1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:

public class A
{
  public bool prop1 { get; set; }
  public string prop2 { get; set; }
  ...
}

public class B
{
  private List<A> _alleA;
   
  // liefert alle Instanzen der Klasse A, für die gilt A.prop1 == param
  public List<A> GetAllA(bool param)
  {
    [b]IEnumerable<A>[/b] result = _alleA.Where(obj => obj.prop1 == param).Select(obj => obj);  
    [b]return ????[/b]
  }
   ...
}

1:

_alleA.FindAll(delegate(A obj) { return (obj.prop1 == param); });