For P/Invoke purposes like you&#39;ve listed, you should use `IntPtr.Zero` in place of `NULL`. Note that this is not equivalent to the C# `null` keyword, however.

You cannot assign null to a value-type. A reference-type can be null, as in, not referring to an object instance, but a value-type always has a value.

IntPtr.Zero is just a constant value that represents a null pointer.

Be aware that there is a bug (feature??) in C# &gt;= 2.0, where

    if (IntPtr.Zero == null)
    {
        // Won&#39;t enter here
    }

will compile correctly, but it won&#39;t ever enter in the `if`.

I opened an issue on the [github of roslyn](https://github.com/dotnet/roslyn/issues/18975) and they replied that they won&#39;t fix it because there are projects that are built with warnings-as-errors. Still there is a partial fix for this: there is a `strict` compilation mode that generates this warning:

    &lt;Features&gt;strict&lt;/Features&gt;

Is it possible to generate short GUID like in YouTube (N7Et6c9nL9w)? 

How can it be done? I want to use it in web app.

YouTube-like GUID

I was wondering what kind of exception should one throw for missing data. For example if an xml node doesn&#39;t contain data. It would be easy to &quot;**throw new Exception(...)**&quot; but this is not recommended. Another option would be to create a new exception class like **MissingDataException** or **InvalidDataException** but isn&#39;t there a built-in exception class for this case?

Exception for missing data

I am trying to setup `ReadFile` to run asynchronously and according to [MSDN][1], I need to set `lpNumberOfBytesRead` to `null`:
&gt; &quot;Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results.&quot;

For example, if I have the following:

      [DllImport(&quot;kernel32.dll&quot;, SetLastError = true, CharSet = CharSet.Auto)]
      public static extern bool ReadFile(
         IntPtr hFile,
         out byte[] aBuffer,
         int cbToRead,
         IntPtr cbThatWereRead,
         ref OVERLAPPED pOverlapped
      );

and I call it like this (with the intention of having the 4th parameter being null):

    Win32API.ReadFile(readHandle, out data_read, Win32API.BUFFER_SIZE, IntPtr.Zero, ref over_lapped);

will that be the same as calling it with null?  If not, what should I change in the declaration or in the function call itself?

I was also curious if I should be using `SafeHandle` or `HandleRef` instead of `IntPtr` for the `hFile` reference?  I know to make sure that I close the handle with `CloseHandle(IntPtr)` when I&#39;m done with it, just not sure if there is any othe reason to use the other two options over `IntPtr`.  I am also tryingn to avoid using unsafe code.

EDIT: As it turns out, I shouldnt be setting the fourth parameter to `IntPtr.Zero` anyway, because even though I am running asynchronously, it could still return right away.  See [Asynchronous Disk I/O][2]. Ahh, I love contradicting stories.

[1]: http://msdn.microsoft.com/en-us/library/aa365467(VS.85).aspx
[2]: http://support.microsoft.com/kb/156932

Is IntPtr.Zero equivalent to null?

I am trying to setup <code>ReadFile</code> to run asynchronously and according to <a href="http://msdn.microsoft.com/en-us/library/aa365467(VS.85).aspx" target="_blank" rel="noopener noreferrer">MSDN</a>, I need to set <code>lpNumberOfBytesRead</code> to <code>null</code>:
> "Use NULL for this parameter if this is an asynchronous operation to avoid potentially erroneous results."
For example, if I have the following:
<pre><code class="hljs language-csharp"> [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
 public static extern bool ReadFile(
 IntPtr hFile,
 out byte[] aBuffer,
 int cbToRead,
 IntPtr cbThatWereRead,
 ref OVERLAPPED pOverlapped
 );
</code></pre>
and I call it like this (with the intention of having the 4th parameter being null):
<pre><code class="hljs language-csharp">Win32API.ReadFile(readHandle, out data_read, Win32API.BUFFER_SIZE, IntPtr.Zero, ref over_lapped);
</code></pre>
will that be the same as calling it with null? If not, what should I change in the declaration or in the function call itself?
I was also curious if I should be using <code>SafeHandle</code> or <code>HandleRef</code> instead of <code>IntPtr</code> for the <code>hFile</code> reference? I know to make sure that I close the handle with <code>CloseHandle(IntPtr)</code> when I'm done with it, just not sure if there is any othe reason to use the other two options over <code>IntPtr</code>. I am also tryingn to avoid using unsafe code.
EDIT: As it turns out, I shouldnt be setting the fourth parameter to <code>IntPtr.Zero</code> anyway, because even though I am running asynchronously, it could still return right away. See <a href="http://support.microsoft.com/kb/156932" target="_blank" rel="noopener noreferrer">Asynchronous Disk I/O</a>. Ahh, I love contradicting stories.

