AddWithValue() isn't accepting DBNull.Value (Exception 0x800A0BB9) - Silverlight - VB - sql

Question

I'm developing an OOB Silverlight application and using the COM Toolkit (http://silverlightcom.codeplex.com), the language is Visual Basic (VB). With this pack, I can connect my app with my Microsoft SQL Server database using ADO.NET. It works, but when I try to insert a null value to a parameter it throws an exception: 0x800A0BB9 (http://prntscr.com/43zwyr).
My code is (I will post the most simple stretch):
Private Const SqlCreateJob As String = "insert into Cargos(Nome_carg, Desc_carg) values(?, ?)"
Public Shared Sub CreateJob(ByVal j As Job)
Dim conn As AdoConnection = DbUtil.GetConn
Dim cmd As AdoCommand = conn.CreateCommand
cmd.CommandText = SqlCreateJob
cmd.Parameters.AddWithValue("Nome_carg", j.Name)
cmd.Parameters.AddWithValue("Desc_carg", j.Desc)
For Each p As AdoParameter In cmd.Parameters
If p.Value = Nothing Then
p.Value = DBNull.Value
End If
Next
cmd.ExecuteNonQuery()
conn.Close()
End Sub
And the class Job:
Public Class Job
Property Code() As Integer
Property Name() As String
Property Desc() As String
End Class
The value Desc, from Job, can be null. And if I put
cmd.Parameters.AddWithValue("Desc_carg", DBNull.Value) it still doesn't work. What can I do? (When the Desc is not null the command works)
The Exception.ToString is here:
System.Runtime.InteropServices.COMException (0x800A0BB9): Exception from HRESULT: 0x800A0BB9 ---> MS.Internal.ComAutomation.ComAutomationObjectException: The arguments are incorrect, are out of acceptable range, or are in conflict.(Source=ADODB.Parameter) (HelpFile=C:\WINDOWS\HELP\ADO270.CHM#1240641) em MS.Internal.ComAutomation.ComAutomationNative.CheckInvokeHResult(UInt32 hr, String memberName, String exceptionSource, String exceptionDescription, String exceptionHelpFile, UInt32 exceptionHelpContext) em MS.Internal.ComAutomation.ComAutomationNative.Invoke(Boolean tryInvoke, String memberName, ComAutomationInvokeType invokeType, ComAutomationInteropValue[] rgParams, IntPtr nativePeer, ComAutomationInteropValue& returnValue) em MS.Internal.ComAutomation.ComAutomationObject.InvokeImpl(Boolean tryInvoke, String name, ComAutomationInvokeType invokeType, Object& returnValue, Object[] args) em MS.Internal.ComAutomation.ComAutomationObject.Invoke(String name, ComAutomationInvokeType invokeType, Object[] args) em System.Runtime.InteropServices.Automation.AutomationMetaObjectProvider.TrySetMember(SetMemberBinder binder, Object value) em System.Runtime.InteropServices.Automation.AutomationMetaObjectProviderBase.<.cctor>b__3(Object obj, SetMemberBinder binder, Object value) em CallSite.Target(Closure , CallSite , Object , Int32 ) em System.Dynamic.UpdateDelegates.UpdateAndExecute2[T0,T1,TRet](CallSite site, T0 arg0, T1 arg1) em CallSite.Target(Closure , CallSite , Object , Int32 ) em ComToolkit.Data.AdoParameter.set_Type(DbType value) em ComToolkit.Data.AdoParameter..ctor(Object adoParameter, String name, Object value) em CallSite.Target(Closure , CallSite , Type , Object , String , Object ) em System.Dynamic.UpdateDelegates.UpdateAndExecute4[T0,T1,T2,T3,TRet](CallSite site, T0 arg0, T1 arg1, T2 arg2, T3 arg3) em CallSite.Target(Closure , CallSite , Type , Object , String , Object ) em ComToolkit.Data.AdoCommand.CreateParameter(String name, Object value) em ComToolkit.Data.AdoParameterCollection.AddWithValue(String name, Object value) em Corporativo.Dao.CargoDao.AddParameters(AdoCommand cmd, Cargo c) em Corporativo.Dao.CargoDao.CriarCargo(Cargo c) em Corporativo.UCCadCargos.Create()
And the .Message:
Exception from HRESULT: 0x800A0BB9

Answer 1

You should not be using AddWithValue if the value is, or van be, NULL. AddWithValue infers the data type of the parameter from the value but, if the value is NULL, a type cannot be inferred. You should be using Add in such cases, specifying the data type explicitly and setting the Value property.