Differences in string compare methods in C#

Question

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 ß 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. ß 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.

From MSDN:

"The CompareTo method was designed primarily for use in sorting or alphabetizing operations. It should not be used when the primary purpose of the method call is to determine whether two strings are equivalent. To determine whether two strings are equivalent, call the Equals method."

They suggest using .Equals instead of .CompareTo when looking solely for equality. I am not sure if there is a difference between .Equals and == for the string class. I will sometimes use .Equals or Object.ReferenceEquals instead of == for my own classes in case someone comes along at a later time and redefines the == operator for that class.

If you are ever curious about differences in BCL methods, Reflector is your friend :-)

I follow these guidelines:

Exact match: _{EDIT: I previously always used == operator on the principle that inside Equals(string, string) the object == operator is used to compare the object references but it seems strA.Equals(strB) is still 1-11% faster overall than string.Equals(strA, strB), strA == strB, and string.CompareOrdinal(strA, strB). I loop tested with a StopWatch on both interned/non-interned string values, with same/different string lengths, and varying sizes (1B to 5MB).}

strA.Equals(strB)

Human-readable match (Western cultures, case-insensitive):

string.Compare(strA, strB, StringComparison.OrdinalIgnoreCase) == 0

Human-readable match (All other cultures, insensitive case/accent/kana/etc defined by CultureInfo):

string.Compare(strA, strB, myCultureInfo) == 0

Human-readable match with custom rules (All other cultures):

CompareOptions compareOptions = CompareOptions.IgnoreCase
                              | CompareOptions.IgnoreWidth
                              | CompareOptions.IgnoreNonSpace;
string.Compare(strA, strB, CultureInfo.CurrentCulture, compareOptions) == 0

As Ed said, CompareTo is used for sorting.

There is a difference, however, between .Equals and ==.

== resolves to essentially the following code:

if(object.ReferenceEquals(left, null) && 
   object.ReferenceEquals(right, null))
    return true;
if(object.ReferenceEquals(left, null))
    return right.Equals(left);
return left.Equals(right);

The simple reason is the following will throw an exception:

string a = null;
string b = "foo";

bool equal = a.Equals(b);

And the following will not:

string a = null;
string b = "foo";

bool equal = a == b;

Good explanation and practices about string comparison issues may be found in the article New Recommendations for Using Strings in Microsoft .NET 2.0 and also in Best Practices for Using Strings in the .NET Framework.

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Each of mentioned method (and other) has particular purpose. The key difference between them is what sort of StringComparison Enumeration they are using by default. There are several options:

• CurrentCulture
• CurrentCultureIgnoreCase
• InvariantCulture
• InvariantCultureIgnoreCase
• Ordinal
• OrdinalIgnoreCase

Each of above comparison type targets different use case:

• Ordinal
  • Case-sensitive internal identifiers
  • Case-sensitive identifiers in standards like XML and HTTP
  • Case-sensitive security-related settings
• OrdinalIgnoreCase
  • Case-insensitive internal identifiers
  • Case-insensitive identifiers in standards like XML and HTTP
  • File paths (on Microsoft Windows)
  • Registry keys/values
  • Environment variables
  • Resource identifiers (handle names, for example)
  • Case insensitive security related settings
• InvariantCulture or InvariantCultureIgnoreCase
  • Some persisted linguistically-relevant data
  • Display of linguistic data requiring a fixed sort order
• CurrentCulture or CurrentCultureIgnoreCase
  • Data displayed to the user
  • Most user input

Note, that StringComparison Enumeration as well as overloads for string comparison methods, exists since .NET 2.0.

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String.CompareTo Method (String)

Is in fact type safe implementation of IComparable.CompareTo Method. Default interpretation: CurrentCulture.

Usage:

The CompareTo method was designed primarily for use in sorting or alphabetizing operations

Thus

Implementing the IComparable interface will necessarily use this method

String.Compare Method

A static member of String Class which has many overloads. Default interpretation: CurrentCulture.

Whenever possible, you should call an overload of the Compare method that includes a StringComparison parameter.

String.Equals Method

Overriden from Object class and overloaded for type safety. Default interpretation: Ordinal. Notice that:

The String class's equality methods include the static Equals, the static operator ==, and the instance method Equals.

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StringComparer class

There is also another way to deal with string comparisons especially aims to sorting:

You can use the StringComparer class to create a type-specific comparison to sort the elements in a generic collection. Classes such as Hashtable, Dictionary, SortedList, and SortedList use the StringComparer class for sorting purposes.

Not that performance usually matters with 99% of the times you need to do this, but if you had to do this in a loop several million times I would highly suggest that you use .Equals or == because as soon as it finds a character that doesn't match it throws the whole thing out as false, but if you use the CompareTo it will have to figure out which character is less than the other, leading to slightly worse performance time.

If your app will be running in different countries, I'd recommend that you take a look at the CultureInfo implications and possibly use .Equals. Since I only really write apps for the US (and don't care if it doesn't work properly by someone), I always just use ==.