对于您的后续问题：
您可以将一个名为 StartUp.vb 的模块添加到您的项目中，如下所示：
Imports System
Imports System.Diagnostics
Imports System.Windows.Forms
Imports MySql.Data.MySqlClient
Module StartUp
'Private Fields
<DebuggerBrowsable(DebuggerBrowsableState.Never)>
Private _LoginDetails As LoginDetails
'Main entry point
Public Sub Main(args As String())
'Standard initialization, copied from a C# project
Application.EnableVisualStyles()
Application.SetCompatibleTextRenderingDefault(False)
'Start login screen
Dim myLoginDetails As LoginDetails = Nothing
Using myLoginForm As New LoginForm()
myLoginForm.ShowDialog()
myLoginDetails = myLoginForm.LoginDetails
End Using
'Assign details
If (myLoginDetails Is Nothing) Then
Environment.ExitCode = -1 'Login cancelled
Application.Exit()
End If
LoginDetails = myLoginDetails
'Start main application
Application.Run(New MainForm())
End Sub
'Public Properties
Public Property LoginDetails As LoginDetails
Get
Return _LoginDetails
End Get
Private Set(value As LoginDetails)
_LoginDetails = value
End Set
End Property
Public ReadOnly Property FooDB As MySqlConnection
Get
Return LoginDetails?.FooDB
End Get
End Property
End Module
然后打开project properties、Application 并取消选中复选框[ ] Enable Application Framework。取消选中复选框后，选择StartUp（我们新添加的模块）作为Startup Object。
还要确保编译选项设置为
Option explicit = On、Option strict = On 和 Option infer = On。
然后您可能想要添加一个名为 LoginDetails.vb 的类，其中包含您需要的登录详细信息，包括具有实际数据库连接的属性。
Imports System
Imports System.Data
Imports System.Diagnostics
Imports MySql.Data.MySqlClient
Public Class LoginDetails
'Private Fields
<ThreadStatic>
<DebuggerBrowsable(DebuggerBrowsableState.Never)>
Private _OpenConnection As MySqlConnection
'Constructors
Public Sub New(extendedLogging As Boolean, userName As String, password As String)
Dim myRawConnectionString As String = If(extendedLogging, My.Settings.TestDB, My.Settings.ProdDB)
Dim myBuilder As New MySqlConnectionStringBuilder(myRawConnectionString)
myBuilder.UserID = userName
myBuilder.Password = password
myBuilder.Pooling = False
If (extendedLogging) Then
myBuilder.UsePerformanceMonitor = True
myBuilder.UseUsageAdvisor = True
End If
ConnectionString = myBuilder.ToString()
Me.UserName = userName
Me.DatabaseName = myBuilder.Database
Me.ServerName = myBuilder.Server
End Sub
'Public Properties
Public ReadOnly Property UserName As String
Public ReadOnly Property DatabaseName As String
Public ReadOnly Property ServerName As String
Public ReadOnly Property ExtendedLogging As Boolean
''' <summary>Returns an open connection to the FooDB or throws an exception if not possible.</summary>
Public ReadOnly Property FooDB As MySqlConnection
Get
'Return from cache
Dim myResult As MySqlConnection = _OpenConnection
If (myResult IsNot Nothing) Then
'Return open conneciton
If (myResult.State = ConnectionState.Open) Then Return myResult
'Try to open it
Try
myResult.Open()
If (myResult.State = ConnectionState.Open) Then Return myResult
Catch
End Try
'Kill corrupted connection
Try
myResult.Dispose()
Catch
End Try
End If
'Initialize and return
Try
myResult = New MySqlConnection(ConnectionString)
myResult.Open()
If (myResult.State = ConnectionState.Open) Then
_OpenConnection = myResult
Return myResult
End If
Catch
End Try
'Throw exception
Throw New ApplicationException($"Unable to connect to database {DatabaseName} on server {ServerName}!")
End Get
End Property
'Private Properties
Private ReadOnly Property ConnectionString As String
End Class
LoginForm 背后的代码再简单不过了：
Imports System
Public Class LoginForm
Public Property LoginDetails As LoginDetails
Private Sub btnOkay_Click(sender As Object, e As EventArgs) Handles btnOkay.Click
LoginDetails = New LoginDetails(ExtLogCheckB.Checked, txtLoginName.Text, txtPassword.Text)
Close()
End Sub
Private Sub btnCancel_Click(sender As Object, e As EventArgs) Handles btnCancel.Click
LoginDetails = Nothing
Close()
End Sub
End Class
一些简化数据访问的扩展方法：
Imports System
Imports System.Data
Imports System.Reflection
Imports System.Runtime.CompilerServices
Imports System.Threading.Tasks
Imports MySql.Data.MySqlClient
'Warning: Don't use string based queries because of the danger of SQL-injection! I told you...
<Extension()>
Module ExtMySqlConnection
<Extension()>
Public Function ExecToString(connection As MySqlConnection, query As String) As String
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myCommand As New MySqlCommand(query, connection)
Dim myResult As Object = myCommand.ExecuteScalar()
Return CType(myResult, String)
End Function
<Extension()>
Public Function ExecToInt32(connection As MySqlConnection, query As String) As Int32
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myCommand As New MySqlCommand(query, connection)
Dim myResult As Object = myCommand.ExecuteScalar()
Return CType(myResult, Int32)
End Function
<Extension()>
Public Function ExecToInt32Nullable(connection As MySqlConnection, query As String) As Int32?
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myCommand As New MySqlCommand(query, connection)
Dim myResult As Object = myCommand.ExecuteScalar()
Return CType(myResult, Int32?)
