Deep Levels of Inheritance and Code Contract Behaviors

i've scaled down , simplified code i've taken big project illustrate problem. however, levels of inheritance correctly represented. code follows:

public interface icontext  	 : iunitofwork, isql, idisposable  {  	/// <summary>  	/// apply changes made in item or related items in graph  	/// </summary>  	/// <typeparam name="tentity">type of item</typeparam>  	/// <param name="item">item changes</param>  	void setchanges<tentity>(tentity item)  		where tentity : class, iobjectwithchangetracker, new();  }    [contractclass( typeof( irepositorycontract<> ) )]  public interface irepository<tentity>  		where tentity : class, new( )  {  	/// <summary>  	/// context in repository  	/// </summary>  	icontext storecontext  	{  		get;  	}  	.....  }  [contractclassfor( typeof( irepository<> ) )]  internal abstract class irepositorycontract<tentity> : irepository<tentity>  	where tentity : class, new( )  {  	#region irepository<tentity> members    	public icontext storecontext  	{  		get  		{  			contract.ensures( contract.result<icontext>( ) != null );  			contract.ensures( contract.result<icontext>( ) iunitofwork );  			throw new notimplementedexception( );  		}  	}  }    public interface ipersonrepository : irepository<person>  {  	....  }    public class genericrepository<tentity> : irepository<tentity>  		where tentity : class,iobjectwithchangetracker, new( )  {  ......    	public icontext storecontext  	{  		get  		{  			//contract.ensures( contract.result<icontext>( ) != null ); removed these contracts since included in irepository  			//contract.ensures( contract.result<icontext>( ) iunitofwork );  			return _context icontext;  		}  	}  	  	[contractinvariantmethod]  	private void objectinvariant( )  	{  		contract.invariant( _context != null );  	}  ......  }    public class personrepository : genericrepository<person>, ipersonrepository  {  	....  }    public class personservice()  {  	public void save( ipersonrepository personrepository )  	{  		//dependency injection substitutes personrepository concrete class  		....  		var unitofwork = ( iunitofwork )_personrepository.storecontext;  		unitofwork.commit( ); // codecontract: possibly calling method on null reference 'unitofwork'  	}  }  

in personservice class, still getting contract warning unitofwork might null, though ensured won't null in irepository interface.

can tell me missing?

i hadn't turned on contracts , set contract reference assembly build projects interfaces reside. since enabling that, working expected.

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