.Conversion error on unique identifier, shouldn't be referenced at all? - sql

Question

I'm working on the shopping cart for a website, I have this function that retrieves a voucher from my database:
Public Shared Function CheckForVoucher(ByVal strVoucherName As String) As DataTable
Dim connect As New SqlConnection
Dim Data As New DataTable ''Connection works, finds number of Vouchers in DB that match either code or ID. ID to be used for randomly generated vouchers.
connect.ConnectionString = "SERVER = SERVER-SQL01; Trusted_Connection=yes; DATABASE=PCSQL"
connect.Open()
Dim query As String
Dim search As String
search = strVoucherName
If search.Length >= 25 Then
query = "SELECT * from PCSQL.dbo.X_WW_VOUCHER_DETAILS WHERE vID='" & search & "' "
Else
query = "SELECT * from PCSQL.dbo.X_WW_VOUCHER_DETAILS WHERE voucherName='" & search & "' "
End If
Dim command = New SqlDataAdapter(query, connect)
command.Fill(Data)
connect.Close()
strVoucherName = query
Return Data
End Function
This all works peachy-keen on the site I'm working on. At the end of a customers order it fires another function, the increase the totalUses of the voucher by one (my boss wants to make sure that people don't spam the codes and run him out of business).
Public Shared Function addUse(ByVal strVoucherName As String, intCurrentUses As Integer) As Boolean
Dim connect As New SqlConnection
Dim data As New DataTable
connect.ConnectionString = "SERVER = SERVER-SQL01; Trusted_Connection = yes; DATABASE=PCSQL"
connect.Open()
Dim query As String
Dim name As String = strVoucherName
Dim current As Integer = intCurrentUses
Dim newint As Integer = current + 1
query = "UPDATE dbo.X_WW_VOUCHER_DETAILS SET currentUses= #currentUses WHERE voucherName=#voucherName"
Dim command As New SqlCommand(query, connect)
command.Parameters.Add("#voucherName", SqlDbType.VarChar)
command.Parameters("#voucherName").Value = name
command.Parameters.Add("#currentUses", SqlDbType.Int)
command.Parameters("#currentUses").Value = newint
command.ExecuteNonQuery()
connect.Close()
Return True
End Function
This also works fine, and I can check the SQL table in SQL SERVER and see that the "currentUses" column has increased by one. Yay.
Problem
When I test the code by submitting again, it fails if the currentUses column is equal to the validUses column. I end up the error "Conversion failed when converting from a character string to uniqueidentifier" even though the Unique Identifier is not directly referenced at all.
I've tested on a voucher that is valid 5 times. Times 1 through 4, no errors but as soon as currentUses was also 5... no go.
Why would equal columns cause my function to crap itself?
SQL Table
-vID Uniqueidentifier
-voucherName Nchar(15)
-expiryDate Date
-validUses Int
-currentUses Int
-discountType Nchar(15)
-appliesTo Nchar(15)
-numberOf Nchar(20)
-Amount Int
-noOfItems Int
-Category Nchar(100)
-freebieID Nchar(15)
-discountAmount Int
-Description Nchar(255)
EDIT
I have narrowed the error down to this if statement.
If search.Length >= 25 Then
query = "SELECT * from PCSQL.dbo.X_WW_VOUCHER_DETAILS WHERE vID='" & search & "' "
Else
query = "SELECT * from PCSQL.dbo.X_WW_VOUCHER_DETAILS WHERE voucherName='" & search & "' "
End If
The Voucher itself is called from cart.aspx using
Protected Sub btnVoucherCheck_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnVoucherCheck.Click
Dim Data As DataTable = Voucher.CheckForVoucher(txtVoucher.text)
If Data.Rows.Count > 0 Then
Dim row As DataRow
row = Data.Rows(0)
Voucher.VoucherID = row.Item("vID").ToString().Trim()
Voucher.VoucherName = row.Item("voucherName").ToString().Trim()
Voucher.ExpiryDate = row.Item("ExpiryDate")
Voucher.ValidUses = row.Item("ValidUses")
Voucher.CurrentUses = row.Item("CurrentUses")
Voucher.DiscountType = row.Item("DiscountType").ToString().Trim()
Voucher.AppliesTo = row.Item("AppliesTo").ToString().Trim()
Voucher.NumberOf = row.Item("NumberOf").ToString().Trim()
Voucher.Amount = row.Item("Amount")
Voucher.noOfItems = row.Item("NoOfItems")
Voucher.Category = row.Item("Category").ToString().Trim()
Voucher.FreebieID = row.Item("FreebieID").ToString().Trim()
Voucher.DiscountAmount = row.Item("DiscountAmount")
Dim count As Int32
count = 0
Dim expiry As DateTime = Voucher.ExpiryDate
Dim today As DateTime = Date.Today()
count = ((expiry - today).Days)
If count <= -1 Then
txtVoucher.Text = "Voucher expired"
Else 'Further checks. One: Valid uses against current uses, checks if too many people have used the voucher.
If (Voucher.CurrentUses >= Voucher.ValidUses) Then
txtVoucher.Text = "Sorry, Voucher is no longer valid"
Else 'Redirects to one of three methods based on discount type.
c.VoucherName = Voucher.VoucherName
If c.discounted = True Then
For Each item As RepeaterItem In rptCart.Items
Dim quantity As TextBox = CType(item.FindControl("txtQuantity"), TextBox)
Dim ProductID As HyperLink = CType(item.FindControl("hlProductID"), HyperLink)
Dim lblPrice As Label = CType(item.FindControl("lblPrice"), Label)
lblPrice.Text = Product.GetProductPrice(ProductID.Text, quantity.Text, c)
Next
End If
Select Case Voucher.DiscountType
Case "Dollar"
txtVoucher.Text = "$ Not yet Supported"
Case "Percentage"
percentageDiscount()
Case "Freebie"
txtVoucher.Text = "Freebie Not Yet Supported"
Case Else
txtVoucher.Text = "Not working."
End Select
End If
End If
Else
End If
End Sub
The two relevant columns are compared in the middle of this function, but my code doesn't execute that far if they are equal to being with. I get the conversion error referencing the line in my CheckForVoucher function that I pasted above. It can't be my comparison that causes the error because the DataTable I use for that doesn't fully exist at the point the error occurs.

Answer 1

The problem you have is when the value of search is 25 characters or longer, you are passing it into the database to compare with the vID column. That column is a uniqueidentifier which means it's a GUID in C# terms. If the value you pass in cannot be converted directly to a GUID, then you will get a conversion error.