Is it definitely a good practice to use it? 

What are some possible situations in a project that need reflection?

Why is the use of reflection in .NET recommended?

In particular 

 1. Create a function to take an array and an index as parameters.
 2. Create a n element array.
 3. Create a n count loop.
 4. Inside the loop on a new thread assign a new instance of the object to the array using the indexer passed in. 

I know how to manage the threads etc. I am interested in know if this is thread safe way of doing something.
 

     class Program
    {
        // bogus object
        class SomeObject
        {
            private int value1;
            private int value2;

            public SomeObject(int value1, int value2)
            {
                this.value1 = value1;
                this.value2 = value2;
            }
        }
       
        static void Main(string[] args)
        {

            var s = new SomeObject[10];
            var threads = Environment.ProcessorCount - 1;
            var stp = new SmartThreadPool(1000, threads, threads);
            for (var i = 0; i &lt; 10; i++)
            {
                stp.QueueWorkItem(CreateElement, s, i);
            }

        }

        static void CreateElement(SomeObject[] s, int index)
        {
            s[index] = new SomeObject(index, 2);
        }
    }

Are C# arrays thread safe?

Say I have:

    IList&lt;Person&gt; people = new List&lt;Person&gt;();

And the person object has properties like FirstName, LastName, and Gender.

How can I convert this to a list of properties of the Person object.  For example, to a list of first names.

    IList&lt;string&gt; firstNames = ???

C#: How to convert a list of objects to a list of a single property of that object?

I have data in a `DataReader` which I want to be converted to a `List&lt;T&gt;`.
What is a possible simple solution for this?

For e.g. in CustomerEntity class, I have CustomerId and CustomerName properties.If my DataReader returns these two columns as data, then how can I convert it into `List&lt;CustomerEntity&gt;`.

How can I easily convert DataReader to List&lt;T&gt;?

I want to execute a command from an Ant buildfile, for each file in a directory.  
I am looking for a platform-independent solution.

How do I do this?

*Sure, I could write a script in some scripting language,
but this would add further dependencies to the project.*

Ant: How to execute a command for each file in directory?

I want to read *only* the first line of a file using Ruby in the fastest, simplest, most idiomatic way possible. What&#39;s the best approach?

(Specifically: I want to read the git commit UUID out of the REVISION file in my latest Capistrano-deployed Rails directory, and then output that to my &lt;meta name=&quot;revision&quot;&gt; tag. This will let me see at an http-glance what version is deployed to my server. If there&#39;s an entirely different &amp; better way to do this, please let me know.)

Reading the first line of a file in Ruby

Reading through the existing unit testing related threads here on Stack Overflow, I couldn&#39;t find one with a clear answer about how to unit test file I/O operations. I have only recently started looking into unit testing, having been previously aware of the advantages but having difficulty getting used to writing tests first. I have set up my project to use NUnit and Rhino Mocks and although I understand the concept behind them, I&#39;m having a little trouble understanding how to use Mock Objects.

Specifically I have two questions that I would like answered. First, what is the proper way to unit test file I/O operations? Second, in my attempts to learn about unit testing, I have come across dependency injection. After getting Ninject set up and working, I was wondering whether I should use DI within my unit tests, or just instantiate objects directly.

Unit Testing File I/O

Say I have something like this:

    new File(&quot;test&quot;).eachFile() { file-&gt;  
    println file.getName()  
    }

This prints the full filename of every file in the `test` directory.  Is there a Groovy way to get the filename without any extension?  (Or am I back in regex land?)

Does groovy have an easy way to get a filename without the extension?

When using files in Ruby, what is the difference between the `r+` and `w+` modes? What about the `a+` mode?

Difference between the access modes of the `File` object (ie. w+, r+)

I am creating an ActionResult in ASP.Net MVC to serve images. With Session state enabled, IIS will only handle one request at a time from the same user. (This is true not just in MVC.) 

Therefore, on a page with multiple images calling back to this Action, only one image request can be handled at a time. It&#39;s synchronous.

I&#39;d like this image Action to be asynchronous -- I&#39;d like multiple image requests to each execute without needing the previous one to complete. (If the images were just static files, IIS would serve them up this way.) 

