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C# difference between == and .Equals()

For comparing two variables we can use == or Equals method. for example,

        string a = new string(new char[] {'a', 'b', 'c', 'd'});
        string b = new string(new char[] {'a', 'b', 'c', 'd'});

        Console.WriteLine (a==b);
        Console.WriteLine (a.Equals(b));

My question is When should I use == and when should I use Equals? Is there any difference between the two?

Answer 1

== is an operator, which, when not overloaded means “reference equality” for classes (and field-wise equality for structs), but which can be overloaded. A consequence of it being an overload rather than an override is that it is not polymorphic.

Equals is a virtual method; this makes it polymorphic, but means you need to be careful not to call it on null instances.

As a rule of thumb:

• if you know the type of something, and know it has an == overload (most core types such as string, int, float, etc have == meanings), then use ==
• if you don’t know the type, Equals(a,b) may be recommended, as this avoids the null issue
• if you really want to check for the same instance (reference equality), consider using ReferenceEquals(a,b), as this will work even if the == is overloaded and Equals overridden
• when using generics, consider EqualityComparer<T>.Default.Equals(a,b) which avoids the null issue, supports Nullable-of-T, and supports IEquatable-of-T