1. Introduction to Exceptions

An Exception is an object delivered by the Exception class. This Exception class is exposed by the System.Exception namespace. Exceptions are used to avoid system failure in an unexpected manner. Exception handles the failure situation that may arise. All the exceptions in the .NET Framework are derived from the System.Exception class.
To understand exception, we need to know two basic things:

1. Somebody sees a failure situation that happened and throws an exception by packing the valid information.
2. Somebody who knows that a failure may happen catches the exception thrown. We call it Exception Handler.

Below is a simple example, which shows what happens when an Exception is not handled:

class ExceptionsEx
{
    //001: Start the Program Execution.
    public void StartProgram()
    {
        Console.WriteLine("Making Call to F1() " );
        F1();
        Console.WriteLine("Successfully returned from F1() " );
    }

    //002: Set of Function that calls each other
    public void F1()
    {
        Console.WriteLine("Making Call to F2() " );
        throw new System.Exception();
        F2();
        Console.WriteLine("Successfully returned from F2() " );
    }
    public void F2()
    {
        Console.WriteLine("Inside F2 " );
    }

    //Client 001: Program Entry
    [STAThread]
    static void Main(string[] args)
    {
        //Create the Object and start the Execution
        ExceptionsEx app = new ExceptionsEx();
        app.StartProgram();
    }
}

In the above code, look at the function F1 that throws an Exception. But, there is no handler to deal with the thrown exception. This situation is called an Un-Handled exception situation. When you execute the code, you get an unhandled exception dialog.
The above dialog is shown in debug mode, so you may get a chance to break the execution to see where the exception is thrown (or) continue ignoring the exception (not advisable).
In release mode, you will get un-handled Exception in the form of Application crash.
So, how do we avoid Un-Handled Exception? Simple, handle it.

2. Handling Exception

To handle the exception, we need to place the code within the try block. When an exception occurs inside the try block, the control looks for the catch block and raises an exception that is handled in the catch block. Below is the simple skeleton for the try and catch block:

try
{
    //Some Code that may expected to raise an exception
}
catch
{
    //Raised exception Handled here
}

The above skeleton handles any exception. But, the main disadvantage is that we don’t know what exception is raised and who raised the exception. Below is the example that handles the Exception raised by the function F1 and avoids the crash:

class ExceptionsEx
{
    //001: Start the Program Execution.
    public void StartProgram()
    {
        Console.WriteLine("Making Call to F1() " );
        try
        {
            F1();
        }
        catch
        {
            Console.WriteLine("Some Exception Occurred. 
  I don't know what Exception it is and where it Occurred. Sorry!");
        }
        Console.WriteLine("Successfully returned from F1() " );
    }

    //002: Set of Function that calls each other
    public void F1()
    {
        Console.WriteLine("Making Call to F2() " );
        throw new System.Exception();
        Console.WriteLine("Successfully returned from F2() " );
    }

    //Client 001: Program Entry
    [STAThread]
    static void Main(string[] args)
    {
        //Create the Object and start the Execution
        ExceptionsEx app = new ExceptionsEx();
        app.StartProgram();
    }
}

3. Exception Bubbling

In the above example, we saw that Exception is handled in the catch block. But, the function call order is simple (Call Stack) that is; StartProgram calls the function F1 and F1 raised exception is handled in the catch block of the StartProgram.
Imagine the situation what happens if there are multiple nested function calls, and exception occurred in the fourth or fifth nested call. Look at the picture below:

• F1(): Calls F2 within the try block and handles the exception in catch block
• F2(): Makes a call to the function F3. But it neither wraps the call for F3 in the try block nor has the exception handler
• F3(): Raises an Exception

