Comments by om3n (Top 10 by date)

thanks all,
I've tried :

Dim Col2 As Nullable(Of Integer)
OpenDatabase()
Command.Connection = Con
Command.CommandType = CommandType.Text
Command.CommandText = "INSERT INTO SIGN(TYPESIGN_ID) VALUES('" + Col2 + "')"
Command.ExecuteNonQuery()
Con.Close()
Still can't, I get error "Conversion from string"')" to type 'Double' is not valid."

if using :
"INSERT INTO SIGN(TYPESIGN_ID) VALUES(" + "null" + ")"
the result is successfully.

however, what if insert null values ​​using variables?

I've tried as you say:
Command.CommandText = "INSERT INTO SIGN(TYPESIGN_ID) VALUES('" + Col2 + "')"
I get error "Conversion from string"')" to type 'Double' is not valid."

I'm also try :
Command.CommandText = "INSERT INTO SIGN(TYPESIGN_ID) VALUES('" + Col2.ToString + "')"
Also get error "The INSERT statement conflicted with the FOREIGN KEY constraint "FK_SIGN_TYPE_SIGN". The conflict occurred in database "database", table "dbo.TYPE_SIGN", coloumn 'TYPESIGN_ID'. The statement has been terminated."

if using Col2.ToString the value is 0

Thank you so much Trak4Net, the problem can be solved. you're a genius. I was motivated to study even harder

Error message :
"AccessViolationException was unhandled"
Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
System.AccessViolationException was unhandled
Message=Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
Source=System.Windows.Forms
StackTrace:
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun()
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel()
at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine)
at Inventory.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81
at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean ignoreSyncCtx)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()
InnerException:

Thanks Trak4net,
As you say, The following is the complete code that I have tried :
==========================
Connection Module
==========================
Imports System.Data
Imports System.Data.SqlClient

Module Connection
Public ConStrInv As String
Public ConStrAuct As String

Public ConInv As New SqlConnection
Public ConAuct As New SqlConnection

Public CommandInv As New SqlCommand
Public CommandAuct As New SqlCommand

Public ObjDtAdapterInv As New SqlDataAdapter
Public ObjDtAdapterAuct As New SqlDataAdapter

Public objDataSetInv As New DataSet
Public objDataSetAuct As New DataSet

Public Sub OpenInventory()
ConStrInv = "Server=h04mc3kn;Database=inventory;user id=sa;password=123;"
ConInv.ConnectionString = ConStrInv
Try
If ConInv.State <> ConnectionState.Closed Then
ConInv.Close()
ConInv.Open()
Else
ConInv.Open()
End If
Catch ex As Exception
MsgBox("Sorry Can not open connection ! ")
End Try
End Sub

Public Sub OpenAuction()
ConStrAuct = "Server=h04mc3kn;Database=auction;user id=sa;password=123;"
ConAuct.ConnectionString = ConStrAuct
Try
If ConAuct.State <> ConnectionState.Closed Then
ConAuct.Close()
ConAuct.Open()
Else
ConAuct.Open()
End If
Catch ex As Exception
MsgBox("Sorry Can not open connection ! ")
End Try
End Sub
End Module

============
Form
============
Imports System.Data
Imports System.Data.SqlClient

Public Class Form1
Private Sub TBArtist_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TBArtist.TextChanged
Call OpenInventory()
CommandInv.Connection = ConInv
CommandInv.CommandType = CommandType.Text
CommandInv.CommandText = "SELECT ART_NAME FROM ARTIST"
ObjDtAdapterInv.SelectCommand = CommandInv
ObjDtAdapterInv.Fill(objDataSetInv, "Artist")
ObjDtAdapterInv.Dispose()
Dim col As New AutoCompleteStringCollection
Dim i As Integer
For i = 0 To objDataSetInv.Tables("Artist").Rows.Count - 1
col.Add(objDataSetInv.Tables("Artist").Rows(i)("ART_NAME").ToString())
Next
TBArtist.AutoCompleteSource = AutoCompleteSource.CustomSource
TBArtist.AutoCompleteCustomSource = col
TBArtist.AutoCompleteMode = AutoCompleteMode.Suggest
CommandInv.Dispose()
ObjDtAdapterInv.Dispose()
ConInv.Close()
End Sub

Private Sub TBOwner_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles TBOwner.TextChanged
Call OpenAuction()
CommandAuct.Connection = ConAuct
CommandAuct.CommandType = CommandType.Text
CommandAuct.CommandText = "SELECT cli_firstname FROM tclient WHERE cli_firstname LIKE '%" & TBOwner.Text & "%'"
ObjDtAdapterAuct.SelectCommand = CommandAuct
ObjDtAdapterAuct.Fill(objDataSetAuct, "Owner")
Dim ColOwner As New AutoCompleteStringCollection
Dim i As Integer
For i = 0 To objDataSetAuct.Tables("Owner").Rows.Count - 1
ColOwner.Add(objDataSetAuct.Tables("Owner").Rows(i)("cli_firstname").ToString())
Next
TBOwner.AutoCompleteSource = AutoCompleteSource.CustomSource
TBOwner.AutoCompleteCustomSource = ColOwner
TBOwner.AutoCompleteMode = AutoCompleteMode.Suggest
CommandAuct.Dispose()
ObjDtAdapterAuct.Dispose()
ConAuct.Close()
End Sub
End Class

First step, I typed in the TBArtist, a result I managed to get the Artist's data after that I typed again in the TBOwner. when I enter the first character of the word "MR" there is no error message, when I enter the second character is "R" I get the error message :
"AccessViolationException was unhandled"
Attempted to read or write