VB.Net reuse sql connection - mysql

Question

I'm a newbie to vb.net, I'd like to ask is there a way to reuse the sql connection command?
Here is the code for my main.vb:
Dim ServerString As String = "Server=localhost;User Id=root;Password=;Database=pos"
Dim SQLConnection As MySqlConnection = New MySqlConnection
Private Sub Main_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
SQLConnection.ConnectionString = ServerString
Try
If SQLConnection.State = ConnectionState.Closed Then
SQLConnection.Open()
Else
SQLConnection.Close()
End If
Catch ex As Exception
MsgBox(ex.ToString)
End Try
Since I'd like to use this in my other classes, i don't want to re-write this code in every form. Any help is truly appreciated. Thanks.

Answer 1

Reusing a connection (or any other unmanaged resource) is normally not a good idea. You should dispose them as soon as possible.
But there's no problem in always creating a new connection anyway, since you're using the ADO.NET connection-pool by default. So you are not creating (and opening) a new physical connection. Actually you're just telling the pool that a conenction is reusable somewhere else now when you close/dispose it. And when you open it, it cannot be used somewhere else, that's why it's important to always close it.
Therefore always use the Using-statement.
Public Shared Function GetColumn1(column2 As Int32) As String
Dim sql = "SELECT Column1 From dbo.tableName WHERE Column2=#Column2 ORDER BY Column1 ASC"
Using con = New SqlConnection(connectionString)
Using cmd = New SqlCommand(sql, con)
cmd.Parameters.AddWithValue("#Column2", column2)
Try
con.Open()
Using rd = cmd.ExecuteReader()
If rd.Read() Then
Dim Column1 As String = rd.GetString(0)
Return Column1
Else
Return Nothing
End If
End Using
Catch ex As Exception
' log the exception here or do not catch it '
' note that you don't need a Finally to close the connection '
' since a Using-Statement disposes the object even in case of exception(which also closes a connection implicitely)
End Try
End Using
End Using
End Function
Above is a sample method to demontrate that you should not reuse anything.

Answer 2

This is what I normally do: I create a class, e.g. ConnectDB, and a method within this class, e.g. GetConnection. Here is the code:
Imports System.Data
Imports System.Data.SqlClient
Public Class ConnectDB
Public Shared Function GetConnection() As SqlConnection
Dim dbConnString As String = "Data Source=(local);Initial Catalog=Northwind;Integrated Security=True"
Return New SqlConnection(dbConnString)
End Function
End Class
Then from the method that needs a connection to the database, I call this function. Here is a sample code:
Imports System.Data.SqlClient
Public Class EmployeeDB
Public Shared Function GetEmployees() As List(Of Employee)
Dim con As SqlConnection = ConnectDB.GetConnection()
Dim selectStmt As String = "SELECT * FROM Employees"
Dim selectCmd As New SqlCommand(selectStmt, con)
Dim employees As New List(Of Employee)
Try
con.Open()
Dim reader As SqlDataReader = selectCmd.ExecuteReader()
Do While reader.Read
Dim employee as New Employee
employee.LastName = reader("LastName").ToString
employee.FirstName = reader("FirstName").ToString
...
employees.Add(employee)
Loop
reader.Close()
Catch ex As Exception
Throw ex
Finally
con.Close()
End Try
Return employees
End Function
End Class
You can also modify the selectStmt string to include filter conditions, parameters, and sort order just like Tim's example above and include selectCmd.Parameters.AddWithValue("#<parameterName>", value) for each of your parameters.