Note, when the exception is thrown by the function F3, even though the caller is F2, as there is no catch handler, the execution comes out of F2 and enters the catch block of F1. Travelling back from F3->F2->F1 is known as Stack Unwind. And exception occurred in F3 is handled in F1 even when there is no handler at F2 is known as Exception Bubbling.
Below is the example that demonstrates the Exception Bubbling:

using System;

namespace ExceptionHandling
{
    class ExceptionsEx
    {
        //001: Start the Program Execution.
        public void StartProgram()
        {
            Console.WriteLine("Making Call to F1() " );
            try
            {
                F1();
            }
            catch
            {
                Console.WriteLine("Some Exception Occurred. 
  I don't know what Exception it is and where it Occurred. Sorry!");
            }
            Console.WriteLine("Successfully returned from F1() " );
        }

        //002: Set of Function that calls each other
        public void F1()
        {
            Console.WriteLine("Making Call to F2() " );
            F2();
            Console.WriteLine("Successfully returned from F2() " );
        }
        public void F2()
        {
            Console.WriteLine("Making Call to F2() " );
            F3();
            Console.WriteLine("Successfully returned from F2() " );
        }
        public void F3()
        {
            Console.WriteLine("Inside F3 " );
            throw new System.Exception();
        }

        //Client 001: Program Entry
        [STAThread]
        static void Main(string[] args)
        {
            //Create the Object and start the Execution
            ExceptionsEx app = new ExceptionsEx();
            app.StartProgram();
        }
    }
}

4. The Importance of Finally Block

In the above example, we saw that when an exception occurs, we directly jump back on the call stack and travel back searching for the catch handler. What about the piece of code that comes next to the exception raising code? If we do releasing resource and releasing the heap memory in the next couple of statements, that will not get reached. Right?
Finally block is the solution for this. Now look at the improved exception-handling skeleton below:

try
{
}
catch
{
}
finally
{
}

Whatever happens inside the try block, it is guaranteed that finally block gets executed. Hence, all resource releases should be in the finally block. Have a look at the below picture:
Exception raised at function F3 is handled in the function F2. Look at the try block, I marked two blocks of code before exception and code after exception. It is obvious that when an exception raised, the set of code inside the code after exception is never executed. If resources are allocated in code before exception and the allocated resources are released in code after exception, we do have a resource leak for each exception occurred. Also, think about business logic that needs to revert back apart from the resource leak. This is why finally block is introduced.
Whether an Exception occurs or not, the code inside finally block always gets executed. So you can keep all the cleaning code inside the finally block. The attached sample solution explains the above situation. Put a break point in the very first statement of the function public void StartProgram() and examine the situation explained above. Note, the usage of try block with only finally and without catch in function F2. Think about it.

5. What is Exception Class?

This class is provided by .NET Framework to handle any exception that occurred. The Exception class is the base class for all other Exception class provided by .NET Framework.
The exception object has some important properties. Some of then are given below:

Property	Usage
Message	Gives detailed information about the message
StackTrace	Gives the function stack to show where exception is thrown
Targetsite	Shows which method throws the exception

In the previous example, we only used the catch block and missed all the above said information. The exception object is thrown by the piece of code, which raises an Exception and the handler code catches that Exception object and makes use of the information packed in it. Consider the below example:

void SomefunctionX()
{
 throw new DivideByZeroException();
}

void SomeFunctionY()
{
 try
 {
  SomefunctionX();
 }
catch (DivideByZeroException Ex)
 {
 //Use the Ex here to get information
 }
}

The example was shown just to understand from where the object that is used in the catch block is coming from. Hope, now you know the exception instance created on the throw statement caught in the catch block and used. Note that, the base class of DivideByZeroException is ArithmaticException, which is derived from the System.Exception. Now the question is, May I use ArithmaticException in the catch block? Yes. Why not? That is the beauty of polymorphism. When you do, always keep in mind that you are moving from more specialized information thrown to the generalized one.

6. Handler for Multiple Exceptions

A code placed in the try block can raise different kinds of exception. It can be say, a Divide by Zero or a Network System Access Denied or a File not exists. How do you handle all the exceptions? The answer is, you can place multiple catch blocks for a try block. When an exception is thrown, type of that exception is examined against each catch block. When a match occurs (even polymorph), the exception enters into the catch block and gets handled.
Let me walk through an example, which will explain the following to you:

1. Make use of Exception object in the catch block
2. Using multiple catch blocks for different exceptions
3. The importance order of the catch blocks

1) Program enters the main function and calls the member function StartProgram after creating the object of type ExceptionsP2.

