Is statement fails from class being in a separate file

I have an is statement failing saying a class name is not found.  The failure only occurs if class c3 is in another fu file.

file1:
```

public class c3
{
  
}

```
file2:
```

class c1
{

}

class c2 : c1
{

}

class c4 : c3
{
  public void test!(c1 value)
  { 
    bool isC2 = value is c2;
  }
}

```

Which then gives the error:
ERROR: 'c2' not found
on the line with the is statement.