Using == or Equals for string comparison

Question

In some languages (e.g. C++) you can't use operators like == for string comparisons as that would compare the address of the string object, and not the string itself. However, in C# you can use == to compare strings, and it will actually compare the content of the strings. But there are also string functions to handle such comparisons, so my question is; should you?

Given two strings:

string aa = "aa"; 
string bb = "bb";

Should you compare them like this:

bool areEqual = (aa == bb); 

Or should you use the Equal function, like this:

bool areEqual = aa.Equals(bb); 

Is there any technical difference anyway? Or reasonable arguments for best practice?

Answer 1

I wouldn't use:

aa.Equals(bb)

unless I knew aa couldn't possibly be null. I might use:

string.Equals(aa,bb)

But I'd mainly use that it I wanted to use one of the specific StringComparison modes (invariant, ordinal, case-insensitive, etc). Although I might also use the StringComparer implementations, since they are a bit easier to abstract (for example, to pass into a Dictionary<string, Foo> for a case-insensitive ordinal dictionary). For general purpose usage,

a == b

is fine.

Answer 2

There is no technical difference (unless aa is null). Use whatever looks better to you. In my opinion, using operator overloads makes the code clearer.

Use functions when you need (or might need in future) their additional arguments (as in CompareTo())

Answer 3

Check http://www.ikriv.com/en/prog/info/dotnet/ObjectEquality.html

Answer 4

Best-practise-wise, I would tend to always use an Equals() function for string comparison. This makes it clear when someone else reads your code that you specifically want the strings compared.

Answer 5

generally speaking, == does pointer equality, while .equals checks whether the attributes are equal. So if you did something like

a = 'a';
b = 'a';
bool c = (a == b);
bool d = (a.Equals(b))

then c should return false and d should be true.

Answer 6

This is the implementation of the operator:

    public static bool operator == (String a, String b) {
       return String.Equals(a, b);
    }

Don't lose any sleep over this.

Answer 7

You should read Jeffrey Richter, CLR via C #.

Answer 8

When there is no techincal difference, opt for the simplest solution. a == b is simply shorter and provides enough explanation on the intent in C#'s context.