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myFoo = myFoo ?? new Foo();
instead of
if (myFoo == null) myFoo = new Foo();
Am I correct in thinking that the first line of code will always perform an assignment? Also, is this a bad use of the null-coalescing operator?
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Object is destroyed, even if the object itself isn't actually null. Null propagation cannot be overridden in this way, and therefore behaves inconsistently with the == operator, because it checks for null in a different way.
The null-coalescing operator ?? returns the value of its left-hand operand if it isn't null ; otherwise, it evaluates the right-hand operand and returns its result. The ?? operator doesn't evaluate its right-hand operand if the left-hand operand evaluates to non-null.
You can use the nullish coalescing assignment operator to apply default values to object properties. Compared to using destructuring and default values, ??= also applies the default value if the property has value null .
The nullish coalescing operator ( ?? ) is a logical operator that returns its right-hand side operand when its left-hand side operand is null or undefined , and otherwise returns its left-hand side operand.
I compared the CIL of the generated code (making sure to do a Release build - with Optimize Code checked in the Project Properties, which corresponds to the /optimize switch on csc.exe). This is what I got (using VS 2008 - note that Foo.MaybeFoo() is a method that sometimes returns null, sometimes a Foo)
GetFooWithIf:
IL_0000: call class Application3.Foo Application3.Foo::MaybeFoo()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: brtrue.s IL_000f
IL_0009: newobj instance void Application3.Foo::.ctor()
IL_000e: stloc.0
IL_000f: ldloc.0
IL_0010: ret
GetFooWithCoalescingOperator:
IL_0000: call class Application3.Foo Application3.Foo::MaybeFoo()
IL_0005: stloc.0
IL_0006: ldloc.0
IL_0007: dup
IL_0008: brtrue.s IL_0010
IL_000a: pop
IL_000b: newobj instance void Application3.Foo::.ctor()
IL_0010: stloc.0
IL_0011: ldloc.0
IL_0012: ret
Thus, the same except for an extra top-of-stack-duplication and pop. If this can be made to make a measurable performance difference, I will purchase a hat specifically for the purpose of eating it; therefore, go with the one that you feel offers better readability.
(edit) oh, and the JITter might be clever enough to get rid of even that difference!
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