Mysql &引用;从字符串到类型Double的转换无效";错误VB.NET
我试图在VB中显示从数据库中登录的用户余额。但是,一旦我单击“检查余额”按钮,它就会产生错误Mysql &引用;从字符串到类型Double的转换无效";错误VB.NET,mysql,vb.net,string,double,type-conversion,Mysql,Vb.net,String,Double,Type Conversion,我试图在VB中显示从数据库中登录的用户余额。但是,一旦我单击“检查余额”按钮,它就会产生错误从字符串“您的余额是”到类型“Double”的转换无效 我尝试了不同的方法将其从字符串转换为双精度,我想可能是因为我将m_decBalance声明为十进制,但这并没有改变任何事情。有人能帮我吗?这是我的密码: Imports MySql.Data Imports MySql.Data.MySqlClient Public Class Form1 Dim dbCon As MySqlConnec
从字符串“您的余额是”到类型“Double”的转换无效
我尝试了不同的方法将其从字符串转换为双精度,我想可能是因为我将m_decBalance声明为十进制,但这并没有改变任何事情。有人能帮我吗?这是我的密码:
Imports MySql.Data
Imports MySql.Data.MySqlClient
Public Class Form1
Dim dbCon As MySqlConnection
Dim strQuery As String = ""
Dim SQLcmd As MySqlCommand
Dim DataReader As MySqlDataReader
Private m_strPass As String
Private m_decBalance As Decimal
Private m_strName As String
Private m_strUserPass As String
Private m_strCardNumber As String
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles btnLogin.Click
'Assign users guessed password to variable
m_strUserPass = txtPass.Text
'Invoke
RetrieveAccountInformation()
' determine if Password is correct or not
If m_strUserPass = m_strPass Then
lblWelcome.Visible = True
lblWelcome.Text = "Welcome" + " " + m_strName
txtPass.Enabled = False
btnBalance.Enabled = True
Else
' indicate that incorrect password was provided
lblWelcome.Visible = True
lblWelcome.Text = "Sorry, Password is incorrect." _
& "Please retry ."
' clear user's previous PIN entry
m_strUserPass = ""
End If
txtPass.Clear() ' clear TextBox
End Sub
' load application Form
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
'Prepare connection and query
Try
dbCon = New MySqlConnection("Server=localhost;Database=test;Uid=root;Pwd=mysql")
strQuery = "SELECT CardNumber " &
"FROM Account"
SQLcmd = New MySqlCommand(strQuery, dbCon)
'Open the connection
dbCon.Open()
' create database reader to read information from database
DataReader = SQLcmd.ExecuteReader
' fill ComboBox with account numbers
While DataReader.Read()
cboAccountNumbers.Items.Add(DataReader("CardNumber"))
End While
'Close the connection
DataReader.Close()
dbCon.Close()
Catch ex As Exception
'Output error message to user with explaination of error
MsgBox("Failure to communicate" & vbCrLf & vbCrLf & ex.Message)
End Try
End Sub
' invoke when user provides account number
Private Sub RetrieveAccountInformation()
' specify account number of record from which data will be retrieved
dbCon = New MySqlConnection("Server=localhost;Database=test;Uid=root;Pwd=mysql")
strQuery = "SELECT Name, Balance, Password " &
"FROM Account WHERE CardNumber='" & Val(cboAccountNumbers.Text) & "' "
SQLcmd = New MySqlCommand(strQuery, dbCon)
dbCon.Open() ' open database connection
' create database reader to read information from database
DataReader = SQLcmd.ExecuteReader
DataReader.Read() ' open data reader connection
' retrieve Password number, balance amount and name information from database
m_strPass = Convert.ToString(DataReader("Password"))
m_decBalance = Convert.ToString(DataReader("Balance"))
m_strName = Convert.ToString(DataReader("Name"))
DataReader.Close() ' close data reader connection
dbCon.Close() ' close database connection
End Sub ' RetrieveAccountInformation
Private Sub btnBalance_Click(sender As Object, e As EventArgs) Handles btnBalance.Click
'Retrieve their account information
RetrieveAccountInformation()
Try
MsgBox("You balance is " + " " + m_decBalance)
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
End Class
使用和进行字符串连接。您正在使用+
MsgBox("You balance is " & " " & m_decBalance)
应始终使用“&”符号将字符串连接到其他字符串
MsgBox("You balance is " & " " & m_decBalance)
问题是您正在用+
连接字符串。您需要将+
替换为&
。我还建议将.ToString
添加到m_decBalance
,因为这将告诉编译器将m_decBalance
视为字符串,如下所示:
MsgBox("You balance is " & " " & m_decBalance.ToString)
出现错误的原因是当+
与数字一起使用时,编译器试图将字符串转换为数值。例如,下面将显示一个值为10
的消息框:
MsgBox("5 " + " " + 5)
及
将导致Val
为35
当您想要连接字符串时,我建议使用&
,因为这会告诉编译器您不希望将字符串转换为数字,而是希望将它们作为字符串连接
我还建议对
使用严格的选项,因为这将有助于防止发生类似错误,因为如果您有任何隐式转换
(编译器必须猜测您要转换为哪种类型),它将阻止您重新编译在我的数据库中,Balance
是一个整数,它的值是200。那么我需要在数据库中将其更改为双精度还是什么?@user3275784由于您的错误,该问题与数据库无关(请参阅下面我的帖子)。您可以将Balance
列保留为整数(除非您需要在数据库中存储双精度)
Dim Val As Integer = "20 " + 15