Question

在组合框中选择问题ID后，相关问题应出现在文本框中。我不确定如何让它工作。我在retrieveQuestion（）上收到错误“类型的值......无法转换为字符串”。感谢任何帮助，谢谢你。

Private Sub cmbQuestion_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmbQuestion.SelectedIndexChanged

            txtExistingQuestion.Text = retrieveQuestion() 'Add question, relevant to Question ID, to text box, DO I NEED .ToString?????????????????      
            loaded = True

        End Sub

        Public Function retrieveQuestion() As List(Of Question) 'Retrieves selected question into text box

            Dim typeList As New List(Of Question)
            Dim Str As String = "SELECT Question_ID, Question_Text FROM Question WHERE Question_ID =" & cmbQuestion.SelectedValue
            Try
                Using conn As New SqlClient.SqlConnection(DBConnection)
                    conn.Open()
                    Using cmdQuery As New SqlClient.SqlCommand(Str, conn)
                        Using drResult As SqlClient.SqlDataReader = cmdQuery.ExecuteReader()
                            While drResult.Read
                                typeList.Add(New Question(drResult("Question_ID"), drResult("Question_Text")))
                            End While
                        End Using 'Automatically closes connection
                    End Using
                End Using

            Catch ex As Exception

                MsgBox("Question List Exception: " & ex.Message & vbNewLine & Str)

            End Try

            Return typeList

        End Function

    Public Class Question 'defining one club within class
            Public Sub New(ByVal questionID As Integer, ByVal questionText As String)

                mQuestionID = questionID 'm is for member of the class
                mQuestionText = questionText

            End Sub

            Private mQuestionID As String = ""
            Private mQuestionText As String = ""

            Public Property QuestionID() As String
                Get
                    Return mQuestionID
                End Get
                Set(ByVal value As String)
                    mQuestionID = value
                End Set
            End Property

            Public Property QuestionText() As String
                Get
                    Return mQuestionText
                End Get
                Set(ByVal value As String)
                    mQuestionText = value
                End Set
            End Property
        End Class

Answer 1

您的问题是这一行：

mQuestionID = questionID

在你的课堂上你已经定义了这个：

Private mQuestionID As String = ""

但是在你的构造函数中，你是说questionID应该是Integer，如下所示：

Public Sub New(ByVal questionID As Integer, ByVal questionText As String)

您需要将班级中的支持变量（mQuestionID）更改为Integer，如下所示：

Private mQuestionID As Integer

这还需要更改QuestionID的属性语法，如下所示：

Public Property QuestionID() As Integer
    Get
        Return mQuestionID
    End Get
    Set(ByVal value As Integer)
        mQuestionID = value
    End Set
End Property

Answer 2

您的错误是由声明为List(Of Question)的 retrieveQuestion 的返回值引起的，但是您尝试设置TextBox的text属性（并且无法自动转换为列表（问题）到字符串）

所以你可以写这样的东西来提取列表中第一个问题的文本

  Dim qList = retrieveQuestion()
  if qList.Count > 0 then
       txtExistingQuestion.Text = qList(0).QuestionText
       loaded = True
  End If

当然，如果您的查询返回零或只有一个问题，则无需返回List(Of Question)，您可以更改 retrieveQuestion 方法，只返回{{1 }或Question

Nothing

但是，关于字符串和整数转换的所有注释都会自动发生在您的代码上，这实际上是一个警钟。您应该努力避免这种转换，因为它们会使代码变弱并容易出现错误的错误。切换到项目属性的Option Strinct On并准备好进行大量的转换修复。

在ComboBox中选择ID时，难以在TextBox中检索文本

2 个答案: