Differences in string compare methods in C#

Question

Comparing string in C# is pretty simple. In fact there are several ways to do it. I have listed some in the block below. What I am curious about are the differences between them and when one should be used over the others? Should one be avoided at all costs? Are there more I haven't listed?

string testString = "Test";
string anotherString = "Another";

if (testString.CompareTo(anotherString) == 0) {}
if (testString.Equals(anotherString)) {}
if (testString == anotherString) {}

(Note: I am looking for equality in this example, not less than or greater than but feel free to comment on that as well)

Answer 1

Here are the rules for how these functions work:

stringValue.CompareTo(otherStringValue)

1. null comes before a string
2. it uses CultureInfo.CurrentCulture.CompareInfo.Compare, which means it will use a culture-dependent comparison. This might mean that ? will compare equal to SS in Germany, or similar

stringValue.Equals(otherStringValue)

1. null is not considered equal to anything
2. unless you specify a StringComparison option, it will use what looks like a direct ordinal equality check, i.e. ? is not the same as SS, in any language or culture

stringValue == otherStringValue

1. Is not the same as stringValue.Equals().
2. The == operator calls the static Equals(string a, string b) method (which in turn goes to an internal EqualsHelper to do the comparison.
3. Calling .Equals() on a null string gets null reference exception, while on == does not.

Object.ReferenceEquals(stringValue, otherStringValue)

Just checks that references are the same, i.e. it isn't just two strings with the same contents, you're comparing a string object with itself.

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Note that with the options above that use method calls, there are overloads with more options to specify how to compare.

My advice if you just want to check for equality is to make up your mind whether you want to use a culture-dependent comparison or not, and then use .CompareTo or .Equals, depending on the choice.