CREATE TABLE [dbo].[TestTable]
(
	ID bigint IDENTITY(1,1) NOT NULL, 
	IDField bigint NOT NULL,
	Value int NOT NULL


,
 CONSTRAINT [PK_TestTable] PRIMARY KEY CLUSTERED 
(
	[ID] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

TestTable

 class TestTableExt : TestTable
        {
            public TestTableExt()
                : base()
            {
            }

            public long GetValue()
            {
                return Value;
            }
        }
        

 GeneralDataContext GDC = new GeneralDataContext();


            TestTable TT = new TestTable();
            TT.IDField = 10;
            TT.Value = 5;

            GDC.TestTables.InsertOnSubmit(TT);
            GDC.SubmitChanges();

GeneralDataContext GDC = new GeneralDataContext();
            TestTableExt TTExt = new TestTableExt();

            TTExt.IDField = 10;
            TTExt.Value = 7;


            TestTable T = (TestTable)TTExt;

            GDC.TestTables.InsertOnSubmit(T);
            GDC.SubmitChanges();