在VB.Net中设置运行时的数据库类

我有以下Sqlite和SqlServer类：

SQLite的类：

Imports System.Data.SQLite
Public Class clsOperDB_SQLite

 Public Shared Function getValue(sql As String) As String

        Try
            Using conn As New SQLiteConnection(strConn_SQLITE)
                Using cmd As New SQLiteCommand()
                    cmd.Connection = conn
                    conn.Open()
                    cmd.CommandText = sql
                    Return cmd.ExecuteScalar
                End Using
            End Using
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try

        Return ""

    End Function

End Class

Imports System.Data.SqlClient
Public Class clsOperDB_SQLSERVER

            Public Shared Function getValue(sql As String) As String

                Try
                    Using conn As New SqlConnection(strConn_SQLSERVER)
                        Using cmd As New SqlCommand()
                            cmd.Connection = conn
                            conn.Open()
                            cmd.CommandText = sql
                            Return cmd.ExecuteScalar
                        End Using
                    End Using
                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try

                Return ""

            End Function

End Class

用于SQLSERVER的类：

Imports System.Data.SQLite
Public Class clsOperDB_SQLite

 Public Shared Function getValue(sql As String) As String

        Try
            Using conn As New SQLiteConnection(strConn_SQLITE)
                Using cmd As New SQLiteCommand()
                    cmd.Connection = conn
                    conn.Open()
                    cmd.CommandText = sql
                    Return cmd.ExecuteScalar
                End Using
            End Using
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try

        Return ""

    End Function

End Class

Imports System.Data.SqlClient
Public Class clsOperDB_SQLSERVER

            Public Shared Function getValue(sql As String) As String

                Try
                    Using conn As New SqlConnection(strConn_SQLSERVER)
                        Using cmd As New SqlCommand()
                            cmd.Connection = conn
                            conn.Open()
                            cmd.CommandText = sql
                            Return cmd.ExecuteScalar
                        End Using
                    End Using
                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try

                Return ""

            End Function

End Class

这是我的测试表：

Public Class Form1

    'form level variable
    Dim dbConnector

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        Dim connectionType As String = "SQLITE"

        ' need something like this or any way to set form level variable

        If connectionType = "SQLITE" Then
            dbConnector = clsOperDB_SQLite
        Else
            dbConnector = clsOperDB_SQLSERVER
        End If


    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click

        'get value from SQLITE
        Dim ValueFromDatabase As String = dbConnector.getValue("select .....")
    End Sub
End Class

我需要帮助定义dbConnector变量并设置其值，同时intellisense应向我显示类方法，使用我要更改数据库的参数，并避免每次使用一个或另一个数据库时使用条件：

   Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click

        Dim Query As String = "Select ..."
        Dim ValueFromDatabase As String = ""

        ' dont want to use if for each sql query

        If connectionType = "SQLITE" Then
            ValueFromDatabase = clsOperDB_SQLite.getValue(Query)
        Else
            ValueFromDatabase = clsOperDB_SQLSERVER.getValue(Query)
        End If

    End Sub

这两个类的其余方法和参数是相同的，仅更改类数据对象（SQLiteConnection、SqlConnection等）

---

谢谢

您应该定义一个对所有公共成员进行分类的接口。然后，您可以为实现该接口的每个数据源创建一个类。在应用程序代码中，可以声明该接口类型的变量，并将实现该接口类型的任何类的实例分配给该变量。然后，您可以使用该变量并调用接口的任何成员，而不必关心实际类实例的类型

接口和实现类的外观如下所示：

Public Interface IDataAccessProvider
    Function GetValue(sql As String) As String
End Interface

Public Class SqliteDataAccessProvider
    Implements IDataAccessProvider

    Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
        'Provide SQLite-specific implementation here.
    End Function

End Class

Public Class SqlServerDataAccessProvider
    Implements IDataAccessProvider

    Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
        'Provide SQL Server-specific implementation here.
    End Function

End Class

Private dataAccessProvider As IDataAccessProvider

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    'Read a value that identifies the data source and store it here.
    'The value might be stored in the config file or wherever else is appropriate.
    Dim dataSourceIdentifier As String

    Select Case dataSourceIdentifier
        Case "SQLite"
            dataAccessProvider = New SqliteDataAccessProvider()
        Case "SQL Server"
            dataAccessProvider = New SqlServerDataAccessProvider()
    End Select
End Sub

然后，您的应用程序代码可能如下所示：

Public Interface IDataAccessProvider
    Function GetValue(sql As String) As String
End Interface

Public Class SqliteDataAccessProvider
    Implements IDataAccessProvider

    Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
        'Provide SQLite-specific implementation here.
    End Function

End Class

Public Class SqlServerDataAccessProvider
    Implements IDataAccessProvider

    Public Function GetValue(sql As String) As String Implements IDataAccessProvider.GetValue
        'Provide SQL Server-specific implementation here.
    End Function

End Class

Private dataAccessProvider As IDataAccessProvider

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    'Read a value that identifies the data source and store it here.
    'The value might be stored in the config file or wherever else is appropriate.
    Dim dataSourceIdentifier As String

    Select Case dataSourceIdentifier
        Case "SQLite"
            dataAccessProvider = New SqliteDataAccessProvider()
        Case "SQL Server"
            dataAccessProvider = New SqlServerDataAccessProvider()
    End Select
End Sub

然后，您可以在代码中调用

dataAccessProvider.GetValue

，而不必关心数据源的实际情况，只是为了确保SQL语法对该数据源有效

---

请注意，虽然您要做什么取决于您自己，但我选择在这段代码中使用一个合理的命名约定。没有人会长期为我使用类名称，比如

clsOperDB\u SQLSERVER

。你在.NET Framework中看不到任何名称与之类似的类型是有原因的。

这与你的问题没有直接关系，但你可能会从阅读中受益。工作很有魅力，还添加了我的其他方法，最后一个问题：如果我只有一个SQLite方法，只需向SQL Server部分添加一个虚拟方法？你可以做到，但我建议在该虚拟实现中抛出一个

invalidoOperationException

。这样，如果您的代码意外调用了不应该调用的方法，您将收到通知。如果您只有一个空方法，那么您可能会在不应该也永远不知道的时候调用它，在这种情况下，您的应用程序可能会做一些不应该做的事情。