How do I define a data adapter?

The data adapter stores your command (query) and connection and using these connect to the database when asked, fetch the result of query and store it in the local dataset.
The DataAdapter class (SqlDataAdapter, OracleDataAdapter, OleDbDataAdapter, OdbcDataAdapter) may be instantiated in three ways:
1. by supplying the command string (SQL Select command) and connection string
2. by supplying the command string (SQL Select command) and a connection object
3. by supplying the command object (SqlCommand, OracleCommand, OleDbCommand, OdbcCommand)
For example, with SQL Server, the data adapter is created as
C# Version
// for Sql Server
SqlDataAdapter dataAdapter = new SqlDataAdapter(commandString, conn);

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What is a data reader?

The data reader is a component that reads the data from the database management system and provides it to the application. The data reader works in the connected manner; it reads a record from the DB, pass it to the application, then reads another and so on.

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What is a database command?

A database command specifies which particular action you want to perform to the database. The commands are in the form of SQL (Structured Query Language). There are four basic SQL statements that can be passed to the database.

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What is a database connection?

A database connection represents a communication channel between you application and database management system (DBMS). The application uses this connection to pass the commands and queries to the database and obtain the results of the operations from the database.

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What is a data adapter?

A data adapter is the component that exists between the local repository (dataset) and the physical database. It contains the four different commands (SELECT, INSERT, UPDATE and DELETE). It uses these commands to fetch the data from the DB and fill into the dataset and to perform updates done in the dataset to the physical database. It is the data adapter that is responsible for opening and closing the database connection and communicates with the dataset.

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What is a dataset?

A dataset is the local repository of the data used to store the tables and disconnected record set. When using disconnected architecture, all the updates are made locally to dataset and then the updates are performed to the database as a batch.

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DataSet or DataReader ?

The data reader is more useful when you need to work with large number of tables, database in non-uniform pattern and you need not execute the large no. of queries on few particular table.
When you need to work on fewer no. of tables and most of the time you need to execute queries on these fewer tables, you should go for the dataset.
It also depends on the nature of application. If multiple users are using the database and the database needs to be updated every time, you must not use the dataset. For this, .Net provides the connection oriented architecture. But in the scenarios where instant update of database is not required, dataset provides optimal performance by making the changes locally and connecting to database later to update a whole batch of data. This also reduces the network bandwidth if the database is accessed through network.
Disconnected data access is suited most to read only services. On the down side, disconnected data access architecture is not designed to be used in the networked environment where multiple users are updating data simultaneously and each of them needs to be aware of current state of database at any time (e.g., Airline Reservation System).

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What's the difference between accessing data with dataset or data reader?

The dataset is generally used when you like to employ the disconnected architecture of the ADO.Net. It reads the data into the local memory buffer and perform the data operations (update, insert, delete) locally to this buffer.
The data reader, on the other hand, is directly connected to the database management system. It passes all the queries to the database management system, which executes them and returns the result back to the application.
Since no memory buffer is maintained by the data reader, it takes up fewer resources and performs more efficiently with small number of data operations. The dataset, on the other hand is more efficient when large number of updates are to be made to the database. All the updates are done in the local memory and are updated to the database in a batch. Since database connection remains open for the short time, the database management system does not get flooded with the incoming requests.

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What does it mean by connected data access architecture of ADO.Net?

In the connected environment, it is your responsibility to open and close the database connection. You first establish the database connection, perform the interested operations to the database and when you are done, close the database connection. All the changes are done directly to the database and no local (memory) buffer is maintained.

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What does it mean by disconnected data access architecture of ADO.Net?

ADO.Net introduces the concept of disconnected data architecture. In traditional data access components, you make a connection to the database system and then interact with it through SQL queries using the connection. The application stays connected to the DB system even when it is not using DB services. This commonly wastes the valuable and expensive database resource as most of the time applications only query and view the persistent data. ADO.Net solves this problem by managing a local buffer of persistent data called data set. Your application automatically connects to the database server when it needs to pass some query and then disconnects immediately after getting the result back and storing it in dataset. This design of ADO.Net is called disconnected data architecture and is very much similar to the connection less services of http over the internet. It should be noted that ADO.Net also provides the connection oriented traditional data access services.
Traditional Data Access Architecture

ADO.Net Disconnected Data Access Architecture

Another important aspect of the disconnected architecture is that it maintains the local repository of data in the dataset object. The dataset object stores the tables, their relationship and different constraints. The user performs operations like update, insert, delete to this dataset locally and finally the changed dataset is stored in actual database as a batch when needed. This greatly reduces the network traffic and results in the better performance.

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Difference between value type and reference type ?

Many programming languages provide built-in data types such as integers and floating-point numbers. These are copied when they are passed in to arguments i.e. they are passed "By Value". In .NET terms, these are called Value Types".
The RunTime supports two kinds of Value Types:
1 Built-in value types
The .NET Framework defines built-in value types such as System.Int32 and System.Boolean which correspond and are identical to primitive data types used in programming languages.
2 User-defined value types
The language you are using will provide functionality to define your own Value Types. These user defined Types derive from System.ValueType. If you want to define a Type representing a value that is a complex number (two floating-point numbers), you might choose to define it as a value type. Why? Because you can pass the Value Type efficiently "By Value". If the Type you are defining could be more efficiently passed "By Reference", you should define it as a class instead. Variables of Reference Types are referred to as objects. These store references to the actual data.
The following are the Reference Types:
• class
• interface
• delegate
This following are the "built-in" Reference Types:
• object
• string

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What is serialization in .NET and what are the ways to control serialization?

Serialization is the process of converting an object into a stream of bytes. On the other hand Deserialization is the process of creating an object from a stream of bytes. Serialization/Deserialization is used to transport or to persist objects. Serialization can be defined as the process of storing the state of an object to a storage medium. During this process, the public and private fields of the object and the name of the class, including the assembly are converted to a stream of bytes. Which is then written to a data stream. Upon the object's subsequent deserialized, an exact clone of the original object is created.
Binary serialization preserves Type fidelity, which is useful for preserving the state of an object between different invocations of an application. For example: An object can be shared between different applications by serializing it to the clipboard.
You can serialize an object to a stream, disk, memory, over a network, and so forth. Remoting uses serialization to pass objects "By Value" from one computer or application domain to another. XML serialization serializes only public properties and fields and does not preserve Type fidelity. This is useful when you want to provide or consume data without restricting the application that uses the data.
As XML is an open standard, it is an attractive choice for sharing data across the Web. SOAP is also an open standard, which makes it an attractive choice too. There are two separate mechanisms provided by the .NET class library - XmlSerializer and SoapFormatter/BinaryFormatter. Microsoft uses XmlSerializer for Web Services, and uses SoapFormatter/BinaryFormatter for remoting. Both are available for use in your own code.

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