HOW TO: Reuse the SqlCommand and OleDbCommand Objects in Visual C# .NET (317559)

SUMMARY

This step-by-step article describes how to reuse the SqlCommand object in your Visual C# .NET code.

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Description of the Technique

You can reuse Command objects (SqlCommand or OleDbCommand) in code. That is, you can create one Command object and then run different commands on that object.

Commands are issued against databases to take actions against data stores. Commands include any statement that can be issued against a database. You can use the OleDbCommand or the SqlCommand class to get a command for your data store.

In Microsoft ActiveX Data Objects (ADO), you can issue commands through the Command, the Connection, or the Recordset object. In Microsoft ADO.NET, only the Command objects (SqlCommand or OleDbCommand) run commands.

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Steps to Reuse a SqlCommand Object

1. Create a new Windows application in Visual C# .NET as follows:
   1. Start Microsoft Visual Studio .NET.
   2. On the File menu, point to New, and then click Project.
   3. In the New Project dialog box, click Visual C# Projects under Project Types, and then click Windows Application under Templates. Form1 is added to the project by default.
2. Verify that your project contains a reference to the System.Data namespace. If it does not, add a reference to this namespace.
3. Drag a Button control from the toolbox to Form1. Change the Name property of the button to btnTest.
4. Use the using statement on the System and the System.Data namespaces so that you are not required to qualify declarations in those namespaces later in your code. Add the following code to the "General Declarations" section of Form1:

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

5. Copy and paste the following code in the btnTest_Click event:

   try
   {
      string myConnString = "User ID=myUID;password=myPWD;Initial Catalog=pubs;Data Source=mySQLServer";
       string mySelectQuery = "SELECT * FROM Titles";
       SqlConnection myConnection = new SqlConnection(myConnString);
       SqlCommand myCommand = new SqlCommand(mySelectQuery, myConnection);
       myConnection.Open();
       SqlDataReader myReader = myCommand.ExecuteReader();
       while (myReader.Read())
       {
           //Process data.
       }
       myReader.Close(); //Close the reader. 
   
       myCommand.CommandText = "SELECT * FROM Sales";
       SqlDataReader myReader1 = myCommand.ExecuteReader();
       while (myReader1.Read())
       {
           //Process data.
       }
       myReader1.Close(); //Close the reader. 
   
       String myInsertQuery = "INSERT INTO Employee " +
             "(emp_id, fname, minit, lname, job_id, job_lvl, pub_id, hire_date)" +
             "VALUES ('MSD12923F', 'JP', 'W', 'Mackenzie', " +
             "10, 82,'0877','2001-01-01')";
       myCommand.CommandText = myInsertQuery;
       myCommand.ExecuteNonQuery();
   
       //You can use the command any number of times.
   
       myConnection.Close();
   }
   catch (Exception ex)
   { 
       MessageBox.Show(ex.ToString());
   }
   					

6. Modify the connection string (myConnString) as appropriate for your environment.
7. Save your project. On the Debug menu, click Start to run your project.

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Troubleshooting

• While the SqlDataReader object is in use, the associated SqlConnection object serves the SqlDataReader, and you cannot perform any other operations on the SqlConnection object other than to close it. This is true until you call the Close method of the SqlDataReader object. This means, for example, that you cannot retrieve output parameters until after you call Close.For additional information about how to handle output parameters, click the article number below to view the article in the Microsoft Knowledge Base:

  308621 PRB: Output Parameters Are Not Returned When You Run an ADO.NET Command in Visual C# .NET

• If the method that runs a SqlCommand generates a SqlException exception, the SqlConnection remains open if the severity level is 19 or less. If the severity level is 20 or greater, the server usually closes the SqlConnection. However, you can reopen the connection and continue.

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