Why RCS1098 is enabled by default?

[AlexeiScherbakov]

Avoid 'null' on the left side of a binary expression

null==a in C# means ReferenceEquals(a,null)

https://stackoverflow.com/questions/34229318/should-null-be-checked-on-the-left-or-right-hand-side

https://stackoverflow.com/questions/970334/is-referenceequalsnull-obj-the-same-thing-as-null-obj

https://stackoverflow.com/questions/4850053/what-is-good-practice-for-null-reference-checks

[jnyrup]

While it often true that null == a and ReferenceEquals(a, null) are interchangeable, it is not always the case.

null == a and ReferenceEquals(a, null) are interchangeable if a is of type object or no public static bool operator == has not been overridden i the type of a.

Here's a class with all possible overrides of operator ==

public class Test
{
    public static bool operator ==(Test x, Test y) => true;
    public static bool operator !=(Test x, Test y) => !(x == y);

    public static bool operator ==(Test x, object y) => true;
    public static bool operator !=(Test x, object y) => !(x == y);

    public static bool operator ==(object x, Test y) => true;
    public static bool operator !=(object x, Test y) => !(x == y);
}

And here you can see in the comments, which operator == is chosen depending on the types.

var a = new Test();

if (null == a) { } // Test == Test
if (a == null) { } // Test == Test

if (null == (object)a) { } // object == object
if (a == (object)null) { } // Test == object

if ((object)null == a) { } // object == Test
if ((object)a == null) { } // object == object

As your links point out having null on the left hand side (sometimes known as Yoda conditions) mostly has its benefits in C-style languages.

[josefpihrt]

Yoda condition is not relevant because this analyzer is applicable only for == and != operators and not for assignment.

Also as Jon Hanna points out "a non-symmetric == is always a bug".

So now we can agree that changing null == x to x == null is only about style and not about semantic change.

Disabling this analyzer by default is worth consideration but before doing that can someone provide c# project (repository) that consistently use null == x over x == null?

[tamlin-mike]

While I can't point to any such repo, I can suggest a possibly cleaner way to do object null check, using is null.

bool IsNullTest1(String s) { return (object)s == null; }
bool IsNullTest2(String s) { return s is null; }

At least the compiler I tested with is generating the exactly same code.