问题
I looked around for this and could not find an answer. Say I have this code:
class Command<T> : ICommand<T>
{
public void Execute(T parameter)
{
var isNull = parameter == null;
// ...
}
}
T could be any class, even a Nullable<>. Does performing the check above cause boxing if T is a value type? My understanding is that this is the same as calling ReferenceEquals which takes two object arguments, either of which would cause boxing if T was a value type, if I understand correctly.
If the above does cause boxing, is there a more preferred way to do this without causing the box to occur? I know there is default(T) but in the case of int that is 0, and I am looking to see if this value is null without boxing it. Also, I am looking to do this in a way that satisfies both value and reference types.
回答1:
No - at least not in my understanding. If T is a non-nullable value type, the parameter == null check is effectively replaced with false by the JIT compiler; no execution-time check is performed.
This is an optimization that only the JIT-compiler can perform, as the C# compiler only generates one form of code. For example, the IL generated for your sample is:
IL_0000: ldarg.1
IL_0001: box !T
IL_0006: ldnull
IL_0007: ceq
That would appear to actually perform the boxing - but I trust a decent JIT compiler to spot that when T is non-nullable, the ceq will always give a false result, and remove the box operation.
来源：https://stackoverflow.com/questions/48685875/does-performing-a-null-check-on-a-generic-cause-boxing