Timeout expired. The timeout period elapsed prior to obtaining a connection from the pool. This may have occurred because all pooled connections were in use and max pool size was reached.

My Research
On the website in question, I use LINQ-to-SQL heavily to perform data operations. Naturally, I suspected that I wasn’t disposing of my DataContext objects correctly, causing me to exceed the number of connections available in the pool. After doing a little research on the subject, I read in several articles (some by folks who are much more knowledgeable than myself) that I shouldn’t have to dispose of them. They all said the following code is just fine:

CORDataContext mcon = new CORDataContext();
COR_Employee emp = mcon.COR_Employees.FirstOrDefault();

In fact, I should be able to put that into an infinite loop and never get the exception I was encountering. According to the experts, LINQ is not only smart enough to automatically open and close the connection for me, but also smart enough to dispose of the object when I’m done. No need for a “using” around my code or even a Connection.Close().

An Exception to the Rule

I noticed, though, that my code was slightly different. In my specific setup, I have a central database and a LOT of other databases I might want to connect to. So, to take advantage of connection pooling more, I always connect to the central database first and change to whatever catalog I need to access. In doing so, it is necessary to manually open the connection, like the following:

CORDataContext mcon = new CORDataContext();mcon.Connection.Open(); mcon.Connection.ChangeDatabase("OtherDB");
COR_Employee emp = mcon.COR_Employees.FirstOrDefault();

My Testing

I began to theorize that manually opening the connection in this way keeps the connection alive after I’ve run my query. I confirmed the belief by trying to run the following code:

for (int i = 0; i < 1000; i++) {

    CORDataContext mcon = new CORDataContext();
    COR_Employee emp = mcon.COR_Employees.FirstOrDefault();
    mcon.Connection.Open();
    mcon.Connection.ChangeDatabase("OtherDB");

    Console.WriteLine("" + (i + 1) + " / 1000");
}

I got the exception after 230 or so connections. By default there are 100 connections available in the pool, but the garbage collector kicked in and helped give me a buffer before I ran out. So, from that I can conclude that manually opened DataContext connections will stay open until the connection is closed or the DataContext is disposed.

The Solution

Once I fully understood the problem, the fix was common sense:

for (int i = 0; i < 1000; i++) {

    using (CORDataContext mcon = new CORDataContext()) {
        COR_Employee emp = mcon.COR_Employees.FirstOrDefault();
        mcon.Connection.Open();
        mcon.Connection.ChangeDatabase("OtherDB");

        Console.WriteLine("" + (i + 1) + " / 1000");
    }
}

Adding a “using” to automatically dispose of my DataContext did the trick, and this loop now finishes in about 3 seconds (even with the overhead of writing to the console). I hope this helps someone else, because it’s been plaguing me for weeks!

With .Net 3.5, Microsoft released the DataContractSerializer. It has a variety of uses, but at the very least it’s an improvement over the XmlSerializer. In Cash Tracker we started out by using the XmlSerializer to save files, but quickly found it inadequate, the reason being XmlSerializer does not play well with mixed access properties.

For an example, let’s look again at our Person class:

public class Person {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Initials { get; private set; }

    private Person() {
    }
    public Person(string first, string last) {
        this.FirstName = first;
        this.LastName = last;

        this.Initials = String.Format("{0}.{1}.", FirstName[0], LastName[0]);
    }
}

Notice I’ve added a new property with a private mutator. Also, I added a constructor to take advantage of the new property and made my default constructor private. Now when we try to serialize using the XmlSerializer we get an error:

Unable to generate a temporary class (result=1).

error CS0200: Property or indexer ‘Person.Initials’ cannot be assigned to — it is read only

What to do?

In .Net 3.5, to get around this problem, Microsoft suggests using the DataContractSerializer, so we will investigate switching to the new technology here. First, we need a reference to System.Runtime.Serialization:

Once that’s done it’s actually pretty simply converting our old code. The new code looks like this:

static void XmlSerializePerson(Person myPerson, string fileName) {
    DataContractSerializer xs = new DataContractSerializer(typeof(Person));

    using (FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.Write)) {
        xs.WriteObject(fs, myPerson);
    }
}

static Person XmlDeserializePerson(string fileName) {
    DataContractSerializer xs = new DataContractSerializer(typeof(Person));

    using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read)) {
        return (Person)xs.ReadObject(fs);
    }
}

Notice we simply re-used the old methods, changing the object and methods used to serialize our Person. Our code will now execute as expected:

Download the Solution

You can download a zip file of the solution by clicking the link below.

