Are string.Equals() and == operator really same? [duplicate]

Answer 1

The apparent contradictions that appear in the question are caused because in one case the Equals function is called on a string object, and in the other case the == operator is called on the System.Object type. string and object implement equality differently from each other (value vs. reference respectively).

Beyond this fact, any type can define == and Equals differently, so in general they are not interchangeable.

Here’s an example using double (from Joseph Albahari’s note to §7.9.2 of the C# language specification):

double x = double.NaN;
Console.WriteLine (x == x);         // False
Console.WriteLine (x != x);         // True
Console.WriteLine (x.Equals(x));    // True

He goes on to say that the double.Equals(double) method was designed to work correctly with lists and dictionaries. The == operator, on the other hand, was designed to follow the IEEE 754 standard for floating point types.

In the specific case of determining string equality, the industry preference is to use neither == nor string.Equals(string) most of the time. These methods determine whether two string are the same character-for-character, which is rarely the correct behavior. It is better to use string.Equals(string, StringComparison), which allows you to specify a particular type of comparison. By using the correct comparison, you can avoid a lot of potential (very hard to diagnose) bugs.

Here’s one example:

string one = "Caf\u00e9";        // U+00E9 LATIN SMALL LETTER E WITH ACUTE
string two = "Cafe\u0301";       // U+0301 COMBINING ACUTE ACCENT
Console.WriteLine(one == two);                                          // False
Console.WriteLine(one.Equals(two));                                     // False
Console.WriteLine(one.Equals(two, StringComparison.InvariantCulture));  // True

Both strings in this example look the same ("Café"), so this could be very tough to debug if using a naïve (ordinal) equality.

Answer 2

Two differences:

• Equals is polymorphic (i.e. it can be overridden, and the implementation used will depend on the execution-time type of the target object), whereas the implementation of == used is determined based on the compile-time types of the objects:

  // Avoid getting confused by interning
  object x = new StringBuilder("hello").ToString();
  object y = new StringBuilder("hello").ToString();
  if (x.Equals(y)) // Yes
  
  // The compiler doesn't know to call ==(string, string) so it generates
  // a reference comparision instead
  if (x == y) // No
  
  string xs = (string) x;
  string ys = (string) y;
  
  // Now *this* will call ==(string, string), comparing values appropriately
  if (xs == ys) // Yes
  

• Equals will go bang if you call it on null, == won't

  string x = null;
  string y = null;
  
  if (x.Equals(y)) // Bang
  
  if (x == y) // Yes
  

Note that you can avoid the latter being a problem using object.Equals:

if (object.Equals(x, y)) // Fine even if x or y is null