Функция для возврата mysqlconnection

Решением вашей проблемы может быть создание свойства, которое определяет, какую строку подключения следует вернуть и к которой осуществляется доступ в кнопке (или ссылке или любом другом следующем элементе пользовательского интерфейса, который пользователь должен вызвать для продолжения).

Но также не компилируйте строки подключения в свой код, пожалуйста. Откройте свойства проекта, Настройки и добавьте туда две строки с двумя строками подключения, например.

Name    Type                 Scope        Value
------  -------------------  -----------  ----------------------------------------------------------------
ProdDB  (Connection string)  Application  server=100.XX.XX.XX;userid=idname;password=idpass;database=my_db
TestDB  (Connection string)  Application  server=200.XX.XX.XX;userid=idname;password=idpass;database=my_db

Это добавляет их в ваш App.config и позволяет вам изменять их в будущем без необходимости перекомпилировать ваше приложение. (Хотя, если ваша строка подключения содержит имя пользователя и пароль, может быть несколько безопаснее скомпилировать их в исполняемый файл.)

Код в вашей форме может выглядеть так:

Private ReadOnly Property ConnectionString As String
    Get
        With My.Settings
            Return If(ExtLogCheckB.Checked, .TestDB, .ProdDB)
        End With
    End Get
End Property

Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Dim myConnection As New MySqlConnection(ConnectionString)
    'Do something with it...
End Sub

На ваш дополнительный вопрос:

Вы можете добавить в свой проект модуль с именем StartUp.vb, который выглядит так:

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

И откройте project properties, Application и снимите флажок [ ] Enable Application Framework. Сняв флажок, выберите StartUp (наш недавно добавленный модуль) как Startup Object.

Также убедитесь, что параметры компиляции установлены на Option explicit = On, Option strict = On и Option infer = On.

Затем вы можете добавить класс с именем LoginDetails.vb, содержащий необходимые данные для входа, включая свойство с фактическим подключением к базе данных.

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

Код LoginForm вряд ли может быть проще:

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

Некоторые методы расширения для упрощения доступа к данным:

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

Образец сущности:

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

И теперь вы можете довольно легко получить доступ к форме данных в вашем приложении (например, 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

Я извиняюсь перед всеми, я знаю, что теперь это стало скорее учебным пособием, чем простым ответом на вопрос. Я обещаю исправиться.

Последний совет: код компилируется, но в остальном не тестируется.