Catch-up the I.T. Bus: May 2012

ADO.NET is an object-oriented set of libraries that allows you to interact with data sources. Commonly, the data source is a database, but it could also be a text file, an Excel spreadsheet, or an XML file. For the purposes of this tutorial, we will look at ADO.NET as a way to interact with a data base.

As you are probably aware, there are many different types of databases available. For example, there is Microsoft SQL Server, Microsoft Access, Oracle, Borland Interbase, and IBM DB2, just to name a few. To further refine the scope of this tutorial, all of the examples will use SQL Server.

Data Providers

We know that ADO.NET allows us to interact with different types of data sources and different types of databases. However, there isn't a single set of classes that allow you to accomplish this universally. Since different data sources expose different protocols, we need a way to communicate with the right data source using the right protocol Some older data sources use the ODBC protocol, many newer data sources use the OleDb protocol, and there are more data sources every day that allow you to communicate with them directly through .NET ADO.NET class libraries.

ADO.NET provides a relatively common way to interact with data sources, but comes in different sets of libraries for each way you can talk to a data source. These libraries are called Data Providers and are usually named for the protocol or data source type they allow you to interact with. Table 1 lists some well known data providers, the API prefix they use, and the type of data source they allow you to interact with.

Provider Name	API prefix	Data Source Description
ODBC Data Provider	Odbc	Data Sources with an ODBC interface. Normally older data bases.
OleDb Data Provider	OleDb	Data Sources that expose an OleDb interface, i.e. Access or Excel.
Oracle Data Provider	Oracle	For Oracle Databases.
SQL Data Provider	Sql	For interacting with Microsoft SQL Server.
Borland Data Provider	Bdp	Generic access to many databases such as Interbase, SQL Server, IBM DB2, and Oracle.

ADO.NET Objects

ADO.NET includes many objects you can use to work with data. This section introduces some of the primary objects you will use. Over the course of this tutorial, you'll be exposed to many more ADO.NET objects from the perspective of how they are used in a particular lesson. The objects below are the ones you must know. Learning about them will give you an idea of the types of things you can do with data when using ADO.NET.

The SqlConnection Object

To interact with a database, you must have a connection to it. The connection helps identify the database server, the database name, user name, password, and other parameters that are required for connecting to the data base. A connection object is used by command objects so they will know which database to execute the command on.

The SqlCommand Object

The process of interacting with a database means that you must specify the actions you want to occur. This is done with a command object. You use a command object to send SQL statements to the database. A command object uses a connection object to figure out which database to communicate with. You can use a command object alone, to execute a command directly, or assign a reference to a command object to an SqlDataAdapter, which holds a set of commands that work on a group of data as described below.

The SqlDataReader Object

Many data operations require that you only get a stream of data for reading. The data reader object allows you to obtain the results of a SELECT statement from a command object. For performance reasons, the data returned from a data reader is a fast forward-only stream of data. This means that you can only pull the data from the stream in a sequential manner This is good for speed, but if you need to manipulate data, then a DataSet is a better object to work with.

The DataSet Object

DataSet objects are in-memory representations of data. They contain multiple Datatable objects, which contain columns and rows, just like normal database tables. You can even define relations between tables to create parent-child relationships. The DataSet is specifically designed to help manage data in memory and to support disconnected operations on data, when such a scenario make sense. The DataSet is an object that is used by all of the Data Providers, which is why it does not have a Data Provider specific prefix.

The SqlDataAdapter Object

Sometimes the data you work with is primarily read-only and you rarely need to make changes to the underlying data source Some situations also call for caching data in memory to minimize the number of database calls for data that does not change. The data adapter makes it easy for you to accomplish these things by helping to manage data in a disconnected mode. The data adapter fills a DataSet object when reading the data and writes in a single batch when persisting changes back to the database. A data adapter contains a reference to the connection object and opens and closes the connection automatically when reading from or writing to the database. Additionally, the data adapter contains command object references for SELECT, INSERT, UPDATE, and DELETE operations on the data. You will have a data adapter defined for each table in a DataSet and it will take care of all communication with the database for you. All you need to do is tell the data adapter when to load from or write to the database.

Summary

ADO.NET is the .NET technology for interacting with data sources. You have several Data Providers, which allow communication with different data sources, depending on the protocols they use or what the database is. Regardless, of which Data Provider used, you'll use a similar set of objects to interact with a data source. The SqlConnection object lets you manage a connection to a data source. SqlCommand objects allow you to talk to a data source and send commands to it. To have fast forward-only read access to data, use the SqlDataReader. If you want to work with disconnected data, use a DataSet and implement reading and writing to/from the data source with a SqlDataAdapter.

Introduction

A SqlCommand object allows you to specify what type of interaction you want to perform with a database. For example, you can do select, insert, modify, and delete commands on rows of data in a database table. The SqlCommand object can be used to support disconnected data management scenarios, but in this lesson we will only use the SqlCommand object alone. A later lesson on the SqlDataAdapter will explain how to implement an application that uses disconnected data. This lesson will also show you how to retrieve a single value from a database, such as the number of records in a table.

