Factory Design Pattern

Define an interface for creating an object, but let sub-classes decide which class to instantiate. Factory Method lets a class defer instantiation to sub-classes.
1. Its a creational pattern.
2. As its name say its to construct some thing.

Problems those solved by this pattern
1. So much of new keywords.
2. Client aware always about all concrete classes.
3. If we need to add / remove a concrete class then client should know about it, code updation required in client code.

Switch(invoiceType)
{
Case : 1
{
objInvoice = new invoiceWithHeader();
}
Case : 2
{
objInvoice = new invoiceWithFooter();
}
:
:
}

Benefits:
1. Reduces no. of new keywords.
2. Client just know about Factory and interface(that is implemented by all concrete classes) only.
3. No changes required at client whenever need to add a new concrete class or need to remove a concrete class.

Conclusion:
When you design an application just think if you really need it a factory to create objects. Maybe using it will bring unnecessary complexity in your application. If you have many objects of the same base type and you manipulate them mostly casted to abstract types, then you need a factory.


Code:
namespace FactoryPatternConsole

{
    //Invoice interface
    public interface IInvoice
    {
        void PrintInvoice();
    }
}

namespace FactoryPatternConsole
{   //Invoice factory
    class InvoiceFactory
    {
        public IInvoice GetInvoice(int invoiceType)
        {
            switch (invoiceType)
            {
                case 1:
                    return new clsInvoiceWithHeader();
                case 2 :
                    return new clsInvoiceWithFooter();
                default :
                    return null;
            }
        }
    }
}

namespace FactoryPatternConsole
{
    //Client
    class Program
    {
        static void Main(string[] args)
        {
            InvoiceFactory objFactory = new InvoiceFactory();
            //Here client just need to pass invoice type code to get required invoice
            IInvoice objInvoice = objFactory.GetInvoice(2);
            objInvoice.PrintInvoice();
            Console.ReadLine();
        }
    }
}

namespace FactoryPatternConsole
{
    //Concrete class invoice with header
    class clsInvoiceWithHeader : IInvoice
    {
        public void PrintInvoice()
        {
            Console.WriteLine("This is an invoice with header.");
        }
    }
}


namespace FactoryPatternConsole
{
    //Concrete class invoice with footer
    class clsInvoiceWithFooter : IInvoice
    {
        public void PrintInvoice()
        {
            Console.WriteLine("This is an invoice with footer.");
        }
    }
}

✅ Benefits

• Decouples object creation logic
• Centralizes creation (easy to change later)
• Supports adding new types easily

---

🧠 Real Uses in .NET Core

1. Dependency Injection Container (built-in factory)

   • When you call builder.Services.AddTransient<IMyService, MyService>(),
     the DI container acts as a factory, creating MyService instances as needed.

2. LoggerFactory

   ILoggerFactory loggerFactory = LoggerFactory.Create(builder =>
   {
       builder.AddConsole();
   });
   var logger =
   loggerFactory.CreateLogger<Program>();
   

3. DbContextFactory (EF Core 5+)

   var factory = new PooledDbContextFactory<AppDbContext>();
   using var context = factory.CreateDbContext();
   
   

Session State Management Strategies

There are two types of session state management strategy

1. In Process
2. out of process

In process – Stores session data on the same server.

Out of process – Stores data on other server like sql server, state server.

1. In process

Its best to choose when requests to come to same physical server rather than split to multiple IIS machines.

Its very fast and easy to implement because it runs in process.

Its wrong choice if your session state data is expensive to rebuild because when ever ASP.Net worker process or IIS restarts data flushes and rebuilds.

In case of multiple server its unsuitable until you implement some form of servers affinity like ‘network load balancing’.

2. Out of process

A. State Server – Its relies on a windows service, which remains stopped by default. You can start it from administrative tools / services / ASP.Net state service.

You can run it on same web server or can have a different ‘state server’ associated with your web server. It runs separately from IIS.

Beneficial is that you can restart IIS your session state data will not loss.

It require plenty of RAM because it runs separately from IIS.

Losses session state data on restart of server.

B. Sql Server - Stores data in table.

Its more reliable and flexible option than other.

You can restart server without losing data, you can even backup session state data.