End Function
<Extension()>
Public Function ExecToDateTime(connection As MySqlConnection, query As String) As DateTime
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myCommand As New MySqlCommand(query, connection)
Dim myResult As Object = myCommand.ExecuteScalar()
Return CType(myResult, DateTime)
End Function
<Extension()>
Public Function ExecToDateTimeNullable(connection As MySqlConnection, query As String) As DateTime?
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myCommand As New MySqlCommand(query, connection)
Dim myResult As Object = myCommand.ExecuteScalar()
Return CType(myResult, DateTime?)
End Function
<Extension()>
Public Function ExecToDataTable(connection As MySqlConnection, ByVal query As String) As DataTable
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myDataSet As DataSet = ExecToDataSet(connection, query)
Return myDataSet.Tables(0)
End Function
<Extension()>
Public Function ExecToDataSet(connection As MySqlConnection, ByVal query As String) As DataSet
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myResult As New DataSet()
Try
Dim myCommand As New MySqlCommand(query, connection)
Dim cmd As New MySqlDataAdapter(myCommand)
cmd.Fill(myResult)
Finally
'CloseConnection()
End Try
Return myResult
End Function
''' <summary>Takes the connection and executes the given query on it and returns the result as a single row of type
''' <see cref="DataRow" />. If the query results in 0 rows, null is returned. If the query results in multiple rows,
''' an <see cref="AmbiguousMatchException" /> is thrown.</summary>
''' <param name="connection">The connection on which to invoke the query (a <see cref="NullReferenceException" /> is thrown if the parameter is null to simulate instance method behavior).</param>
''' <param name="query">The SQL statement to be executed.</param>
''' <returns>The according <see cref="DataRow" /> or null (or an <see cref="AmbiguousMatchException" /> may be thrown).</returns>
<Extension()>
Public Function ExecToDataRow(connection As MySqlConnection, ByVal query As String) As DataRow
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Return ExecToDataRow(connection, query, False)
End Function
''' <summary>Takes the connection and executes the given query on it and returns the result as a single row of type
''' <see cref="DataRow" />. If the query results in 0 rows, null is returned. If the query results in multiple rows,
''' it depends on parameter <paramref name="ignoreAdditionalRows"/> whether the first record is returned (true) or
''' an <see cref="AmbiguousMatchException" /> is thrown (false).</summary>
''' <param name="connection">The connection on which to invoke the query (a <see cref="NullReferenceException" /> is thrown if the parameter is null to simulate instance method behavior).</param>
''' <param name="query">The SQL statement to be executed.</param>
''' <param name="ignoreAdditionalRows">Determines whether additional rows should be silently ignored if more than one rows are returnd (true) or whether an <see cref="AmbiguousMatchException" /> should be thrown (false).</param>
''' <returns>The according <see cref="DataRow" /> or null (or an <see cref="AmbiguousMatchException" /> may be thrown).</returns>
<Extension()>
Public Function ExecToDataRow(connection As MySqlConnection, ByVal query As String, ignoreAdditionalRows As Boolean) As DataRow
'Check args
If (connection Is Nothing) Then Throw New NullReferenceException()
If (String.IsNullOrWhiteSpace(query)) Then Throw New ArgumentNullException(NameOf(query), "The query is null or empty!")
'Execute query
Dim myDataTable As DataTable = ExecToDataTable(connection, query)
'Handle 1 or 0 records
Select Case myDataTable.Rows.Count
Case 1
Return myDataTable.Rows(0)
Case 0
Return Nothing
End Select
'Determine what to do if there are more than one record
If (Not ignoreAdditionalRows) Then
Throw New AmbiguousMatchException()
End If
Return myDataTable.Rows(0)
End Function
End Module
一个示例实体：
Imports System
Imports System.Data
Imports System.Diagnostics
<DebuggerDisplay("{DebuggerDisplayValue}")>
Public Class SampleEntity
'Private Fields
Private ReadOnly _Record As DataRow
'Constructors
Public Sub New(record As DataRow)
If (record Is Nothing) Then Throw New ArgumentNullException(NameOf(record))
_Record = record
End Sub
'Public Properties
Public ReadOnly Property RecID As Int32
Get
Return _Record.Field(Of Int32)("RecID")
End Get
End Property
Public ReadOnly Property LastName As String
Get
Return _Record.Field(Of String)("LastName")
End Get
End Property
Public ReadOnly Property FirstName As String
Get
Return _Record.Field(Of String)("FirstName")
End Get
End Property
Public ReadOnly Property FullName As String
Get
Return If(LastName, "") & " " & If(FirstName, "").Trim()
End Get
End Property
Public ReadOnly Property EntryDate As DateTime
Get
Return _Record.Field(Of DateTime)("EntryDate")
End Get
End Property
Public ReadOnly Property LeavingDate As DateTime?
Get
Return _Record.Field(Of DateTime?)("LeavingDate")
End Get
End Property
Public ReadOnly Property IsActive As Boolean
Get
Return _Record.Field(Of Boolean)("IsActive")
End Get
End Property
'Private Properties
Private ReadOnly Property DebuggerDisplayValue As String
Get
Return $"{RecID}: {FullName}"
End Get
End Property
End Class
现在您可以非常轻松地访问应用程序中的数据表单（例如 MainForm）：
Imports System.Collections.Generic
Imports System.Data
Public Class MainForm
Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
Dim myAppUsers = GetUsers()
'Bind the users to some control
End Sub
Private Iterator Function GetUsers() As IEnumerable(Of SampleEntity)
Dim myDT = FooDB.ExecToDataTable("SELECT * FROM AppUser ORDER BY LastName, FirstName")
For Each myRecord As DataRow In myDT.Rows
Yield New SampleEntity(myRecord)
Next
End Function
End Class
我向大家道歉，我知道它现在更像是一个教程，而不是一个问题的简单答案。我保证改革。
最后提示：代码可以编译，但未经测试。