Represents an open connection to a SQL Server database. This class cannot be inherited.
Assembly: System.Data (in System.Data.dll)
A object represents a unique session to a SQL Server data source. With a client/server database system, it is equivalent to a network connection to the server. is used together with SqlDataAdapter and SqlCommand to increase performance when connecting to a Microsoft SQL Server database. For all third-party SQL server products, and other OLE DB-supported data sources, use OleDbConnection.
When you create an instance of , all properties are set to their initial values. For a list of these values, see the constructor.
If the goes out of scope, it won't be closed. Therefore, you must explicitly close the connection by calling Close or Dispose. Close and Dispose are functionally equivalent. If the connection pooling value Pooling is set to true or yes, the underlying connection is returned back to the connection pool. On the other hand, if Pooling is set to false or no, the underlying connection to the server is actually closed.
Login and logout events will not be raised on the server when a connection is fetched from or returned to the connection pool, because the connection is not actually closed when it is returned to the connection pool. For more information, see SQL Server Connection Pooling (ADO.NET).
To ensure that connections are always closed, open the connection inside of a using block, as shown in the following code fragment. Doing so ensures that the connection is automatically closed when the code exits the block.
To deploy high-performance applications, you must use connection pooling. When you use the .NET Framework Data Provider for SQL Server, you do not have to enable connection pooling because the provider manages this automatically, although you can modify some settings. For more information, see SQL Server Connection Pooling (ADO.NET).
If a SqlException is generated by the method executing a SqlCommand, the remains open when the severity level is 19 or less. When the severity level is 20 or greater, the server ordinarily closes the . However, the user can reopen the connection and continue.
An application that creates an instance of the object can require all direct and indirect callers to have sufficient permission to the code by setting declarative or imperative security demands. makes security demands using the SqlClientPermission object. Users can verify that their code has sufficient permissions by using the SqlClientPermissionAttribute object. Users and administrators can also use the Code Access Security Policy Tool (Caspol.exe) to modify security policy at the machine, user, and enterprise levels. For more information, see Security in the .NET Framework. For an example demonstrating how to use security demands, see Code Access Security and ADO.NET.
If you are using Microsoft .NET Framework version 1.0, the FullTrust named permission set is required to connect to SQL Server by using Open. This requirement does not apply if you are using .NET Framework version 1.1 or later. For more information, see Requesting Permissions and Named Permission Sets.
For more information about handling warning and informational messages from the server, see Connection Events (ADO.NET). SQL Server engine errors and error messages are documented in SQL Server Books Online.
ADO.NET 2.0 does not support Asynchronous commands over shared memory for SQL Server 2000 or lower. However, you can force TCP instead of shared memory. You can do that by prefixing tcp: to the server name in the connection string or you can use localhost.
|How to: Create Connections to SQL Server Databases||Data Access in Visual Studio|
The following example creates a SqlCommand and a . The is opened and set as the Connection for the SqlCommand. The example then calls ExecuteNonQuery. To accomplish this, the ExecuteNonQuery is passed a connection string and a query string that is a Transact-SQL INSERT statement. The connection is closed automatically when the code exits the using block.
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The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.