[STAThread]
public static void Main(string[] args)
{
 ExceptionsP2 start = new ExceptionsP2();
 start.StartProgram();
}

2) The StartProgram function makes a call to Calculate function. And this call is placed in the try block as it is expected that there be some arithmetic exception and possibly a Divide by Zero exception. Two catch blocks are placed for the try block of code. One is DivideByZeroException and other one is System.Exception. Inside each catch block, the exception object caught was used to display the Exception message, and function call stack of where the exception raised. Below is the code:

//001: Function that Handles the Exception
public void StartProgram()
{
 Console.WriteLine("Calling the Function Calculate");
 try
 {
  Calculate();
 }
 catch (DivideByZeroException Ex)
 {
  Console.WriteLine("Divide By Zero. 
   Look at the Call stack for More information");
  Console.WriteLine("Packed Message: " +  Ex.Message );
  Console.WriteLine("Thrown By: " + Ex.TargetSite );
  Console.Write("Call Stack: " + Ex.StackTrace );
 }
 catch (Exception Ex)
 {
  Console.WriteLine("General Exception Occurred");
  Console.WriteLine("Packed Message: " +  Ex.Message );
  Console.Write("Call Stack: " + Ex.StackTrace );
 }
}

3) The calculate is just an intermediate function which makes a call to Divide. Note how the stack rewind and exception bubbling is happening. Below is the function which makes a call to divide, and does not have any Exception handler code:

//002: An intermediate function, just for Demo Purpose
public void Calculate()
{
 Console.WriteLine("Calling Divide Function ");
 Divide();
}

4) The divide function tries to divide a number by zero. The .NET environment detects the situation and raises the exception for us. Note there is no throw statement here. In my previous examples (actually in the previous part), I forced the system to throw an Exception for my demonstration purpose. We usually use the throw statement for Custom defined exceptions. The .NET environment is smart enough to detect the exception at right time and raise it. Below is the code for divide:

//003: The Divide function which actually raises an Exception
public void Divide()
{
 int x = 10;
 int y = 0;
 y = x / y;   
}

7. Closing Notes

The DivideByZeroException thrown in the Divide function is caught by the StartProgram function. Then the exception object is matched against the first catch block. As there is a match, the exception enters that catch block.
What happens if System.Exception catch block is before the DivideByZeroException? We do enter the System.Exception catch block even though a more perfect match exists. Because, the exception caught is matched from top to bottom. As the System.Exception is on the top (that is; Before DivideByZero) and the caught Exception DivideByZeroException is polymorphically a System.Exception the execution control just enters the System.Exception catch block.
So always keep in mind to place more specific catch statement first and move down with more Generalized Exceptions. Look at the code below how a smart developer placed his catch block based on what his Team Lead told.
Lead: “The code block may raise a DivideByZeroException or Any ArithmeticException. It is possible to have FileNotFound Exception also. These are the possibilities of Exception I am seeing when I am reviewing your code. But, for safety, try to handle all exceptions in the world and it is OK if it is not more specific”.
That's all, the Lead went to a meeting.
Forget what meeting he is going for, see how the developer placed his catch block:

try
{
 // The set of code and function here
 // reviewed by an Experienced lead.
}
catch (System.FileNotFoundException Ex)
{
 //File Not Found Exception is Different from Divide by zero.
 //Look at the Inheritance hierarchy in MSDN for FileNotFoundException
 //and DivideByZeroException.
 //So I don't care Which one First, FileNotFond or DivideByZero
}
catch( System.DivideByZeroException Ex) 
{
 //Divide by Zero Exception. Need More Specialized care
}
catch (System.ArithmeticException Ex)
{
 // For any ArithmaticException except DivideByZero. Because I already handled it 
 // before this catch statement
}
catch(System.Exception Ex)
{
 //I am not expecting that this would Occur. As per my Lead
 //I placed it for safe to handle any Exception in the world 
 // That derived from Exception.
}
finally
{
 //Clean-up code
}

Using Linq DefaultIfEmpty() extension method

By Using DefaultIfEmpty() extension method you can check the sequence for empty and return something for it and do some manipulation on it afterwards.