So, I&#39;d like to disable Session just for calls to that Action, or to specify that certain requests do not have Session state. Anyone know how this is done in MVC? Thanks!

Disable Session state per-request in ASP.Net MVC

I have been trying to solve this &quot;Concurrent Programming&quot; exam exercise (in C#):

&gt; Knowing that `Stream` class contains `int Read(byte[] buffer, int offset, int size)` and `void Write(byte[] buffer, int offset, int size)` methods, implement in C# the `NetToFile` method that copies all data received from `NetworkStream net` instance to the `FileStream file` instance. To do the transfer, use asynchronous reads and synchronous writes, avoiding one thread to be blocked during read operations. The transfer ends when the `net` read operation returns value 0. To simplify, it is not necessary to support controlled cancel of the operation.

    void NetToFile(NetworkStream net, FileStream file);

I&#39;ve been trying to solve this exercise, but I&#39;m struggling with a question related with the question itself. But first, here is my code:

    public static void NetToFile(NetworkStream net, FileStream file) {
        byte[] buffer = new byte[4096]; // buffer with 4 kB dimension
        int offset = 0; // read/write offset
        int nBytesRead = 0; // number of bytes read on each cycle
    
        IAsyncResult ar;
        do {
            // read partial content of net (asynchronously)
            ar = net.BeginRead(buffer,offset,buffer.Length,null,null);
            // wait until read is completed
            ar.AsyncWaitHandle.WaitOne();
            // get number of bytes read on each cycle
            nBytesRead = net.EndRead(ar);
    
            // write partial content to file (synchronously)
            fs.Write(buffer,offset,nBytesRead);
            // update offset
            offset += nBytesRead;
        }
        while( nBytesRead &gt; 0);
    }

The question I have is that, in the question statement, is said:
&gt; To do the transfer, use asynchronous
&gt; reads and synchronous writes, avoiding
&gt; one thread to be blocked during read
&gt; operations

I&#39;m not really sure if my solution accomplishes what is wanted in this exercise, because I&#39;m using `AsyncWaitHandle.WaitOne()` to wait until the asynchronous read completes.

On the other side, I&#39;m not really figuring out what is meant to be a &quot;non-blocking&quot; solution in this scenario, as the `FileStream` write is meant to be made synchronously... and to do that, I have to wait until `NetworkStream` read completes to proceed with the `FileStream` writing, isn&#39;t it?

Can you, please, help me out with this?

-------------------------------------

[ EDIT 1 ] **Using *callback* solution**

Ok, if I understood what [Mitchel Sellers][1] and [willvv][2] replied, I&#39;ve been counseled to use a callback method to turn this into a &quot;non-blocking&quot; solution. Here is my code, then:

    byte[] buffer; // buffer
    
    public static void NetToFile(NetworkStream net, FileStream file) {
        // buffer with same dimension as file stream data
        buffer = new byte[file.Length];
        //start asynchronous read
        net.BeginRead(buffer,0,buffer.Length,OnEndRead,net);
    }
    
    //asynchronous callback
    static void OnEndRead(IAsyncResult ar) {
        //NetworkStream retrieve
        NetworkStream net = (NetworkStream) ar.IAsyncState;
        //get number of bytes read
        int nBytesRead = net.EndRead(ar);
        
        //write content to file
        //... and now, how do I write to FileStream instance without
        //having its reference??
        //fs.Write(buffer,0,nBytesRead);
    }

As you may have noticed, I&#39;m stuck on the callback method, as I don&#39;t have a reference to the `FileStream` instance where I want to invoke the &quot;Write(...)&quot; method.

Additionally, this is not a thread-safe solution, as the `byte[]` field is exposed and may be shared among concurrent `NetToFile` invocations. I don&#39;t know how to solve this problem without exposing this `byte[]` field in the outer-scope... and I&#39;m almost sure it may not be exposed this way.

I don&#39;t want to use a lambda or anonymous method solution, because that&#39;s not in the curriculum of &quot;Concurrent Programing&quot; course.

  [1]: https://stackoverflow.com/users/13279/mitchel-sellers
  [2]: https://stackoverflow.com/users/84167/willvv

.NET Asynchronous stream read/write

Aside from the Microsoft documentation, is there a good introduction and tutorial to the Microsoft Reactive (Rx) framework?

Also, what is a good example (with code) that Reactive makes easier of a programming problem that is challenging to solve using conventional asynchronous coding techniques?


