NicolBlog : c#

NicolTIP#006: how to show previous and next item using LINQ

NicolD — Wed, 03 Jun 2009 17:04:58 GMT

I had a table on SQL 2005 I am accessing it via LINQ. I had to select an item via a “where” condition (see below).

var item = (from snap in adc.Snapshots
where snap.file == image
select snap).First();

The problem was that I needed to identify the previous item and the next item too because we had to show them on a web page. Talking with some collgues of mine we found the following way that looks quite smart:

var previous = adc.Snapshots.OrderByDescending(s => s.InsertDateTime).Where(s => string.Compare(s.file, image) < 0).FirstOrDefault();
var item = adc.Snapshots.OrderBy(s => s.InsertDateTime).Where(s => s.file == image).SingleOrDefault();
var next = adc.Snapshots.OrderBy(s => s.InsertDateTime).Where(s => string.Compare(s.file, image) > 0).FirstOrDefault();

Update WinForm interface from a different thread

NicolD — Sat, 29 Dec 2007 20:56:00 GMT

Well, this is a typical issue when you have a thread that works (i.e. a Workflow) and a UI that needs to be updated.

Let assume that you have a WinFom and you need to update its windows Title from another thread. The other thread needs to call "UpdateTitle" public method of current Form instance. If you write the following code:

public class MainForm : Form
{
   ...
   
   void UpdateTitle(string newTitle)
   {
      this.Text = newTitle;         
   }

   ...
}

UpdateTitle will work only if you call the method from the UI thread.

In order to make this call thread safe, I found two ways, choose the one that fits your needs:-)

Method ONE: Using a delegate

public class MainForm : Form
{
   ...
   
   private delegate void UpdateTitleDelegate(string s);
   void UpdateTitle(string newTitle)
   {
   if (this.InvokeRequired)             
      this.Invoke(new UpdateTitleDelegate                 
         (this.UpdateTitle), newTitle);         
   else         
      {             
      this.Text = newTitle;         
      }
   }

   ...
}

Method TWO: using MethodInvoker

public class MainForm : Form
{
   ...
      
   void UpdateTitle(string newTitle)
   {
   if (this.InvokeRequired)             
      this.Invoke(new MethodInvoker(delegate()
      {
      this.Text = newTitle;    
      }));
   else         
      {             
      this.Text = newTitle;         
      }
   }
   ...
}

For more information have a look to Control.InvokeRequired property on MSDN.

Avoid threads deadlock

NicolD — Wed, 03 Oct 2007 16:28:00 GMT

On this month's issue of MSDN Magazine, Stephen Toub describe the class DdMonitor. DdMonitor implements almost all interfaces exposed by .NET System.Threading.Monitor class but includes deadlock detection capabilities too.

With the objective to make available a lock(...) keyword replacement, a static DdMonitor.Lock(...) is implemented too.

A System.Threading.Monitor replacement to avoid threads deadlocks

How to digitally sign a string

NicolD — Mon, 03 Sep 2007 11:22:00 GMT

The first step is to create a pair of key(pulic/private):

RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
string publicKey = RSA.ToXmlString(false);
string privateKey = RSA.ToXmlString(true);

Private key is required to sign the string. Public key is required to verify if the sign is valid or not.

Sequence required to create a sign is:

select a private key
select an HASH algorithm to create one starting from the string to sign (you'll sign the hash, not the string)
create a sign starting from the hash

As shown bellow:

RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
RSA.FromXmlString(privateKey);
RSAPKCS1SignatureFormatter RSAFormatter = new RSAPKCS1SignatureFormatter(RSA);
RSAFormatter.SetHashAlgorithm("SHA1");
SHA1Managed SHhash = new SHA1Managed();
byte[] SignedHashValue = RSAFormatter.CreateSignature( SHhash.ComputeHash(new UnicodeEncoding().GetBytes(stringToBeSigned)));
string signature = System.Convert.ToBase64String(SignedHashValue);

Sequence of operations needed to verify a signature is instead:

select the proper public key
select the HASH algorithm to create one starting from the string to be verified
veirfy the sign

as shown below:

RSACryptoServiceProvider RSA = new RSACryptoServiceProvider();
RSA.FromXmlString(publicKey);
RSAPKCS1SignatureDeformatter RSADeformatter = new RSAPKCS1SignatureDeformatter(RSA);
RSADeformatter.SetHashAlgorithm("SHA1");
SHA1Managed SHhash = new SHA1Managed();
if (RSADeformatter.VerifySignature( 
 SHhash.ComputeHash(new UnicodeEncoding().GetBytes(stringToBeVerified)),
 System.Convert.FromBase64String(signature))
 )
 {
 /// The signature is valid.
 }
else
 {
 /// The signature is not valid.
 }