Use the LINQ to SQL IsPrimaryKey property on the ColumnAttribute attribute to designate a property or field to represent the primary key for a database column.
For code examples, see IsPrimaryKey.
LINQ to SQL does not support computed columns as primary keys.
Add the IsPrimaryKey property to the ColumnAttribute attribute.
Specify the value as true.
The LINQ to SQL Object Model How to: Customize Entity Classes by Using the Code Editor