这个问题在这里已经有答案了：

什么是nullreferenceexception，如何修复它(28个答案）
两年前关门了。
我有两组函数，它们的工作原理是相似的，但第二组使用的是结构。我正在尝试使用结构设置mysql连接。第一个可行，它使用在另一个模块中声明的公共变量。第二个没有，它使用另一个模块中声明为public的结构。设置connectionstring属性时，它总是给我nullreferenceexception。
第一（工作）

Public Function CheckConnection(server As String, port As String, username As String, password As String) As Boolean
    Try
        sqlConnection.ConnectionString = BuildConnectionString(server, port, username, password)
        sqlConnection.Open()
        Return True
    Catch ex As Exception
        Return False
    Finally
        If Not sqlDataReader Is Nothing Then sqlDataReader.Close()
        If sqlConnection.State = ConnectionState.Open Then sqlConnection.Close()
    End Try        
End Function

第二（不工作）

Public Function CheckConnection(server As String, port As String, username As String, password As String) As Boolean

    Try
        Dim structQuery As New MyQueryStructure
        With structQuery
            .Server = server
            .Port = port
            .Username = username
            .Password = password
            .Connection.ConnectionString = BuildConnectionString(.Server, .Port, .Username, .Password)
        End With
        structQuery.Connection.Open()
        Return True
    Catch ex As Exception
        GenerateErrorReport(ex)
        Return False
    End Try

End Function

附加代码和变量

Public Function BuildConnectionString(stringHost As String, stringPort As String, stringUsername As String, stringPassword As String)
    Return "Server=" & stringHost & ";Port=" & stringPort & ";Uid=" & stringUsername & ";Pwd=" & stringPassword & ";Allow User Variables=True;Persist Security Info=true;SslMode=none;"
End Function

Public sqlConnection As New MySqlConnection
Public sqlQuery As String
Public sqlCommand As New MySqlCommand
Public sqlDataReader As MySqlDataReader
Public sqlDataAdapter As New MySqlDataAdapter
Public sqlDataset As New DataSet
Public sqlDataTable As New DataTable
Public sqlServer, sqlPort, sqlUsername, sqlPassword As String

Public Structure MyQueryStructure
    Dim Connection As MySqlConnection
    Dim Command As MySqlCommand
    Dim DataReader As MySqlDataReader
    Dim DataAdapter As MySqlDataAdapter
    Dim DataSet As DataSet
    Dim DataTablee As DataTable
    Dim Line As String
    Dim Server As String
    Dim Port As String
    Dim Username As String
    Dim Password As String
End Structure

我总是可以回到第一个。但我想学习如何使用结构。另外，我需要我的连接字符串在运行时是可变的。