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F# – Adding custom indexer and slicer to your type

Indexer

If your have a type that represents a collection of values, adding a custom indexer gives you a natural way to index directly into the object using the .[ ] operator.

Take this simple Calendar class for instance, which keeps a map (F# equivalent of a Dictionary<TKey, TValue>) of notes against DateTime values:

By adding a custom indexer here I’m now able to write code like this:

Also, as you may have noticed already, F# allows you to use non-integer parameters in your indexer! In this case, I’m using a tuple of int * string * int. You can use this indexer from your C# code too:

One-Dimensional Slicer

That’s pretty cool, but we can go a step further by allowing you to use slicers on our types too. To allow users to get all the notes associated with a range of years, e.g. from 1999 to 2009, let’s add a slicer:

So now we can do this:

Two-Dimensional Slicer

Pretty awesome, right? But what if we want to refine our search criteria even more and let you specify a year range as well a range of months, e.g. get me all notes for April/May/June in the years 1999 to 2009. Thankfully, F# lets you define a two-dimensional slicer too:

This two-dimensional slicer allows me to query the calendar with a year range as well as a month range:

As you can see, indexers and slicers give the consumers of your code a much more intuitive way to interact with the data encapsulated in your type. Keep in mind though, that F# does not allow you to add slicer with more than two dimensions, but I think two-dimensional slicers should be good enough for most day-to-day requirements.

Update 2011/11/08:

Swapped out my verbose optional argument handling with defaultArg as Arseny Kapoulkine pointed out in the comments.

Find out which process is listening on a port

I needed to find out which process is listening on a port the other day and a quick search on SO showed me the way and all you need is to run:

netstat –aon | find /i “listening”

and you will get a list of the ports and the PID of the process listening on that port:

Sweet!

.Net Tips – Getting the default value of a type outside of generics

To get the default value of a type, you’ve probably used the default keyword in .Net already:

   1: var defaultInt = default(int);        // 0

   2: var defaultObj = default(string);     // null

However, the use of the default keyword requires a type name to be specified at compile time, so you won’t be able to use it on a type which you discover at runtime. Instead, you will need to do something along the line of:

   1: public object GetDefaultValue(Type type)

   2: {

   3:     // Handle value types and reference types differently

   4:     if (type.IsValueType)

   5:     {

   6:         return Activator.CreateInstance(type);

   7:     }

   8:     else

   9:     {

  10:         return null;

  11:     }

  12: }

It’ll yield the same result as using the default keyword:

   1: var defaultInt = GetDefaultValue(typeof(int));        // 0

   2: var defaultObj = GetDefaultValue(typeof(string));     // null

.Net Tips – Finding GAC

By default the GAC folder in Windows is located at %windir%\assembly, you can find all the registered DLLs in that folder. Whilst you can open it in windows explorer and view it like any other folder, it is a somewhat special and allows you to have different versions of the same DLL registered in the GAC.

Open up DOS prompt and navigate to the GAC folder, for instance:

and you can see that the GAC is actually a folder inside the assembly folder, and drilling a little deeper reveals that each DLL has its own folder which contains all the registered versions, each as a folder that contains the actual DLL inside:

.Net 4

For .Net 4, the GAC location is now %windir%\Microsoft.Net\assembly.

Global Assembly Cache Tool

You can use gacutil.exe from the command line to view (e.g. gacutil /l), add (e.g. gacutil /i SomeAssembly.dll) or remove (e.g. gacutil /u SomeAssembly) the contents of the GAC, this of course, supports multiple versions of the same assembly too.

S3 — Use using block to get the stream

When you’re using the Amazon S3 client, have you come across the occasional exception that says something like one of these exception messages:

“The request was aborted: The connection was closed unexpectedly”

“Unable to read data from the transport connection: A blocking operation was interrupted by a call to WSACancelBlockingCall”

“Unable to read data from the transport connection: An established connection was aborted by the software in your host machine “

If you do, then you’re probably attempting to return the response stream directly back to the rest of your application with something like this:

   1: var response = _s3Client.GetObject(request);

   2: return response.ResponseStream;

However, because the stream is coming from the Amazon S3 service and is fed to your code in chunks, your code needs to ensure that the connection to S3 stays open until all the data has been received. So as mentioned in the S3 documentation (which incidentally, most of us don’t read in great details…) here, you should be wrapping the response you get from the GetObject method in a using clause.

Depends on what it is you want to do with the stream, you might have to handle it differently. For instance, if you just want to read the string content of a text file, you might want to do this:

   1: using (var response = _s3Client.GetObject(request))

   2: {

   3:     using (var reader = new StreamReader(response.ResponseStream))

   4:     {

   5:         return reader.ReadToEnd();

   6:     }

   7: }

Alternatively, if you want to return the response stream itself, you’ll need to first load the stream in its entirety and return the loaded stream. Unfortunately, at the time of this writing, the AWSSDK library still hasn’t been migrated to .Net 4 and therefore doesn’t have the uber useful CopyTo method added in .Net 4, so you will most likely have to do the heavy lifting yourself and read the data out manually into a memory stream:

   1: using (var response = _s3Client.GetObject(request))

   2: {

   3:     var binaryData = ReadFully(response.ResponseStream);

   4:     return new MemoryStream(binaryData);

   5: }

6:

   7: /// <summary>

   8: /// See Jon Skeet's article on reading binary data:

   9: /// http://www.yoda.arachsys.com/csharp/readbinary.html

  10: /// </summary>

  11: public static byte[] ReadFully (Stream stream, int initialLength = -1)

  12: {

  13:     // If we've been passed an unhelpful initial length, just

  14:     // use 32K.

  15:     if (initialLength < 1)

  16:     {

  17:         initialLength = 32768;

  18:     }

19:

  20:     byte[] buffer = new byte[initialLength];

  21:     int read=0;

22:

  23:     int chunk;

  24:     while ( (chunk = stream.Read(buffer, read, buffer.Length-read)) > 0)

  25:     {

  26:         read += chunk;

27:

  28:         // If we've reached the end of our buffer, check to see if there's

  29:         // any more information

  30:         if (read == buffer.Length)

  31:         {

  32:             int nextByte = stream.ReadByte();

33:

  34:             // End of stream? If so, we're done

  35:             if (nextByte==-1)

  36:             {

  37:                 return buffer;

  38:             }

39:

  40:             // Nope. Resize the buffer, put in the byte we've just

  41:             // read, and continue

  42:             byte[] newBuffer = new byte[buffer.Length*2];

  43:             Array.Copy(buffer, newBuffer, buffer.Length);

  44:             newBuffer[read]=(byte)nextByte;

  45:             buffer = newBuffer;

  46:             read++;

  47:         }

  48:     }

  49:     // Buffer is now too big. Shrink it.

  50:     byte[] ret = new byte[read];

  51:     Array.Copy(buffer, ret, read);

  52:     return ret;

  53: }

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