Good introduction to the .NET Reactive Framework

If I set up multiple event handlers, like so:

    _webservice.RetrieveDataCompleted += ProcessData1;
    _webservice.RetrieveDataCompleted += ProcessData2;

what order are the handlers run when the event `RetrieveDataCompleted` is fired? Are they run in the same thread and sequentially in the order that are registered?

Order of event handler execution

A (long) while ago I wrote a web-spider that I multithreaded to enable concurrent requests to occur at the same time.  That was in my Python youth, in the days before I knew about the [GIL][1] and the associated woes it creates for multithreaded code (IE, most of the time stuff just ends up serialized!)...

I&#39;d like to rework this code to make it more robust and perform better.  There are basically two ways I could do this: I could use the new [multiprocessing module][2] in 2.6+ or I could go for a reactor / event-based model of some sort.  I would rather do the later since it&#39;s far simpler and less error-prone.

So the question relates to what framework would be best suited to my needs.  The following is a list of the options I know about so far:

- [Twisted][3]: The granddaddy of Python reactor frameworks: seems complex and a bit bloated however.  Steep learning curve for a small task.
- [Eventlet][4]: From the guys at [lindenlab][5].  Greenlet based framework that&#39;s geared towards these kinds of tasks. I had a look at the code though and it&#39;s not too pretty: non-pep8 compliant, scattered with prints (why do people do this in a framework!?), API seems a little inconsistent.
- [PyEv][6]: Immature, doesn&#39;t seem to be anyone using it right now though it is based on libevent so it&#39;s got a solid backend.
- [asyncore][7]: From the stdlib: &#252;ber low-level, seems like a lot of legwork involved just to get something off the ground.
- [tornado][8]: Though this is a server oriented product designed to server dynamic websites it does feature an [async HTTP client][9] and a simple [ioloop][10].  Looks like it could get the job done but not what it was intended for. 
[edit: doesn&#39;t run on Windows unfortunately, which counts it out for me - its a requirement for me to support this lame platform]

Is there anything I have missed at all?  Surely there must be a library out there that fits the sweet-spot of a simplified async networking library!

[edit: big thanks to [intgr][13] for his pointer to [this page][14].  If you scroll to the bottom you will see there is a really nice list of projects that aim to tackle this task in one way or another.  It seems actually that things have indeed moved on since the inception of Twisted: people now seem to favour a [co-routine][15] based solution rather than a traditional reactor / callback oriented one.  The benefits of this approach are clearer more direct code: I&#39;ve certainly found in the past, especially when working with [boost.asio][16] in C++ that callback based code can lead to designs that can be hard-to-follow and are relatively obscure to the untrained eye.  Using co-routines allows you to write code that looks a little more synchronous at least.  I guess now my task is to work out which one of these many libraries I like the look of and give it a go!  Glad I asked now...]

[edit: perhaps of interest to anyone who followed or stumbled on this this question or cares about this topic in any sense: I found a really great writeup of the current state of [the available tools][17] for this job]


  [1]: http://www.dabeaz.com/python/GIL.pdf
  [2]: http://docs.python.org/library/multiprocessing.html
  [3]: http://twistedmatrix.com/trac/
  [4]: http://eventlet.net/
  [5]: http://lindenlab.com/
  [6]: http://code.google.com/p/pyev/
  [7]: http://docs.python.org/library/asyncore.html
  [8]: http://www.tornadoweb.org/
  [9]: http://github.com/facebook/tornado/blob/master/tornado/httpclient.py
  [10]: http://github.com/facebook/tornado/blob/master/tornado/ioloop.py
  [11]: http://doc.scrapy.org/intro/overview.html
  [12]: http://dev.scrapy.org/wiki/SEP-004
  [13]: https://stackoverflow.com/users/177663/intgr
  [14]: http://code.google.com/p/cogen/
  [15]: http://en.wikipedia.org/wiki/Coroutine
  [16]: http://www.boost.org/doc/libs/1_41_0/doc/html/boost_asio.html
  [17]: http://nichol.as/asynchronous-servers-in-python

Content Type	Original Author	Original Content on Stackoverflow
Question	SwDevMan81	View Question on Stackoverflow
Solution 1 - C#	Michael	View Answer on Stackoverflow
Solution 2 - C#	Yannick Motton	View Answer on Stackoverflow
Solution 3 - C#	xanatos	View Answer on Stackoverflow

Is IntPtr.Zero equivalent to null?

C# Problem Overview

C# Solutions

Solution 1 - C#

Solution 2 - C#

Solution 3 - C#

Exception for missing data

YouTube-like GUID

Attributions