C# Refreshers

I had an interview recently that really challenged me. The interview was a collaborative coding challenge using C#. I was reminded of many C# features (datatypes, operators, etc), and I was taught about principles & performance characteristics. I outline these below because they inspired me!

Reserved Keywords

Some reserved keywords:

• event is a reserved keyword

Null Coalescing Operator ?? and ??=

More here Some examples:

TODO

Javascript also has the “nullish coalescing” and “nullish coalescing assignment” operators.

Javascript calls it “nullish” because Javascript null is distinct from Javascript undfined, Javascripts “nullish coalescing” operator works in both scenarios (emphasis added):

…when its left-hand side operand is null or undefined…

Null-Conditional Operator ?. (the “Elvis Operator”)

A variation on the . member access operator It can also be used for index access operator [...] becomes ?[...]

Comparison with Javascript

The C# “null-conditional operator” is called the “optional chaining” operator in Javascript.

Both of the objects short-circuit – if any object higher in the chain is null, it ignores all subsequent operators. In this Javascript example, I don’t use the optional chaining operator on p2 or p3 because using it on p1 is sufficient.

If p1 is not defined, this expression returns 'name' without erroring about p2 and p3 nonexistence. window.p1?.p2.p3 ?? 'name'

However, if p1 did exist, but then its p2 was not defined, the above expression would cause an TypeError:.

window.p1 = {};
window.p1?.p2.p3 ?? 'name'
 > Uncaught TypeError: Cannot read properties of undefined (reading 'p3')

Error with Chaining Null-Conditional

I thought this operator couldn’t be used on every type. I got an error on this line:

SortedEventsByTicker // a SortedList<TKey,TValue>
            .GetValueOrDefault(EventTicker)
            ?.LastOrDefault() 
            ?.Value // <--------- error

The error was:

Operator ‘?’ cannot be applied to operand of type ‘KeyValuePair<long, Solution.TradingEvent>’

When I look closer, it’s not complaining about the operator ?. being used on a KeyValuePair, it’s complaining about operator ? (the conditional/ternary operator?, maybe the nullability operator?)

• Maybe the error is because ?. is executed right-to-left (does that mean it’s right-associative?)
• Is this error specific to replit; maybe the C# interpreter it is using??
• TODO: Check a simpler case on replit; check the same case with different C# interpreter

I fixed the error by adding parentheses, which is why I thought associativity was part of the problem…

(SortedEventsByTicker // a SortedList<TKey,TValue>
            .GetValueOrDefault(EventTicker)
            ?.LastOrDefault()) // <------- parentheses
            ?.Value 

Short-Circuiting

It’s also interesting to learn that ?. is “short-circuiting”.

• Does short-circuiting mean I can avoid “chaining” the operator like A?.B?.DoSomething(C), can I just do A?.B.DoSomething(C)?
  • Yes, can do that, it means if A is null it won’t attempt to access B or C or anything else to the right of the operator:

  • If A might be null but B and C wouldn’t be null if A isn’t null, you only need to apply the null-conditional operator to A:

  • A?.B.C();

  • Note this part particularly: if “B and C wouldn’t be null”
• Don’t get confused, short-circuiting does not mean there are “implied” null conditional operators. There is no implied nullish operator after the B; these are not equivalent: A?.B.C(); and A?.B?.C();
  • In other words, if B or C were null, it would fail. -Also note short-circuiting is interrupted with parentheses:

• if the chained member access is interrupted, for example by parentheses as in (A?.B).C(), short-circuiting doesn’t happen.

• So these are not equivalent either: A?.B.C(); and (A?.B).C();

Also compare C# and Javascript C# has “null conditional” operators for both “members” and “elements”: a member access (?.) or element access (?[]) C# does not have a null conditional operator for methods, like ?.(...) or ?(...)

Consider this code…

StringMaker s = null;
// this fails with a syntax error and "Compilation error: 'method group' cannot be made nullable."
Console.WriteLine(s?.MakeString?.());
// this line fails with "Identifier expected" and "Compilation error: 'Program.StringMaker.MakeString()' is a method, which is not valid in the given context"
Console.WriteLine(s?.MakeString?());

Javascript’s operator works on functions/methods, too:

obj.val?.prop
obj.val?.[expr]
obj.func?.(args)

As explained in the documentation:

You can use optional chaining when attempting to call a method which may not exist

This makes sense, I guess C# would never have null methods, right? It might have null delegates though

LastOrDefault

More on the LastOrDefault method here

The default value for reference and nullable types is null. The LastOrDefault method does not provide a way to specify a default value. If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.

Classes (Reference Types) are Nullable

More here. Just a quick reminder. Other “primitive” types must be made nullable using the ? operator like float -> float?

System.Collections.Generic Datatypes & Performance Characteristics

More on the System.Collections.Generic here and performance characteristics here

List

Inserting into a List is:

• TODO Confirm
• TODO Link the Python algorithms book
• TODO describe “amortized”
• O(1) if list need not be reallocated
• O(n) if list needs to be reallocated

Dictionary

TODO

SortedList

I hadn’t used SortedList before… TODO