"Conversion from string to type Double is not valid" error VB.NET

Question

I'm trying to display a logged in users balance from a database in VB. However once I click the Check Balance button it produces the error Conversion from string "Your balance is " to type 'Double' is not valid.

I've tried different ways a converting it from a string to a double, I thought maybe it was because I had m_decBalance declared as a decimal, but that didn't change anything. Can anyone help me? Here's my code:

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

Solution

The problem is that you are joining a string with a +. You need to replace the + with &. I also recommend adding .ToString to your m_decBalance, as this will tell the compiler to treat m_decBalance as a string, like so:

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

The reason you are getting the error is that the compiler tries to convert the string to a numeric value when + is used with a number. For example, the following will display a messagebox with the value of 10:

MsgBox("5 " + " " + 5)

and

Dim Val As Integer = "20 " + 15

will result in Val being 35

When you want to join strings, I recommend using the &, as this tells the compiler that you don't wish to convert the string to a number, and instead you wish to join them as strings.

I would also like to suggest using Option Strict On, as this will help prevent errors like this from happening, as it prevent you from recompiling if you have any implicit conversions (where the compiler has to guess which type you want to convert to)