Creating a SqlCommand Object

Similar to other C# objects, you instantiate a SqlCommand object via the new instance declaration, as follows:

    SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

The line above is typical for instantiating a SqlCommand object. It takes a string parameter that holds the command you want to execute and a reference to a SqlConnection object. SqlCommand has a few overloads, which you will see in the examples of this tutorial.

Querying Data

When using a SQL select command, you retrieve a data set for viewing. To accomplish this with a SqlCommand object, you would use the ExecuteReader method, which returns a SqlDataReader object. We'll discuss the SqlDataReader in a future lesson. The example below shows how to use the SqlCommand object to obtain a SqlDataReader object:
// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

// 2. Call Execute reader to get query results
SqlDataReader rdr = cmd.ExecuteReader();
In the example above, we instantiate a SqlCommand object, passing the command string and connection object to the constructor. Then we obtain a SqlDataReader object by calling the ExecuteReader method of the SqlCommand object, cmd.
This code is part of the ReadData method of Listing 1 in the Putting it All Together section later in this lesson.

Inserting Data

To insert data into a database, use the ExecuteNonQuery method of the SqlCommand object. The following code shows how to insert data into a database table:
// prepare command string
string insertString = @"
     insert into Categories
     (CategoryName, Description)
     values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";

// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand(insertString, conn);

// 2. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
The SqlCommand instantiation is just a little different from what you've seen before, but it is basically the same. Instead of a literal string as the first parameter of the SqlCommand constructor, we are using a variable, insertString. The insertString variable is declared just above the SqlCommand declaration.
Notice the two apostrophes ('') in the insertString text for the word "doesn''t". This is how you escape the apostrophe to get the string to populate column properly.
Another observation to make about the insert command is that we explicitly specified the columns CategoryName and Description. The Categories table has a primary key field named CategoryID. We left this out of the list because SQL Server will add this field itself. trying to add a value to a primary key field, such as CategoryID, will generate an exception.
To execute this command, we simply call the ExecuteNonQuery method on the SqlCommand instance, cmd.
This code is part of the Insertdata method of Listing 1 in the Putting it All Together section later in this lesson.

Updating Data

The ExecuteNonQuery method is also used for updating data. The following code shows how to update data:
// prepare command string
string updateString = @"
     update Categories
     set CategoryName = 'Other'
     where CategoryName = 'Miscellaneous'";

// 1. Instantiate a new command with command text only
SqlCommand cmd = new SqlCommand(updateString);

// 2. Set the Connection property
cmd.Connection = conn;

// 3. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
Again, we put the SQL command into a string variable, but this time we used a different SqlCommand constructor that takes only the command. In step 2, we assign the SqlConnection object, conn, to the Connection property of the SqlCommand object, cmd.
This could have been done with the same constructor used for the insert command, with two parameters. It demonstrates that you can change the connection object assigned to a command at any time.
The ExecuteNonQuery method performs the update command.
This code is part of the UpdateData method of Listing 1 in the Putting it All Together section later in this lesson.

Deleting Data

You can also delete data using the ExecuteNonQuery method. The following example shows how to delete a record from a database with the ExecuteNonQuery method:
// prepare command string
string deleteString = @"
delete from Categories
where CategoryName = 'Other'";

// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand();

// 2. Set the CommandText property
cmd.CommandText = deleteString;

// 3. Set the Connection property
cmd.Connection = conn;

// 4. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
This example uses the SqlCommand constructor with no parameters. Instead, it explicity sets the CommandText and Connection properties of the SqlCommand object, cmd.
We could have also used either of the two previous SqlCommand constructor overloads, used for the insert or update command, with the same result. This demonstrates that you can change both the command text and the connection object at any time.
The ExecuteNonQuery method call sends the command to the database.
This code is part of the DeleteData method of Listing 1 in the Putting it All Together section later in this lesson.

Getting Single values

Sometimes all you need from a database is a single value, which could be a count, sum, average, or other aggregated value from a data set. Performing an ExecuteReader and calculating the result in your code is not the most efficient way to do this. The best choice is to let the database perform the work and return just the single value you need. The following example shows how to do this with the ExecuteScalar method:
// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);

// 2. Call ExecuteNonQuery to send command
int count = (int)cmd.ExecuteScalar();
The query in the SqlCommand constructor obtains the count of all records from the Categories table. This query will only return a single value. The ExecuteScalar method in step 2 returns this value. Since the return type of ExecuteScalar is type object, we use a cast operator to convert the value to int.
This code is part of the GetNumberOfRecords method of Listing 1 in the Putting it All Together section later in this lesson.

Putting it All Together

For simplicity, we showed snippets of code in previous sections to demonstrate the applicable techniques. It is also useful to have an entire code listing to see how this code is used in a working program. Listing 1 shows all of the code used in this example, along with a driver in the Main method to produce formatted output.

