Insert exception when using linqtosql - linq-to-sql

Question

When I want to Insert data in my table this Exception appeared
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Message_Subject". The conflict occurred in database "C:\DOCUMENTS AND SETTINGS\TEHRANI\DESKTOP\MESSAGEADMINPAGE\APP_DATA\ASPNETDB.MDF", table "dbo.Subject", column 'ID_Subject'.
The statement has been terminated.
This Code for Insert :
string[] a = UserIDtxt.Text.Split(',');
foreach (String b in a)
{
Message M = new Message();
Guid i = (from q in MDB.aspnet_Memberships
where q.aspnet_User.UserName.ToString() == b.ToString()
select q).Single().UserId;
M.ID_Receiev = i;
M.ID_Message = Guid.NewGuid();
M.ID_Sender = (Guid)Admin.ProviderUserKey;
M.ID_Message_Parent = Guid.Empty;
if (SubjectDDL.SelectedItem.ToString() != "Other")
{
M.ID_Subject = new Guid(SubjectDDL.SelectedValue);
}
else
{
M.Other_Subject = Othertxt.Text;
}
M.Body = TEXTtxt.Text;
M.Date = DateTime.Now;
M.IsFinished = false;
M.IsRead = false;
MDB.Messages.InsertOnSubmit(M);
}
MDB.SubmitChanges();

Answer 1

you must set value all of feild
if (SubjectDDL.SelectedItem.ToString() != "Other")
{
M.ID_Subject = new Guid(SubjectDDL.SelectedValue);
M.Other_Subject = null;
}
else
{
M.ID_Subject = new Guid(SubjectDDL.SelectedValue);
M.Other_Subject = Othertxt.Text;
}

Answer 2

For what I can tell, based in the FOREIGN KEY constraint "FK_Message_Subject", you also have a table to Subjects. If this assumption is correct, when you assign M.ID_Subject a new Guid, it might not exist as a FOREIGN KEY in the Subjects table. You must find any existing Subject with the SubjectDDL.SelectedValue and retrieve the existing ID for the FOREIGN KEY. If it doesn't exist, create a new Subject and assign it directly to the Message M.
The same applies to when SubjectDDL.SelectedItem.ToString() == "Other". In this case, the FOREIGN KEY is null and it might be causing this error also.