database - DBnull 到整数转换错误

Question

我得到 从类型 'DBNull' 到类型 'Integer' 的转换无效。”错误行“Dim avgObject As string = Cstr(avgCom.ExecuteScalar())

当删除 where module_ID='"& moduleSelect & "' 语句时，该命令有效，我不知道如何解决这个问题，有人可以帮忙吗？

    Dim moduleSelect As String = moduleRadio.SelectedValue
    Using connection As New SqlConnection("Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True")
        Using avgCom As New SqlCommand("SELECT AVG(exam) FROM completed_module where module_ID='" & moduleSelect & "' ", _
                                        connection)
            connection.Open()
            Dim avgObject As Integer = CInt(avgCom.ExecuteScalar())
            Dim averageVar As String
            averageVar = avgObject.ToString
            avgLabel.Text = averageVar
        End Using

Answer 1

我相信你正在寻找这样的东西，首先检查它是否为 dbnull:

Dim moduleSelect As String = moduleRadio.SelectedValue

Using connection As New SqlConnection("Data Source=.\SQLEXPRESS;AttachDbFilename=|DataDirectory|\Database.mdf;Integrated Security=True;User Instance=True")
    Using avgCom As New SqlCommand("SELECT AVG(exam) FROM completed_module where module_ID='" & moduleSelect & "' ", _
                                    connection)
        connection.Open()
        Dim result = avgCom.ExecuteScalar()
        If IsDBNull(result) then return
        Dim avgObject As Integer = CInt(result)
        Dim averageVar As String
        averageVar = avgObject.ToString
        avgLabel.Text = averageVar
    End Using