display a warning message when an exception occurs - vb.net

Question

i have a webservice that inserts, delete, updates the data from or to a database. so in this particular form it has a foreign key. so basically it should only accept values which are present in the primarty table.
when user enters any other value a exception occurs.
How do I make sure that such type of SQL exception when caught, a user friendly custom error message(Such as the entered ID doesn't exist.Data insertion failed!) is passed from my web service.
Thanks a lot.
Imports System.Web.Services
Imports System.Web.Services.Protocols
Imports System.ComponentModel
Imports System
Imports System.Collections.Generic
Imports System.Configuration
Imports System.Data.SqlClient
Imports System.Web.Script.Serialization
Imports System.Web.Script.Services
' To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
<System.Web.Script.Services.ScriptService()>
<System.Web.Services.WebService(Namespace:="http://tempuri.org/")>
<System.Web.Services.WebServiceBinding(ConformsTo:=WsiProfiles.BasicProfile1_1)>
<ToolboxItem(False)>
Public Class WebService2
Inherits System.Web.Services.WebService
Public Class Details
Public Property CardID As Integer
Public Property BookID As Integer
Public Property IssuedBy As String
End Class
<WebMethod>
Public Sub AddBook(ByVal emp As Details)
Dim cs1 As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con1 As SqlConnection = New SqlConnection(cs1)
Dim thequery As String = "select * from BookIssue where CardID=#CardID"
Dim cmd1 As SqlCommand = New SqlCommand(thequery, con1)
cmd1.Parameters.Add(New SqlParameter() With {
.ParameterName = "#CardID",
.Value = emp.CardID
})
con1.Open()
Dim reader As SqlDataReader = cmd1.ExecuteReader()
If reader.HasRows Then
MsgBox("ID Number is Already in use")
con1.Close()
Else
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("spInsertIntoBookIssue", con)
cmd.CommandType = CommandType.StoredProcedure
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#CardID",
.Value = emp.CardID
})
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#BookID",
.Value = emp.BookID
})
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#IssuedBy",
.Value = emp.IssuedBy
})
con.Open()
cmd.ExecuteNonQuery()
MsgBox("Data Inserted")
End Using
End If
End Using
End Sub
<WebMethod>
Public Sub GetAllDetails()
Dim listEmployees As List(Of Details) = New List(Of Details)()
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("Select * from BookIssue", con)
con.Open()
Dim rdr As SqlDataReader = cmd.ExecuteReader()
While rdr.Read()
Dim details As Details = New Details()
details.CardID = Convert.ToInt32(rdr("CardID"))
details.BookID = Convert.ToInt32(rdr("BookID"))
details.IssuedBy = rdr("IssuedBy").ToString()
listEmployees.Add(details)
End While
End Using
Dim js As JavaScriptSerializer = New JavaScriptSerializer()
Context.Response.Write(js.Serialize(listEmployees))
End Sub
<WebMethod>
Public Sub GetAllDetails1()
Dim listEmployees As List(Of Details) = New List(Of Details)()
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("Select * from BookIssue", con)
con.Open()
Dim rdr As SqlDataReader = cmd.ExecuteReader()
While rdr.Read()
Dim details As Details = New Details()
details.CardID = Convert.ToInt32(rdr("CardID"))
details.BookID = Convert.ToInt32(rdr("BookID"))
details.IssuedBy = rdr("IssuedBy").ToString()
listEmployees.Add(details)
End While
End Using
Dim js As JavaScriptSerializer = New JavaScriptSerializer()
Context.Response.Write(js.Serialize(listEmployees))
End Sub
<WebMethod>
Public Function GetdetailsById(ByVal CardID As Integer) As Details
Dim details As Details = New Details()
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("spGetBookIssueDetailsByCardID", con)
cmd.CommandType = CommandType.StoredProcedure
Dim parameter As SqlParameter = New SqlParameter()
parameter.ParameterName = "#CardID"
parameter.Value = CardID
cmd.Parameters.Add(parameter)
con.Open()
Dim rdr As SqlDataReader = cmd.ExecuteReader()
While rdr.Read()
details.CardID = Convert.ToInt32(rdr("CardID"))
details.BookID = Convert.ToInt32(rdr("BookID"))
details.IssuedBy = rdr("IssuedBy").ToString()
End While
End Using
Return details
End Function
<WebMethod>
Public Function GetdetailsById1(ByVal CardID As Integer) As Details
Dim details As Details = New Details()
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("spGetBookIssueDetailsByCardID1", con)
cmd.CommandType = CommandType.StoredProcedure
Dim parameter As SqlParameter = New SqlParameter()
parameter.ParameterName = "#CardID"
parameter.Value = CardID
cmd.Parameters.Add(parameter)
con.Open()
Dim rdr As SqlDataReader = cmd.ExecuteReader()
While rdr.Read()
details.CardID = Convert.ToInt32(rdr("CardID"))
details.BookID = Convert.ToInt32(rdr("BookID"))
details.IssuedBy = rdr("IssuedBy").ToString()
End While
End Using
Return details
End Function
<WebMethod>
Public Sub DeleteRecord1(ByVal emp As Details)
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("spDeleteByID", con)
cmd.CommandType = CommandType.StoredProcedure
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#CardID",
.Value = emp.CardID
})
con.Open()
cmd.ExecuteNonQuery()
End Using
End Sub
<WebMethod>
Public Sub Update(ByVal emp As Details)
Dim cs As String = ConfigurationManager.ConnectionStrings("library management systemConnectionString").ConnectionString
Using con As SqlConnection = New SqlConnection(cs)
Dim cmd As SqlCommand = New SqlCommand("spUpdateBookIssue", con)
cmd.CommandType = CommandType.StoredProcedure
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#CardID",
.Value = emp.CardID
})
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#BookID",
.Value = emp.BookID
})
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#IssuedBy",
.Value = emp.IssuedBy
})
con.Open()
cmd.ExecuteNonQuery()
End Using
End Sub
End Class

Answer 1

I suggest modeling your code off this example, i modified the delete sub you provided to include a catch with a custom ErrorRoutine for unhandled cases, otherwise you can obtain the number of deleted rows from the executeNonQuery() function. I also chnaged so we return the number of deleted rows, this way if its less than 1 you can do whatever you want with messages after calling the delete.
Public Sub DeleteRecord1(ByVal emp As Details) As Integer
Dim cs As String = ConfigurationManager.ConnectionStrings("library management
systemConnectionString").ConnectionString
Dim m_RowsDeleted As Integer
Dim con As SqlConnection = New SqlConnection(cs)
Try
Using con
Dim cmd As SqlCommand = New SqlCommand("spDeleteByID", con)
cmd.CommandType = CommandType.StoredProcedure
cmd.Parameters.Add(New SqlParameter() With {
.ParameterName = "#CardID",
.Value = emp.CardID
})
con.Open()
m_RowsDeleted = cmd.ExecuteNonQuery()
End Using
Catch ex As Exception
ErrorRoutine(ex, "Delete")
Finally
If Not IsNothing(con) Then
If con.State = ConnectionState.Open Then
con.Close()
End If
End If
End Try
Return m_RowsDeleted
End Sub