calling trygetmember on chained null references

Question

Is it possible to have a DynamicObject implementation that can be called in chain keeping a null reference to end if anywhere in the path a null reference is encountered, without throwing any exceptions?

a.b.c.e

for example: if a is null then a.b.c.e is null, or if c is null c.e is null etc.?

Very much like the Maybe monad from Haskell.

Answer 1

You can do something like that, but not for the outermost object, i.e. if a is null, you can't access a.b.

You can make an empty instance of the A class, that returns empty instances for all it's properties. Then a.b would return an empty instance of B, which for the c property would return an empty instance of C, which for the e property would return an empty instance of E.

You would not get a null value, but you would get an empty instance, which you could check with:

E e = a.b.c.e;
if (e != E.Empty) { ... }

If any of the properties along the way returns an empty instance, the end result would be E.Empty.

public class A {

  public B b;

  public A(B newB) { b = newB; }

  private static A _empty = new A(B.Empty);
  public static A Empty { get { return _empty; }}

}

public class B {

  public C c;

  public B(C newC) { c = newC; }

  private static B _empty = new B(C.Empty);
  public static B Empty { get { return _empty; } }

}

public class C {

  public E e;

  public C(E newE) { e = newE; }

  private static C _empty = new C(E.Empty);
  public static C Empty { get { return _empty; } }

}

public class E {

  public string name;

  public E(string newName) { name = newName; }

  private static E _empty = new E(null);
  public static E Empty { get { return _empty; } }

}

Example:

A a1 = new A(new B(new C(new E("Hello world!"))));
A a2 = new A(new B(new C(E.Empty)));
A a3 = new A(B.Empty);

E e1 = a1.b.c.e; // e1.name returns "Hello world!"
E e2 = a2.b.c.e; // e2 == E.Empty
E e3 = a3.b.c.e; // e3 == E.Empty