C# has || just like any other language, but objects aren't truthy and nulls aren't falsy. This was a good design decision, since it catches the common = vs. == error if(a=b) while doing the type check rather than silently doing the (probably) wrong thing. As a consequence, the || doesn't work as it does in other places. Hence ??.

Every language?

Java, as far as I know, does not allow if (someObject) where someObject is a class type. AFIAK, java only allows that if someObject is a bool, or possibly an int as well.

In strongly typed languages in general, null is not the same thing as false and not-null is not the same thing as true.

The Expression tree part is powerful (you can inspect it with code, pull property names out, etc), but it's not needed in most cases. Your example in C# is:

  Func<int, int> f = x => x + 1;
  int b = f(42);

This is shorter than the JavaScript. The C# is more verbose where has to specify a type for f because it cannot work it out from context (in fact the left side of the '=' supplies type information to the right side), but the actual expression on the right is less verbose than the JavaScript with its "function" and "return" keywords.

If you want, the second line can be

  var b = f(42);

But it's the same number of chars. I don't think that you gain anything from letting the compiler work out that b is an int.

The best thing about expression trees isn't that it lets you declare a method like Func<Func<int>, int> = ... (or whatever), but that you can inspect (and modify) the expression tree at runtime. This is what powers LINQ, and the 'static reflection' used by fluent APIs like Fluent nHibernate.

well you can write it as

  var f = ((x) => x + 1).Compile();
  int b = f.Invoke(42);