1. I want to use code to check the duplicate primary key , then i can write a msg.But how to check ?

And i mainly problem , i can't use Me.CompanyBindingSource.EndEdit() to checking , use this statement will error...
Becuase my dataset ( set a primary , b not allow null ), if primary key has entry ,but b not entry will error in
Me.CompanyBindingSource.EndEdit() , no body how to check it ?