C#中的“！=”和“不是”有區別嗎？

這是：

if(x != y)
{

}

與此不同：

if (x is not y)
{

}

或者這兩個條件之間沒有區別嗎？

uj5u.com熱心網友回復：

要直接和具體地回答 OP 的問題：

if( x != y ) { }
// vs:
if( x is not y ) { }

• 如果x是整數值型別（例如int/ Int32）并且y是const-expression(例如const int y = 123;) 那么no，沒有區別，并且兩個陳述句都會生成相同的 .NET MSIL 位元組碼（啟用和不啟用編譯器優化）：

  

• 如果y是一個型別的名稱（而不是值名稱），那么就是一個區別：第一個if宣告是無效的，并不會進行編譯，并且if( x is not y )宣告是一個型別的模式匹配，而不是一個恒定的模式匹配。

腳注：

1. "Constant Pattern" : "當輸入值不是開放型別時，常量運算式被隱式轉換為匹配運算式的型別"。

2. x is not null is more analogous to !(x == null) than x != null.

3. C# 7.0 introduced some limited forms of constant-pattern matching, which was further expanded by C# 8.0, but it wasn't until C# 9.0 that the not negation operator (or is it a modifier?) was added.

4. Given a non-constrained generic method, like so:

   void Foo<T>( T x )
   {
       if( x == null ) { DoSomething(); }
   
       DoSomethingElse();
   }
   

   ...when the JIT instantiates the above generic method (i.e.: monomorphization) when T is a value-type (struct) then the entire if( x == null ) { DoSomething(); } statement (and its block contents) will be removed by the JIT compiler ("elision"), this is because a value-tupe can never be equal to null. While you'd expect that to be handled by any optimizing compiler, I understand that the .NET JIT has specially hardcoded rules for that particular scenario.

   • Curiously in earlier versions of C# (e.g. 7.0) the elision rule only applied to the == and != operators, but not the is operator, so while if( x == null ) { DoSomething(); } would be elided, the statement if( x is null ) { DoSometing(); } would not, and in fact you would get a compiler error unless T was constrained to where T : class. Since C# 8.0 this seems to now be allowed for unconstrained generic types.

5. Surprisingly I couldn't find an authoritative source on this (as the published C# specs are now significantly outdated; and I don't want to go through the csc source-code to find out either).

   • If neither the C# compiler or JIT do apply impossible-branch-elision in generic code with Constant-pattern expressions then I think it might simply because it's too hard to do at-present.

6. Note that a constant-expression does not mean a literal-expression: you can use named const values, enum members, and so on, even non-trivial raw expressions provided all sub-expressions are also constant-expressions.

   • I'm curious if there's any cases where static readonly fields could be used though.

7. Note that in the case of typeof(X) != y.GetType(), this expression will return true when X is derived from y's type (as they are different types), but x is not Y is actually false because x is Y (because x is an instance of a subclass of Y). When using Type it's better to do something like typeof(X).IsSubclassOf(y.GetType()), or the even looser y.GetType().IsAssignableFrom(typeof(X)).

   • Though in this case, as Char is a struct and so cannot participate in a type-hierarchy, so doing !x.IsSubclassOf(typeof(Char)) would just be silly.

uj5u.com熱心網友回復：

在優異的接受的答案列出的額外的不同之處在于（因為C＃7.0），is2個NaN值之間是一個圖案匹配，因為x.Equals(y)是true當兩個x和y為NaN，和NaN值不具有整數型別。因此，is not在兩個 NaN 值之間回傳模式不匹配。

然而，C＃遵循IEEE浮點和C通過指定一個!=2個NaN值之間的比較是true與一個==在它們之間的比較是false。這主要是因為早在 1980 年的 Intel 8087 浮點協處理器就沒有其他方法來測驗 NaN。

uj5u.com熱心網友回復：

Nan 和 null 是變數可以包含的沒有值的屬性。相等性檢查需要一個實際值來確定相等性。畢竟，在沒人知道莎莉和彼得有多少個蘋果的情況下，關于他們是否擁有相同數量的蘋果的問題毫無意義。

有時你想檢查一個變數是否有一個沒有值的屬性。為此，基本的平等檢查是不夠的。那是什么時候 is / is not 運算子很有用。可以說 != 是一個值檢查，其中 is / 不是一個屬性檢查。

標籤：C＃ if 语句 句法

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