Question

How to specify that type should be a class inherited from Interface? I tried various variants but no sense.

public class CarDal<TCarMark> where TCarMark:class//also tried ICarMark
{
    ...
    private static readonly DataContext Context = new DataContext(ConnectionString);
    private Table<ICarMark> _tblCarMarks = Context.GetTable<TCarMark>();//Error is "Cannot convert source type 'Table<TCarMark> to target type Table<ICarMark>'"
    ...
}

Answer 1

Your constraint, especially combining the two forms you cite, is fine:

 where TCarMark : class, ICarMark

The problem is variance.

A Table<TCarMark> has no covariant/contravariant relationship with a Table<ICarMark>

You must use Table<TCarMark> throught this class:

private Table<TCarMark> _tblCarMarks = Context.GetTable<TCarMark>();
// emphasis: ^^^

However! Don’t use a static data-context:

• it will not be thread-safe
• it will mean your data-context grows over time (it is meant to be a unit of work, not an application lifetime object)