If the sequence is empty it will return 0 otherwise it will sum and return that.

By Default if you want to return some value other then 0 it will return that as well

In this case it will return 100 if the sequence is empty.

Every ISAPI filter is contained in a separate DLL that must export two entry-point functions, GetFilterVersionand HttpFilterProc, and optionally export the TerminateFilter function. The metabase property, FilterLoadOrder, contains a list of all filters that IIS loads when the Web service is started.

The following steps outline the interaction between ISAPI filters and IIS:

GetFilterVersion is called exactly once, when the ISAPI filter is initially loaded. Any per-connection initializations the developer wants to perform must be managed internally within the context of the HttpFilterProc function call.

Introduction

In this article we will discuss about the IEnumerable interface. We will discuss how IEnumerable interface facilitate the use of foreach statement to iterate through a set of data. We will then look how to implement our own collections that implement IEnumerable interface. The use of yield keyword and Enumerating generic collections will also be discussed in this article.

Background 

Whenever we work with collection of objects, we might find ourselves in need to iterate the collection. The best way to iterate through a collection is by implementing the Iterator pattern. (refer: Understanding and Implementing the Iterator Pattern in C# and C++[^]). C# provides a very clean construct of foreach statement to iterate in a read only manner over a collection.
C# also provides us the possibility of using the same foreach construct and all the enumeration techniques on our custom collection objects by implementing the IEnumerable interface. So let us see how we can implement IEnumerable interface with our custom collection classes.

Using the code

Enumerating the Collection classes

Before starting the discussion let us see how we can use the Built-in classes and iterate over them. Lets start by looking into the ArrayList class that implements IEnumerable and see how we can have read only iteration over that using foreach statement.

// Let us first see how we can enumerate an object implementing IEnumerable
ArrayList list = new ArrayList();

list.Add("1");
list.Add(2);
list.Add("3");
list.Add('4');

foreach (object s in list)
{
    Console.WriteLine(s);
}

Enumerating the Generic Collection classes

The Arraylist class is a generalized class that let us keep a collection. We can also have a generic class in which we can provide the type along with the data. Iterating over generic collection classes is also possible because they implement IEnumerable<T> interface. Lets see how we can enumerate over a generic collection.

// Let us first see how we can enumerate an object implementing IEnumerable<T>
List<string> listOfStrings = new List<string>();

listOfStrings.Add("one");
listOfStrings.Add("two");
listOfStrings.Add("three");
listOfStrings.Add("four");

foreach (string s in listOfStrings)
{
    Console.WriteLine(s);
}

Now our objective is to have our own custom collection class and a generic collection class that should implement the IEnumerable and IEnumerable<T> interface respectively to provide the possibility of enumerating over them.

Understanding the yield keyword

Before jumping into the Implementation of these classes, we need to understand a very important keyword yield which actually facilitate the enumeration over collection. yield statement is used while returning a value from a function.
A normal method call like the one shown below will return only the first value no matter how many times it is called.

static int SimpleReturn()
{
    return 1;
    return 2;
    return 3;
}

static void Main(string[] args)
{
    // Lets see how simeple return works
    Console.WriteLine(SimpleReturn());
    Console.WriteLine(SimpleReturn());
    Console.WriteLine(SimpleReturn());
    Console.WriteLine(SimpleReturn());
}

The reason for this is that the normal return statement does not preserve the state of the function while returning. i.e. every call to this function is a new call and it will return the first value only.
Where as if I replace the return keyword by yield return then the function will become capable of saving its state while returning the value. i.e. when the function is called second time, it will continue the processing from where is has returned in the previous call.

static IEnumerable<int> YieldReturn()
{
    yield return 1;
    yield return 2;
    yield return 3;
}
static void Main(string[] args)
{
    // Lets see how yield return works
    foreach (int i in YieldReturn())
    {
        Console.WriteLine(i);
    }
}

When we run the above code it will return 1,2 and then 3. The only catch while using the yield return statement is that the function should return an IEnumerable and should be called from an iteration block i.e. foreach statement.

