VB.NET函數返回錯誤並退出子

Question

我有這方面的功能在我的VB.NET工程

Public Function OpenMysqlCon() As MySqlConnection 
    Dim mMysqlconnection = New MySqlConnection() 
    Try 
     Dim strconDB As String = "server='192.168.100.2'; database='mydb';Port=3306; UID='epb'; password='hahaha'; pooling=true" 
     mMysqlconnection = New MySqlConnection 
     mMysqlconnection.ConnectionString = strconDB 
     mMysqlconnection.Open() 
     OpenMysqlCon = mMysqlconnection 
    Catch exceptionThatICaught As System.Exception 
     OpenMysqlCon = Nothing 
    End Try 
End Function 

，我會調用該函數在我的VB項目類似

Private Sub frmTest_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
    Using Con=OpenMysqlCon() 
     'mycode here 
    End Using 
End Sub 

然而當連接不可用時，它會拋出異常。

我如何通過給msgbox類似connection not available at the moment, please try again later的東西來避免異常，然後退出使用該函數的子？

End Sub

Answer 1

它會是這樣的。

Public Function OpenMysqlCon() As MySqlConnection 
    Dim mMysqlconnection As MySqlConnection() 
    Try 
     Dim strconDB As String = "server='192.168.100.2'; database='mydb';Port=3306; UID='epb'; password='hahaha'; pooling=true" 
     mMysqlconnection = New MySqlConnection 
     mMysqlconnection.ConnectionString = strconDB 
     mMysqlconnection.Open() 
    Catch exceptionThatICaught As System.Exception 
     mMysqlconnection = Nothing 
    End Try 
    Return mMysqlconnection 
End Function 

Private Sub frmTest_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
    Dim con As MySqlConnection = OpenMysqlCon 
    If con Is Nothing Then 
     MessageBox.Show("connection not available at the moment, please try again later") 
     'Me.Close 'uncomment if the app should end 
    End If 
End Sub 

編輯 - 這不是測試

Using con As MySqlConnection = OpenMysqlCon 
     If con Is Nothing Then 
      MessageBox.Show("connection not available at the moment, please try again later") 
      'Me.Close 'uncomment if the app should end 
     Else 
      'your code 
     End If 
    End Using 

Answer 2

最好的辦法是不要持有形式的現場連接，但創造何地您需要它initalize它。連接池將確保不需要創建物理連接。

所以不要在這樣所有，但代碼中使用OpenMysqlCon方法（GetAllUsers只是一個例子）：

Public Function GetAllUsers() As List(Of User) 
    Dim userList = New List(Of User) 

    Try 
     Using mMysqlconnection = New MySqlConnection("server='192.168.100.2'; database='mydb';Port=3306; UID='epb'; password='hahaha'; pooling=true") 
      mMysqlconnection.Open() 
      Using command = New MySqlCommand("SELECT * FROM USER ORDER By UserName", mMysqlconnection) 
       Using rdr = command.ExecuteReader() 
        While rdr.Read() 
         Dim user = New User() 
         user.UserName = rdr.GetString(0) 
         userList.Add(user) 
        End While 
       End Using 
      End Using 
     End Using 
    Catch exceptionThatICaught As System.Exception 
     MessageBox.Show("meaningful message here, logging would be useful too") 
     Return Nothing 
    End Try 

    Return userList 
End Function 

也許有用，因爲相關的（你還重用連接）：ExecuteReader requires an open and available Connection. The connection's current state is Connecting