从字符串到双精度的转换无效

Posted 2023-02-25

【中文标题】从字符串到双精度的转换无效

【英文标题】：conversion from string to double is not valid

【发布时间】：2015-07-08 16:12:03

【问题描述】：

导入 mysql.Data.MySqlClient

公开课预约表

Dim MySqlConn As MySqlConnection
Dim Command As MySqlCommand
Dim Valid As Boolean = False




Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    Call Input_Validation()

    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Insert into venuesdb.Event(VenueID, EventName, EventDate, EventStartTime, EventEndTime, EventID, CustomerID) Values ('" & TextBox1.Text & "', '" & TextBox2.Text & "', '" & DateTimePicker1.Text & "', '" & TextBox4.Text & "', '" & TextBox5.Text & "', '" & TextBox6.Text & "', '" & TextBox7.Text & "')"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader

        MessageBox.Show("Booking Added")
        MySqlConn.Close()
        If TextBox1.Text = 0 Then
            MessageBox.Show("Please enter text")
        End If
    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()

        TextBox1.Clear()
        TextBox2.Clear()

        TextBox4.Clear()
        TextBox5.Clear()
        TextBox6.Clear()
        TextBox7.Clear()
    End Try

End Sub

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Delete from venuesdb.Event where EventName = '" & ComboBox1.Text & "'"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader

        MessageBox.Show("Booking Deleted")
        MySqlConn.Close()

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()

    End Try
End Sub

Private Sub TabPage2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Select * from venuesdb.Event"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader
        While Reader.Read
            Dim Bandname = Reader.GetString("EventName")
            ComboBox1.Items.Add(Bandname)
        End While

        MySqlConn.Close()

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()
    End Try

End Sub

Private Sub BookingForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load

End Sub

Private Sub TabPage3_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Select * from venuesdb.Event"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader
        While Reader.Read
            Dim Bandname = Reader.GetString("EventName")
            ComboBox2.Items.Add(Bandname)
        End While

        MySqlConn.Close()

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()

    End Try
End Sub

Private Sub ComboBox2_SelectedIndexChanged(sender As Object, e As EventArgs) Handles ComboBox2.SelectedIndexChanged
    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Select * from venuesdb.Event Where EventName = '" & ComboBox2.Text & "'"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader
        While Reader.Read
            TextBox14.Text = Reader.GetString("EventName")
            TextBox13.Text = Reader.GetString("VenueID")

            TextBox11.Text = Reader.GetString("EventStartTime")
            TextBox10.Text = Reader.GetString("EventEndTime")
            TextBox9.Text = Reader.GetString("EventID")
            TextBox8.Text = Reader.GetString("CustomerID")
            DateTimePicker2.Text = Reader.GetDateTime("EventDate")

        End While

        MySqlConn.Close()

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()

    End Try
End Sub

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
    MySqlConn = New MySqlConnection
    MySqlConn.ConnectionString = "Server = Localhost; database = venuesdb; user id = root; Password = "
    Dim Reader As MySqlDataReader

    Try
        MySqlConn.Open()
        Dim Query As String
        Query = "Update venuesdb.Event set EventName = '" & TextBox1.Text & "', VenueID = '" & TextBox2.Text & "', EventDate = '" & DateTimePicker2.Text & "'where EventName = '" & TextBox1.Text & "'"
        Command = New MySqlCommand(Query, MySqlConn)
        Reader = Command.ExecuteReader

        MessageBox.Show("Booking Updated")
        MySqlConn.Close()

    Catch ex As Exception
        MessageBox.Show(ex.Message)
    Finally
        MySqlConn.Dispose()

    End Try

    TextBox1.Clear()
    TextBox2.Clear()

    TextBox4.Clear()
    TextBox5.Clear()
    TextBox6.Clear()
    TextBox7.Clear()
End Sub

我有一个错误，当我单击一个按钮时不断弹出错误提示“从字符串“转换为双精度类型无效” 如您所见，我没有尝试将任何内容转换为双精度

请帮忙

【问题讨论】：

你确定这是抛出异常的地方吗？没有其他课程可以转换为Double？

不，我已经检查了所有表格，但我还没有将任何东西转换成双倍？

Switch Option Strict On 开始...

【参考方案1】：

这将隐式尝试转换为 Double 以尝试运行比较：

If TextBox1.Text = 0 Then
    MessageBox.Show("Please enter text")
End If

您可能希望将其（以及任何其他事件）包装在

If Textbox1.Text <> "" Then
    If TextBox1.Text = 0 Then
        MessageBox.Show("Please enter text")
    End If
End If

或者，也许更好，将其更改为：

If TextBox1.Text.Length = 0 Then 
    ....

或

If TextBox1.Text = "" Then
    ....

比较长度不会在比较之前尝试将其转换为双精度数。

【讨论】：

非常感谢，我放错地方了