;
k = -1;

task.Wait();
Console.ReadLine();

Results:

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

i=’2′
j=’2′
k=’2′

What happened here is that all the loops completed before any of the tasks had a chance to run (all the loops incrementing incremented to 2 before failing the for condition).

 

We can get around the problem a couple of ways, first we can reassign the values to an unchanging variable that shares the same scope as our Task. This will ensure that when ours Tasks are created the values that they use will be assigned uniquely for that instance. If we choose this route then our code will look like this:

List<Task> tasks = new List<Task>();

for (int i = 0; i < 2; i++)
{
	for (int j = 0; j < 2; j++)
	{
		for (int k = 0; k < 2; k++)
		{
			int I = i;
			int J = j;
			int K = k;

			tasks.Add(Task.Factory.StartNew(() =>
			{
				Console.WriteLine(string.Format("i='{1}'{0}j='{2}'{0}k='{3}'{0}{0}", Environment.NewLine, I, K, J));
			}));
		}
	}
}

Results:

i=’0′
j=’0′
k=’0′

i=’0′
j=’1′
k=’0′

i=’0′
j=’1′
k=’1′

i=’1′
j=’0′
k=’0′

i=’1′
j=’1′
k=’0′

i=’1′
j=’0′
k=’1′

i=’0′
j=’0′
k=’1′

i=’1′
j=’1′
k=’1′

Though the Tasks may not have completed in order, all the tasks are present and all the values have been accounted for.

 

Finally, my favourite solution, passing all the variables as an Anonymous Type to the Task [1]. Within the Task the object will have to be casted to a dynamic type to access all the Anonymous Type’s members [2]. The reason why I like this solution is that it lends itself well to working with more complicated data types without having to reassign a large number of variables locally. For our above example our solution will look like this:

List<Task> tasks = new List<Task>();

for (int i = 0; i < 2; i++)
{
    for (int j = 0; j < 2; j++)
    {
        for (int k = 0; k < 2; k++)
        {
            tasks.Add(Task.Factory.StartNew((Object obj) =>
            {
                var data = (dynamic)obj;
                Console.WriteLine(string.Format("i='{1}'{0}j='{2}'{0}k='{3}'{0}{0}", Environment.NewLine, data.i, data.k, data.j));
            }, new { i = i, j = j, k = k }));
        }
    }
}

Task.WaitAll(tasks.ToArray());
Console.ReadLine();

And the results will be similar to those above.

 

Paul

 

References

 

[1] Anonymous Types (C# Programming Guide). MSDN Library.

[2] Using Type dynamic (C# Programming Guide). MSDN Library.

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Renaming File with Team Foundation Server API: Extending Workspace to Rename a Large Group of Files

The Problem

We have TFS check-in policies that will enforce naming conventions for certain files. The policy will allow the developer to programmatically rename any files that do not match the acceptable naming convention. Behind the scene the renaming is done using the WorkSpace.PendRename method [1]. This policy works fine for one or two files but for a large number of files we start having performance issues. Anyone who has experience renaming multiple files on TFS knows that it is a painfully slow ordeal. So how can we speed it up.

The Solution

I have solved this issue through the use of extending methods in the Microsoft.TeamFoundation.VersionControl.Client.Workspace class, and incorporating Task Parallelism [2][3][4]. Since the original Workspace.PendRename returns the number of files that it has renamed as an Int32, we want to make sure that we preserve that functionality.  I have only created a solution for the Workspace.PendRename(string,string) case since this is the only variation of the method that I am currently using; to expand this to all Workspace.PendRename implementations is trivial.

This the extended method:

public static class WorkSpaceExtension
{
    // Extend PendRename(String, String)
    public static int PendRename(this Workspace workspace,
        string[] oldPaths,
        string[] newPaths)
    {
        // Make sure that the oldPath and new Paths match
        if (oldPaths.Count() != newPaths.Count())
            throw new ArgumentException("Every oldPath must have corresponding newPath");

        // This list will contain all our tasks
        List pendRenameTasks = new List();

        // Loop throuh all newPath and oldPath pairs
        for (int i = 0; i < oldPaths.Count(); i++)
        {
            // Add each new task in out task container
            pendRenameTasks.Add(Task.Factory.StartNew((Object obj) =>
            {
                // We are going to pass the current old path and the
                // current new path into the path as an anonymous type
                var path = (dynamic)obj;
                return workspace.PendRename(path.oldPath, path.newPath);
            }, new { oldPath = oldPaths[i], newPath = newPaths[i] }));
        }

        // Wait for all tasks to complete
        Task.WaitAll(pendRenameTasks.ToArray());

        // Sum up the result of all the original PendRename method
        int taskCount=0;
        foreach (Task pendRenameTask in pendRenameTasks)
        {
            taskCount += pendRenameTask.Result;
        }

        // Return the number of file names changed
        return taskCount;
    }
}

References

[1] Workspace.PendRename Method. MSDN Library.

[2] Extension Methods (C# Programming Guide). MSDN Library.

[3] WorkSpace Class. MSDN Library.

[4] Extension Methods (C# Programming Guide). MSDN Library.