Given two classes

```
class Contract
{
    public int ID {get; set;}
    // . . . 
}

class DBContract
{
    public int FromID {get; set;}
    // . . . 
}

```

And two IEnumerables

```
IEnumerable<Contract> ValidContracts = Application.GetContracts(//. . .
IEnumerable<DBContract> ExportedContracts = DBAdapter.GetRows(// . . .

```

I need to find intersection of theese IEnumerables. But how do I implement IEqualityComparer, if it only has one type argument?