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

Answer 2

Check this great article: Chained null checks and the Maybe monad

A great many programmers have met a situation where, while accessing a nested object property (e.g., person.Address.PostCode), they have to do several null checks. This requirement frequently pops up in XML parsing where missing elements and attributes can return null when you attempt to access them (and subsequently trying to access Value throws a NullReferenceException). In this article, I’ll show how a take on the Maybe monad in C#, coupled with use of extension methods, can be used to improve readability.