Listing 1. SqlConnection Demo

using System;
using System.Data;
using System.Data.SqlClient;

/// <summary>
/// Demonstrates how to work with SqlCommand objects
/// </summary>
class SqlCommandDemo
{
     SqlConnection conn;

     public SqlCommandDemo()
     {
         // Instantiate the connection
         conn = new SqlConnection(
            "Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");
     }

     // call methods that demo SqlCommand capabilities
     static void Main()
     {
         SqlCommandDemo scd = new SqlCommandDemo();

         Console.WriteLine();
         Console.WriteLine("Categories Before Insert");
         Console.WriteLine("------------------------");

         // use ExecuteReader method
         scd.ReadData();

         // use ExecuteNonQuery method for Insert
         scd.Insertdata();
         Console.WriteLine();
         Console.WriteLine("Categories After Insert");
         Console.WriteLine("------------------------------");

        scd.ReadData();

         // use ExecuteNonQuery method for Update
         scd.UpdateData();

         Console.WriteLine();
         Console.WriteLine("Categories After Update");
         Console.WriteLine("------------------------------");

         scd.ReadData();

         // use ExecuteNonQuery method for Delete
         scd.DeleteData();

         Console.WriteLine();
         Console.WriteLine("Categories After Delete");
         Console.WriteLine("------------------------------");

         scd.ReadData();

         // use ExecuteScalar method
         int numberOfRecords = scd.GetNumberOfRecords();

         Console.WriteLine();
         Console.WriteLine("Number of Records: {0}", numberOfRecords);
     }

     /// <summary>
     /// use ExecuteReader method
     /// </summary>
     public void ReadData()
     {
        SqlDataReader rdr = null;

         try
         {
             // Open the connection
             conn.Open();

             // 1. Instantiate a new command with a query and connection
             SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

            // 2. Call Execute reader to get query results
            rdr = cmd.ExecuteReader();

            // print the CategoryName of each record
            while (rdr.Read())
            {
                 Console.WriteLine(rdr[0]);
            }
       }
       finally
       {
           // close the reader
           if (rdr != null)
           {
                 rdr.Close();
             }

             // Close the connection
             if (conn != null)
             {
                 conn.Close();
            }
        }
     }

     /// <summary>
     /// use ExecuteNonQuery method for Insert
     /// </summary>
     public void Insertdata()
     {
         try
         {
             // Open the connection
             conn.Open();

           // prepare command string
           string insertString = @"
                 insert into Categories
                 (CategoryName, Description)
                 values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";

             // 1. Instantiate a new command with a query and connection
             SqlCommand cmd = new SqlCommand(insertString, conn);

             // 2. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
         }
         finally
         {
             // Close the connection
           if (conn != null)
             {
                 conn.Close();
             }
         }
     }

     /// <summary>
     /// use ExecuteNonQuery method for Update
     /// </summary>
     public void UpdateData()
     {
         try
         {
           // Open the connection
           conn.Open();

             // prepare command string
             string updateString = @"
               update Categories
               set CategoryName = 'Other'
               where CategoryName = 'Miscellaneous'";

           // 1. Instantiate a new command with command text only
             SqlCommand cmd = new SqlCommand(updateString);

             // 2. Set the Connection property
             cmd.Connection = conn;

             // 3. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
      }
         finally
         {
             // Close the connection
           if (conn != null)
             {
                 conn.Close();
             }
       }
     }

     /// <summary>
     /// use ExecuteNonQuery method for Delete
     /// </summary>
     public void DeleteData()
     {
         try
       {
             // Open the connection
             conn.Open();

             // prepare command string
             string deleteString = @"
                 delete from Categories
                 where CategoryName = 'Other'";

             // 1. Instantiate a new command
             SqlCommand cmd = new SqlCommand();

           // 2. Set the CommandText property
             cmd.CommandText = deleteString;

           // 3. Set the Connection property
             cmd.Connection = conn;

             // 4. Call ExecuteNonQuery to send command
             cmd.ExecuteNonQuery();
         }
         finally
         {
             // Close the connection
             if (conn != null)
           {
               conn.Close();
             }
         }
     }

     /// <summary>
     /// use ExecuteScalar method
     /// </summary>
     /// <returns>number of records</returns>
     public int GetNumberOfRecords()
     {
       int count = -1;

       try
       {
             // Open the connection
             conn.Open();

             // 1. Instantiate a new command
             SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);

             // 2. Call ExecuteScalar to send command
             count = (int)cmd.ExecuteScalar();
       }
         finally
         {
          // Close the connection
             if (conn != null)
             {
               conn.Close();
           }
       }
         return count;
     }
}
In Listing 1, the SqlConnection object is instantiated in the SqlCommandDemo structure. This is okay because the object itself will be cleaned up when the CLR garbage collector executes. What is important is that we close the connection when we are done using it. This program opens the connection in a try block and closes it in a finally block in each method.
The ReadData method displays the contents of the CategoryName column of the Categories table. We use it several times in the Main method to show the current status of the Categories table, which changes after each of the insert, update, and delete commands. Because of this, it is convenient to reuse to show you the effects after each method call.

Catch-up the I.T. Bus

Tuesday 22 May 2012

ADO.NET