Its also most expensive to build and maintain.

IEnumerable and IEnumerator

IEnumerable and IEnumerator are used to iterate over non generic collection.
IEnumerable exposes an enumerator.

IEnumerator has a property 'Current' that returns value of list item at current cursor position.
It has 2 methods
MoveNext() - Moves cursor to next item.
Reset() -  Reset cursor and send it to first item.

You can type cast a collection to IEnumerable or IEnumerator like list, array, array list.

List<int> myCol = List<int>();
mycol.add(5);
mycol.add(15);
mycol.add(20);

IEnumerable<int> ienmb = (IEnumerable<int>) myCol ;
IEnumerator<int> ienmr = (IEnumerator<int>) myCol ;

Now you can use these objects to iterate like this
Itration of IEnumerable
foreach(int i in ienmb) console.writeline(i);

Itration of IEnumerator
while(ienmr.MoveNext()) Console.Writeline(ienmr.Current.tostring());

You can also get enumrator object from enumrable object
IEnumerator<int> IEnumrObject = ienmb.GetEnumerator();

Differance b\w IEnumerable and IEnumerator
IEnumerable actually uses IEnumrator internally.
IEnumerator remember it's state, means it knows where is it's cursor position currently.
Syntactically IEnumerable easy to use.
If there is not any requirement that your collection object know about it's state than you can use IEnumerable.

SDLC

SDLC phases
1. Preliminary Analysis: The objective of phase 1 is to conduct a preliminary analysis, propose alternative solutions, describe costs and benefits and submit a preliminary plan with recommendations.
The 1st stage of SDLC is the investigation phase (Feasibility study). During this stage, business opportunities and problems are identified, and information technology solutions are discussed. Multiple alternative projects may be suggested and their feasibility analyzed. Operational feasibility is assessed, and it is determined whether or not the project fits with the current business environment, and to what degree it addresses business objectives. In addition, an economic feasibility investigation is conducted to judge the costs and benefits of the project. Technical feasibility must also be analyzed to determine if the available hardware and software resources are sufficient to meet expected specifications. A legal feasibility study is important to discover any potential legal ramification. The results of the feasibility study can then be compiled into a report, along with preliminary specifications. When the investigation stage ends, a decision whether or not to move forward with the project should be made. If it is decided to move ahead, a proposal should have been produced that outlines the general specifications of the project.
Conduct the preliminary analysis: in this step, you need to find out the organization's objectives and the nature and scope of the problem under study. Even if a problem refers only to a small segment of the organization itself then you need to find out what the objectives of the organization itself are. Then you need to see how the problem being studied fits in with them.
Propose alternative solutions: In digging into the organization's objectives and specific problems, you may have already covered some solutions. Alternate proposals may come from interviewing employees, clients , suppliers, and/or consultants. You can also study what competitors are doing. With this data, you will have three choices: leave the system as is, improve it, or develop a new system.
Goal of this phase is to prepare BRS (Business Resource specification) or URS (User Resource specification) or CRS (Customer Resource specification). It is good practice if BRS is prepared by company owner that describes what is his business, so analyst can decide and suggest appropriate solution for the business.
Describe the costs and benefits.

2. Systems analysis, requirements definition: Defines project goals into defined functions and operation of the intended application. Analyzes end-user information needs.
Goal of this phase is to prepare SRS (System Requirement Specification).

3. Systems design: Describes desired features and operations in detail, including screen layouts, business rules, process diagrams, pseudo-code and other documentation.
Goal of this phase is to prepare HLD (High Level Document) / Architectural Design and LLD (Low Level Design).

4. Development: The real code is written here. Goal of this phase is to prepare code for divided modules and unit testing (N unit).

5. Integration and testing: Brings all the pieces together into a special testing environment, then checks for errors, bugs and interoperability. Goal of this phase is to integrate different prepared modules from coding phase and testing of integrated module (Product).

6. Acceptance, installation, deployment: The final stage of initial development, where the software is put into production and runs actual business.

7. Maintenance: What happens during the rest of the software's life: changes, correction, additions, moves to a different computing platform and more. This is often the longest of the stages.

Waterfall Model

V & V - Validation and Varification