I want to create a C# API for ASP.NET Application. How to Create the API in C# for Storing and retrieving the data in sql.

Am in confusion of how to define the API, whether I have to define separate class for connection of sql, and insert I have to define a separate class likewise for update and delete or we can define all the functionality in single class. Can anyone guide me in right in which which way I have to go.

What is the best way for building Database API.

Separate classes for each. This adheres to the Single Responsibility Principle and the Open/Closed Principle.

• http://en.wikipedia.org/wiki/Single_responsibility_principle
  
• http://en.wikipedia.org/wiki/Open/closed_principle

Hopefully I have not misunderstood your question - I read it several times to make sure I understood what you need.  It sounds like the 'API' you are attempting to build is really a DAL or Data Access Layer.  The DAL in its simplest form takes care of all communication to the Data Layer which could include maintingin your connections, CRUD, etc.  There are a lot of methods to designing the DAL and retrieving data, but in its basic form it can be a single binary (.dll) containing 1 or many classes that do the work that you need it to do to get back your data.

Now typically this data is then translated into some type of business object to be used with the UI in a traditional UI -> BLL -> DAL arcitecture.  So to get you started I have some links that should help you with your design.

Building an N-Tier Application in .NET:

http://msdn.microsoft.com/en-us/library/ms973279.aspx

Tutorial 1: Creating a Data Access Layer:

http://msdn.microsoft.com/en-us/library/aa581778.aspx

Implement a Data Access Layer for Your App with ADO.NET:

http://msdn.microsoft.com/en-us/magazine/cc188750.aspx

.NET Application Architecture: the Data Access Layer:

http://www.simple-talk.com/dotnet/.net-framework/.net-application-architecture-the-data-access-layer/

If you need more information or were talking about something different, please post back to this thread.

Hello

Also you can refer to the following books on understanding how to architecture your application.

1.) Architecturing Microsoft .net Solutins for the Enterprise by Dino Esposito , Andrea Saltarello

In this book author clearly explains what should go in which layer . Also they explain design patterns etc with .Net framework in context there by making this book a must read.

>Architecturing Microsoft .net Solutins for the Enterprise by Dino Esposito , Andrea Saltarello

That is an excellent book!