Implementing IEnumerable in our custom Collection class

Now in our custom collection classes, if we define a function that will iterate over all the elements in the collection and return then using the yield keyword, we will be able to get hold of all the elements in the the collection.
So let us define our own MyArrayList class and implement IEnumerable interface, which will force us to implement the GetEnumerator function. This function will iterate over the collection and do a yield return on all the elements.

class MyArrayList : IEnumerable
{
    object[] m_Items = null;
    int freeIndex = 0;

    public MyArrayList()
    {
        // For the sake of simplicity lets keep them as arrays
        // ideally it should be link list
        m_Items = new object[100];
    }

    public void Add(object item)
    {
        // Let us only worry about adding the item 
        m_Items[freeIndex] = item;
        freeIndex++;
    }

    // IEnumerable Member
    public IEnumerator GetEnumerator()
    {
        foreach (object o in m_Items)
        {
            // Lets check for end of list (its bad code since we used arrays)
            if(o == null)
            {
                break;
            }

            // Return the current element and then on next function call 
            // resume from next element rather than starting all over again;
            yield return o;
        }
    }
}

This class will now let us enumerate all the elements using a foreach stemement.

static void Main(string[] args)
{
    //Let us now go ahead and use our custom MyArrayList with IEnumerable implemented
    MyArrayList myList = new MyArrayList();

    myList.Add("1");
    myList.Add(2);
    myList.Add("3");
    myList.Add('4');

    foreach (object s in myList)
    {
        Console.WriteLine(s);
    }
}

Note: This class is neither complete not a very good implementation. The only purpose of the sample implementation is to demonstrate the implementation of IEnumerable interface.

Implementing IEnumerable<T> in our custom Generic Collection class

Let us now take this approach a little further and define a generic collection class capable of being enumerated. To do this we need to implement IEnumerable<T> interface.

class MyList<T> : IEnumerable<T>
{
    T[] m_Items = null;
    int freeIndex = 0;

    public MyList()
    {
        // For the sake of simplicity lets keep them as arrays
        // ideally it should be link list
        m_Items = new T[100];
    }

    public void Add(T item)
    {
        // Let us only worry about adding the item 
        m_Items[freeIndex] = item;
        freeIndex++;
    }

    #region IEnumerable<T> Members

    public IEnumerator<T> GetEnumerator()
    {
        foreach (T t in m_Items)
        {
            // Lets check for end of list (its bad code since we used arrays)
            if (t == null) // this wont work is T is not a nullable type
            {
                break;
            }

            // Return the current element and then on next function call 
            // resume from next element rather than starting all over again;
            yield return t;
        }
    }

    #endregion

    #region IEnumerable Members

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        // Lets call the generic version here
        return this.GetEnumerator();
    }

    #endregion
}

This class will now let us enumrate all the elements using a foreach stemement.

static void Main(string[] args)
{
    // Let us first see how we can enumerate an custom MyList<t> class implementing IEnumerable<T>
    MyList<string> myListOfStrings = new MyList<string>();

    myListOfStrings.Add("one");
    myListOfStrings.Add("two");
    myListOfStrings.Add("three");
    myListOfStrings.Add("four");

    foreach (string s in myListOfStrings)
    {
        Console.WriteLine(s);
    }
}
</t>

So now we have a collection class and a generic collectio class that implement IEnumerable and IEnumerable<T> respectively. Althogh These class is neither complete not a very good implementation but they do serve the purpose of the article i.e. to demonstrate the implementation of IEnumerable interface.

Point of interest

What we have tried to do in this article is see how can we implement the IEnumerable and IEnumberable<T> interface. We have looked into the significance of yield keyword. This stuff is known to most experienced C# programmers but the beginners might find it useful. I hope this has been informative.

