C# – the difference in .NET between developing a multithreaded application and parallel programming

cconcurrencymultithreadingparallel-processingtask-parallel-library

Recently I've read a lot about parallel programming in .NET but I am still confused by contradicting statements over the texts on this subject.

For example, tThe popup (upon pointing a mouse on tag's icon) description of the stackoverflow.com task-parallel-library tag:

"The Task Parallel Library is part of .NET 4. It is a set of APIs tpo
enable developers to program multi-core shared memory processors"

Does this mean that multi-core-d and parallel programming applications impossible using prior versions of .NET?

Do I control a multicore/parallel usage/ditribution between cores in .NET multithreaded application?

How can I identify a core on which a thread to be run and attribute a thread to a specific core?

What has the .NET 4.0+ Task Parallel Library enabled that was impossible to do in previous versions of .NET?

Update:
Well, it was difficult to formulate specific questions but I'd like to better understand:

What is the difference in .NET between developing a multi-threaded application and parallel programming?

So far, I could not grasp the difference between them

Update2:
MSDN "Parallel Programming in the .NET Framework" starts from version .NET 4.0 and its article Task Parallel Library tells:

"Starting with the .NET Framework 4, the TPL is the preferred way to
write multithreaded and parallel code"

Can you give me hints how to specifically create parallel code in pre-.NET4 (in .NET3.5), taking into account that I am familiar with multi-threading development?

Best Answer

I see "multithreading" as just what the term says: using multiple threads.

"Parallel processing" would be: splitting up a group of work among multiple threads so the work can be processed in parallel.

Thus, parallel processing is a special case of multithreading.

Does this mean that multi-core-d and parallel programming applications impossible using prior versions of .NET?

Not at all. You could do it using the Thread class. It was just much harder to write, and much much harder to get it right.

Do I control a multicore/parallel usage/ditribution between cores in .NET multithreaded application?

Not really, but you don't need to. You can mess around with processor affinity for your application, but at the .NET level that's hardly ever a winning strategy.

The Task Parallel Library includes a "partitioner" concept that can be used to control the distribution of work, which is a better solution that controlling the distribution of threads over cores.

How can I identify a core on which a thread to be run and attribute a thread to a specific core?

You're not supposed to do this. A .NET thread doesn't necessarily correspond with an OS thread; you're at a higher level of abstraction than that. Now, the default .NET host does map threads 1-to-1, so if you want to depend on an undocumented implementation detail, then you can poke through the abstraction and use P/invoke to determine/drive your processor affinity. But as noted above, it's not useful.

What has the .NET 4.0+ Task Parallel Library enabled that was impossible to do in previous versions of .NET?

Nothing. But it sure has made parallel processing (and multithreading) much easier!

Can you give me hints how to specifically create parallel code in pre-.NET4 (in .NET3.5), taking into account that I am familiar with multi-threading development?

First off, there's no reason to develop for that platform. None. .NET 4.5 is already out, and the last version (.NET 4.0) supports all OSes that the next older version (.NET 3.5) did.

But if you really want to, you can do simple parallel processing by spinning up Thread objects or BackgroundWorkers, or by queueing work directly to the thread pool. All of these approaches require more code (particularly around error handling) than the Task type in the TPL.

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

C# – the difference between const and readonly in C#

Apart from the apparent difference of

having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.

There is a subtle difference. Consider a class defined in AssemblyA.

public class Const_V_Readonly
{
  public const int I_CONST_VALUE = 2;
  public readonly int I_RO_VALUE;
  public Const_V_Readonly()
  {
     I_RO_VALUE = 3;
  }
}

AssemblyB references AssemblyA and uses these values in code. When this is compiled:

in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.

So if you are confident that the value of the constant won't change, use a const.

public const int CM_IN_A_METER = 100;

But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.

public readonly float PI = 3.14;

Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner

Best Answer

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