Using “is” keyword with “null” keyword c# 7.0

问题:

Recently i find out, that the following code compiles and works as expected in VS2017. But i can't find any topic/documentation on this. So i'm curious is it legit to use this syntax:

class Program
{
    static void Main(string[] args)
    {
        var o = new object();              
        Console.WriteLine(o is null);
        o = null;
        Console.WriteLine(o is null);
        Console.ReadLine();
    }
}

BTW this is not working in VS2015

回答1:

Yes, it's entirely valid. This uses the pattern matching feature of C# 7, which is available with is expressions and switch/case statements. (The fact that it requires C# 7 is why it isn't working for you in VS2015.) For example:

// Type check, with declaration of new variable
if (o is int i)
{
    Console.WriteLine(i * 10);
}
// Simple equality check
if (o is 5)  {}

Equality checks like the latter - particularly for null - aren't likely to be very useful for is pattern matching, but are more useful for switch/case:

switch (o)
{
    case int i when i > 100000:
        Console.WriteLine("Large integer");
        break;
    case null:
        Console.WriteLine("Null value");
        break;
    case string _:
        Console.WriteLine("It was a string");
        break;
    default:
        Console.WriteLine("Not really sure");
        break;
}

For more details of C# 7 features, see the MSDN blog post by Mads Torgersen.

回答2:

Yes, it is valid to write o is null, but this is not equivalent to o == null. The code

static bool TestEquality(object value) => value == null;

compiles into following IL instructions.

  IL_0000:  ldarg.0
  IL_0001:  ldnull
  IL_0002:  ceq
  IL_0004:  ret

The pattern matching case compiled in following manner:

static bool TestPatternMatching(object value) => value is null;

  IL_0000:  ldnull
  IL_0001:  ldarg.0
  IL_0002:  call       bool [System.Runtime]System.Object::Equals(object, object)
  IL_0007:  ret

So, pattern matching o is null is equivalent to

Object.Equals(value, null);

So, in most cases o is null and o == null will behave in same way. Except equality variant is a bit faster. BUT! Things will change dramatically, if we replace object with following class.

class TestObject
{
    public static bool operator ==(TestObject lhs, TestObject rhs) => false;
    public static bool operator !=(TestObject lhs, TestObject rhs) => false;
}

and methods with

static bool TestEquality(TestObject value) => value == null;
static bool TestPatternMatching(TestObject value) => value is null;

The pattern matching will stay same, but the equality variant will use following IL

  IL_0000:  ldarg.0
  IL_0001:  ldnull
  IL_0002:  call       bool PatternMatchingTest.TestObject::op_Equality(class PatternMatchingTest.TestObject, class PatternMatchingTest.TestObject)
  IL_0007:  ret

Here we can see, that == operator is using TestObject's overload as expected. But o is null and o==null will give different results. So be careful using pattern matching is operator.