為什么我在連接 mysql 時在 Vb.net 中收到此錯誤“從字符串轉換為類型 'Double' 無效”？

Question

我分別嘗試了這個等式。 有效。 但是我加入后，我得到了轉換的錯誤。 我的目標是獲取 MySql 數據到 vb.net 所以我可以檢查一些值並開發項目。 我必須在一周內完成這個項目，我不知道如何完成這個。 如果這看起來很容易，請原諒我。

Imports MySql.Data.MySqlClient
Public Class Form3
    Dim conn As MySqlConnection
    Dim command As MySqlCommand
    Dim cmd As MySqlCommand
    Dim Da As New MySqlDataAdapter
    Dim ds As New DataSet

    Private Sub Form3_Load(sender As Object, e As EventArgs) Handles MyBase.Load

        'To check whether the date is same
        TextBox1.Text = System.DateTime.Now.ToString(("MM/dd/yyyy"))

    End Sub

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

        conn = New MySqlConnection
        conn.ConnectionString = "server=localhost;user=root;password=1234;database=attendance"
        Dim reader As MySqlDataReader
        Try
            ds.Clear()
            conn.Open()
            'Checking Subject Now
            cmd = New MySqlCommand("select Subject_Name from dateverification", conn)
            Da = New MySqlDataAdapter(cmd)
            Da.Fill(ds, "dateverification")
            TextBox2.Text = ds.Tables(0).Rows(0).Item(0)

            'Checking Todays Date
            cmd = New MySqlCommand("select Today_Date from dateverification", conn)
            Da = New MySqlDataAdapter(cmd)
            Da.Fill(ds, "dateverification")
            Label1.Text = ds.Tables(0).Rows(0).Item(0)

            'Checking Count1
            cmd = New MySqlCommand("select Count1 from dateverification", conn)
            Da = New MySqlDataAdapter(cmd)
            Da.Fill(ds, "dateverification")
            Label2.Text = ds.Tables(0).Rows(0).Item(0)

            'Checking Count2
            cmd = New MySqlCommand("select Count2 from dateverification", conn)
            Da = New MySqlDataAdapter(cmd)
            Da.Fill(ds, "dateverification")
            Label3.Text = ds.Tables(0).Rows(0).Item(0)

            'If the days are Different, Total days will be counted and Date will be updated
            If Label1.Text <> TextBox1.Text Then
                Label1.Text = System.DateTime.Now.ToString(("yyyy-MM-dd"))
                Label2.Text = Label2.Text + 1
                Dim query1 As String
                query1 = "UPDATE attendance.dateverification SET Today_Date = '" & Label1.Text & "' , Count1 = '" & Label2.Text & "' WHERE Subject_Name = '" & TextBox3.Text & "'; "
                command = New MySqlCommand(query1, conn)
                reader = command.ExecuteReader
                MessageBox.Show("Welcome to New Day")

            Else
            'If the date are equal, then the number of counts which register wasopen in same day will be increased
                Label3.Text = Label3.Text + 1
                Dim query1 As String
                query1 = "UPDATE attendance.dateverification SET Count2 = '" & Label3.Text & "' WHERE Subject_Name = '" & TextBox3.Text & "'; "
                command = New MySqlCommand(query1, conn)
                reader = command.ExecuteReader
                MessageBox.Show("You are still on the same day")

            End If

            Dim query As String
            query = "UPDATE attendance.dateverification SET Subject_selected = '" & TextBox3.Text & "' WHERE Subject_Name = '" & TextBox3.Text & "'; "
            command = New MySqlCommand(query, conn)
            reader = command.ExecuteReader
            conn.Close()

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

        End Try

Answer 1

錯誤可能是因為此代碼並不總是合法的：

Label3.Text = Label3.Text + 1

Label3.Text是一個字符串。 根據編譯器選項，您不能只期望編譯器為您將其轉換為數字。 這些選項可以幫助您，您應該使用它們。

繼續。

在 MySql 上不是 100% 確定，但在 Sql 服務器中，您可以確定您可以將所有這些作為數據庫中的一個連接命令作業來完成。 這將表現得更好，並幫助我大大簡化了代碼。

特別注意我是如何使用參數的。 使用字符串連接將參數值放入查詢中是絕對不行的。 即使對於學習或概念驗證代碼，這也太重要了。

Option Strict On
Option Infer On

Imports System
Imports MySql.Data.MySqlClient

Public Class Form3

    'Don't try to keep the connection/command objects at the form level!
    'Do keep the connection string here:
    Private ConnectionString As String = "server=localhost;user=root;password=1234;database=attendance"

    Private Sub Form3_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        TextBox1.Text = Today.ToString("MM/dd/yyyy")
    End Sub

    Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Dim conn As SqlConnection

        'I put two statements in one command string,
        ' and the CASE expressions let me get all of the updates into one statement
        Dim sql As String = 
"UPDATE attendance.dateverification 
    SET Count1 = CASE WHEN Today_Date <> current_date THEN Count1 + 1 ELSE Count1 END,
        Count2 = CASE WHEN Today_Date = current_date THEN Count2 + 1 ELSE Count2 END,   
        Today_Date = current_date,
        Subject_selected = @subject 
WHERE Subject_Name = @subject;

SELECT Subject_Name, Today_Date, Count1, Count2 
FROM dateverification"
' Also note: this string could be a constant if we wanted to!

        Try
            conn = New MySqlConnection(ConnectionString)
            Dim cmd As New MySqlCommand(sql, conn)
            'Use actual type and length here
            cmd.Parameters.Add("@subject", MySqlDbType.VarString, 50).Value = TextBox3.Text

            conn.Open()
            Dim rdr As MySqlDataReader = cmd.ExecuteReader()
            rdr.Read()

            TextBox2.Text = rdr("Subject_Name").ToString()
            Label1.Text = rdr("Today_Date").ToString()
            Label2.Text = rdr("Count1").ToString()
            Label3.Text = rdr("Count2").ToString()
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally 
            conn.Dispose()
        End Try
    End Sub
End Class