DataContractSerializerDemo.zip

Class: Person

Below is a simple class Person. My person has a name and an age:

public class Person {
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public Person() {
    }
}

The XmlSerializer requires that I leave an empty constructor in my class. I can make the constructor private if I don’t want it used for other things. Now, let’s create an instance of a Person.

class Program {
    static void Main(string[] args) {
        Person myPerson = new Person();
        myPerson.FirstName = "John";
        myPerson.LastName = "Smith";
    }
}

How to serialize our class

I will add a couple methods now to handle serializating and deserializing myPerson.

static void XmlSerializePerson(Person myPerson, string fileName) {
    XmlSerializer xs = new XmlSerializer(typeof(Person));

    using (FileStream fs = new FileStream(fileName, FileMode.Create, FileAccess.Write)) {
        xs.Serialize(fs, myPerson);
    }
}

static Person XmlDeserializePerson(string fileName) {
    XmlSerializer xs = new XmlSerializer(typeof(Person));

    using (FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read)) {
        return (Person)xs.Deserialize(fs);
    }
}

Code in action

Now, saving myPerson is simple:

static void Main(string[] args) {
    Person myPerson = new Person();
    myPerson.FirstName = "John";
    myPerson.LastName = "Smith";

    XmlSerializePerson(myPerson, @"C:\MyPerson.xml");
}

Let’s take a look at the resulting XML:

<?xml version="1.0"?>
<Person xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <FirstName>John</FirstName>
  <LastName>Smith</LastName>
</Person>

Now, let’s deserialize our person and display its data:

static void Main(string[] args) {
    Person myPerson = XmlDeserializePerson(@"C:\MyPerson.xml");

    Console.WriteLine("Name: " + myPerson.LastName + ", " + myPerson.FirstName);
    Console.ReadLine();
}

The result:

As you can see, the XmlSerializer did its job.

Download the Solution

You can download a zip file of the solution by clicking the link below.

XmlSerializerDemo.zip

using System;

namespace VersionChecker {
    class Program {
        static void Main(string[] args) {
            Version v1 = new Version("1.0.0.0");
            Version v2 = new Version("1.0.0.1");

            if (v1 > v2) {
                Console.WriteLine("v1 is higher");
            } else {
                Console.WriteLine("v2 is higher");
            }

            Console.ReadLine();
        }
    }
}

Here’s the result:

Here’s the method:

public static byte[] AppendImageToPdf(byte[] pdf, byte[] img, Point position, double scale)
{
    using (System.IO.MemoryStream msPdf = new System.IO.MemoryStream(pdf))
    {
        using (System.IO.MemoryStream msImg = new System.IO.MemoryStream(img))
        {
            System.Drawing.Image image = System.Drawing.Image.FromStream(msImg);
            PdfSharp.Pdf.PdfDocument document = PdfSharp.Pdf.IO.PdfReader.Open(msPdf);
            PdfSharp.Pdf.PdfPage page = document.Pages[0];
            PdfSharp.Drawing.XGraphics gfx = PdfSharp.Drawing.XGraphics.FromPdfPage(page);
            PdfSharp.Drawing.XImage ximg = PdfSharp.Drawing.XImage.FromGdiPlusImage(image);

            gfx.DrawImage(
                ximg,
                position.X,
                position.Y,
                ximg.Width * scale,
                ximg.Height * scale
            );

            using (System.IO.MemoryStream msFinal = new System.IO.MemoryStream())
            {
                document.Save(msFinal);
                return msFinal.ToArray();
            }

        }
    }
}

I will now take you briefly through a pretty simple control that has saved hours of my time. First, you may want to download the sample code, images, and style sheet at the end of this article.

Using MessageControl
The code here is pretty simple. If you already have a helper or control library you reference from most of your web projects, you may want to add this file to it. Once that is done, just register the assembly on a page (or in your web.config), implement the css classes, add images to your images folder, and start using it. For example:

<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="Default.aspx.cs"
    Inherits="MessageControlTest._Default" %>
<%@ Register Assembly="Controls" TagPrefix="CC" Namespace="Controls" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
    "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
    <head runat="server">
        <link href="style.css" rel="stylesheet" type="text/css" />
        <title></title>
    </head>
    <body><form id="form1" runat="server"><div>
            Our test MessageControl lives below:
            <CC:MessageControl ID="msgInfo" runat="server" IconType="Info"
                MessageText="Testing the Message Control" />
            <CC:MessageControl ID="msgWarn" runat="server" IconType="Warn"
                MessageText="Testing the Message Control" />
            <CC:MessageControl ID="msgError" runat="server" IconType="Error"
                MessageText="Testing the Message Control" />
    </div></form></body>
</html>

Yields the following:

Download the Solution

You can download a zip file of the sample solution by clicking the link below.

MessageControl.zip