Question

I got the error “invalid cast exception unhandled.” I’m using SQL Server 2008 and Visual Studio 2008. I get conversion from String “Jack” to type Boolean is not valid.

See the code of my form below:

Imports System.Boolean

Imports System.Data.SqlClient

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim username As String
        Dim pswd As String
        Dim conn As SqlConnection
        Dim cmd As SqlCommand
        Dim reader As SqlDataReader

        username = txt_username.Text
        pswd = txt_password.Text
        txt_username.Text = Focus()
        txt_password.Visible = "False"
        Try
            conn = New SqlConnection("data source=TAMIZHAN\SQLEXPRESS;persistsecurity info=False;initial catalog=log;User ID=sa;Password=123");
            cmd = New SqlCommand("Select usename,passw from Userlog where usename='" + username + "' & passw='" + pswd + "'")
            conn.Open()
            reader = cmd.ExecuteReader()
            If (String.Compare(pswd and '"+passw+"')) Then
                MsgBox("Success")
            End If
            If (reader.Read()) Then

                MsgBox("Login success")

            End If
            conn.Close()
        Catch ex As Exception
            MsgBox("Error"+ex.Message());
        End Try
    End Sub
End Class

Answer 1

The string.Compare returns an Integer not a boolean and should be called in this way

If (String.Compare(pswd,passw) = 0) Then 
    MsgBox("Success") 
End If 

Please see the references on MSDN

However your code has many problems as it stands now:

• You are using string concatenation to build your sql text
  (SqlInjection, Quoting problems).
• You don’t use the Using statement (connection remains open in case of
  exception).
• The compare logic seems to be absolutely not needed.