DataAdapter不会自行关闭连接

Question

As we know that DataAdapter opens and closes a Connection if it is not already open. But with my code it opens but not close..I am using MySql.Data.MySqlClient.MySqlDataAdapter, not sure what I am doing wrong. below is my code

Public Function GetDT(ByVal SqlQuery As String, ByVal ConString As String) As DataTable
    Dim da As New MySql.Data.MySqlClient.MySqlDataAdapter(SqlQuery, ConString)
    Dim ds As New DataSet
    da.Fill(ds)
    GetDT = ds.Tables(0)
    da.Dispose()
    ds.Dispose()
    da = Nothing
    ds = Nothing
End Function

I am using this connection string: "server=localhost;port=3306;user=someuser;pwd=somepassword;database=mydatabasename;Allow Zero Datetime=True;"

while debugging the code I found as soon as DataAdapter.fill executes its start a connection thread, but no where connection is getting close as thread remains in SLEEP state . Please check the below image.

Can any one please help me on this ?

Thanks

Answer 1

Its normal behavior because ADO.NET uses Connection Pooling with SQL Server by default. From MSDN:

http://msdn.microsoft.com/en-US/library/8xx3tyca(v=vs.110).aspx

Connecting to a database server typically consists of several time-consuming steps. A physical channel such as a socket or a named pipe must be established, the initial handshake with the server must occur, the connection string information must be parsed, the connection must be authenticated by the server, checks must be run for enlisting in the current transaction, and so on.

In practice, most applications use only one or a few different configurations for connections. This means that during application execution, many identical connections will be repeatedly opened and closed. To minimize the cost of opening connections, ADO.NET uses an optimization technique called connection pooling.

Connection pooling reduces the number of times that new connections must be opened. The pooler maintains ownership of the physical connection. It manages connections by keeping alive a set of active connections for each given connection configuration. Whenever a user calls Open on a connection, the pooler looks for an available connection in the pool. If a pooled connection is available, it returns it to the caller instead of opening a new connection. When the application calls Close on the connection, the pooler returns it to the pooled set of active connections instead of closing it . Once the connection is returned to the pool, it is ready to be reused on the next Open call.

....

The connection pooler removes a connection from the pool after it has been idle for approximately 4-8 minutes,...

The effect of "using" or "dispose" in the DataAdapter is the same. "using" ensures that dispose is executed, even if an exception happends. Is the same as using a try / finally statment and put the "dispose" on the finally section.

Answer 2

The connection only closes itself when you use a Using directive, the Using statement can be used on any object that implements the IDisposable interface.

In the Dispose() method of the DataAdapter the connection is closed before disposing the object. using it correctly would be something like this.

Public Function GetDT(ByVal SqlQuery As String, ByVal ConString As String) As DataTable
    Using da As New MySql.Data.MySqlClient.MySqlDataAdapter(SqlQuery, ConString)
        Dim ds As New DataSet
        da.Fill(ds)
        GetDT = ds.Tables(0)
        da.Dispose()   //because you are now using a 'Using' statement, this method is not necessary anymore.
        ds.Dispose()
        da = Nothing
        ds = Nothing
    End Using
End Function