Temp Table or Table variable or CTE are commonly used for storing data temporarily in SQL Server. In this article, you will learn the differences among these three.

CTE

CTE stands for Common Table expressions. It was introduced with SQL Server 2005. It is a temporary result set and typically it may be a result of complex sub-query. Unlike temporary table its life is limited to the current query. It is defined by using WITH statement. CTE improves readability and ease in maintenance of complex queries and sub-queries. Always begin CTE with semicolon.

A sub query without CTE is given below :

SELECT * FROM (
 SELECT Addr.Address, Emp.Name, Emp.Age From Address Addr
 Inner join Employee Emp on Emp.EID = Addr.EID) Temp
WHERE Temp.Age > 50
ORDER BY Temp.NAME

By using CTE above query can be re-written as follows :

;With CTE1(Address, Name, Age)--Column names for CTE, which are optional
AS
(
SELECT Addr.Address, Emp.Name, Emp.Age from Address Addr
INNER JOIN EMP Emp ON Emp.EID = Addr.EID
)
SELECT * FROM CTE1 --Using CTE 
WHERE CTE1.Age > 50
ORDER BY CTE1.NAME

When to use CTE

1. This is used to store result of a complex sub query for further use.
2. This is also used to create a recursive query.

Temporary Tables

In SQL Server, temporary tables are created at run-time and you can do all the operations which you can do on a normal table. These tables are created inside Tempdb database. Based on the scope and behavior temporary tables are of two types as given below-

1. Local Temp Table

   Local temp tables are only available to the SQL Server session or connection (means single user) that created the tables. These are automatically deleted when the session that created the tables has been closed. Local temporary table name is stared with single hash ("#") sign.
   

   
   CREATE TABLE #LocalTemp
   (
    UserID int,
    Name varchar(50), 
    Address varchar(150)
   )
   GO
   insert into #LocalTemp values ( 1, 'Shailendra','Noida');
   GO
   Select * from #LocalTemp
   
   

   
   The scope of Local temp table exist to the current session of current user means to the current query window. If you will close the current query window or open a new query window and will try to find above created temp table, it will give you the error.
   

2. Global Temp Table

   Global temp tables are available to all SQL Server sessions or connections (means all the user). These can be created by any SQL Server connection user and these are automatically deleted when all the SQL Server connections have been closed. Global temporary table name is stared with double hash ("##") sign.
   

   
   CREATE TABLE ##GlobalTemp
   (
    UserID int,
    Name varchar(50), 
    Address varchar(150)
   )
   GO
   insert into ##GlobalTemp values ( 1, 'Shailendra','Noida');
   GO
   Select * from ##GlobalTemp
    
   
   

   
   Global temporary tables are visible to all SQL Server connections while Local temporary tables are visible to only current SQL Server connection.
   

Table Variable

This acts like a variable and exists for a particular batch of query execution. It gets dropped once it comes out of batch. This is also created in the Tempdb database but not the memory. This also allows you to create primary key, identity at the time of Table variable declaration but not non-clustered index.

 GO
 DECLARE @TProduct TABLE
 (
 SNo INT IDENTITY(1,1),
 ProductID INT,
 Qty INT
 ) 
 --Insert data to Table variable @Product 
 INSERT INTO @TProduct(ProductID,Qty)
 SELECT DISTINCT ProductID, Qty FROM ProductsSales ORDER BY ProductID ASC 
 --Select data
 Select * from @TProduct
 
 --Next batch
 GO
 Select * from @TProduct --gives error in next batch
 

Note

1. Temp Tables are physically created in the Tempdb database. These tables act as the normal table and also can have constraints, index like normal tables.
2. CTE is a named temporary result set which is used to manipulate the complex sub-queries data. This exists for the scope of statement. This is created in memory rather than Tempdb database. You cannot create any index on CTE.
3. Table Variable acts like a variable and exists for a particular batch of query execution. It gets dropped once it comes out of batch. This is also created in the Tempdb database but not the memory.