"从字符串转换为Double类型无效"错误VB.NET

Question

我正在尝试在VB中显示来自数据库的登录用户余额.但是,一旦我单击"检查余额"按钮,就会产生错误Conversion from string "Your balance is " to type 'Double' is not valid.

我尝试过将它从字符串转换为double的不同方法,我想也许是因为我将m_decBalance声明为小数,但这并没有改变任何东西.谁能帮我？这是我的代码:

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

Answer 1

问题是你要加入一个字符串+.你需要更换+用&.我还建议添加.ToString到您的m_decBalance,因为这将告诉编译器将其m_decBalance视为一个string,如下所示:

MsgBox("You balance is " & " " & m_decBalance.ToString)

您收到错误的原因是编译器在与数字一起+使用时尝试将字符串转换为数值.例如,以下内容将显示一个值为的消息框10:

MsgBox("5 " + " " + 5)

和

Dim Val As Integer = "20 " + 15

将导致Val存在35

当你想连接字符串时,我建议使用&,因为这告诉编译器你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起来.

我还建议使用Option Strict On,因为这将有助于防止这样的错误发生,因为它阻止你重新编译,如果你有任何implicit conversions(编译器必须猜测你想要转换为哪种类型)