String.Equals vs. == in C#: Which one to use?

Introduction

When comparing string values in C#, developers often have to choose between using String.Equals or the == operator. While both options seem to achieve the same result, there are subtle differences that must be considered when choosing which one to use. In this article, we will explore those differences and offer guidance on which option to use in different situations.

The Differences

The == operator compares the reference of the two strings, whereas String.Equals compares their contents. This means that == will return true if the two strings point to the same memory location, while String.Equals will return true if the two strings contain the same characters.

Here's an example to illustrate this difference:

string a = "hello";
string b = "hello";
string c = string.Copy(a);

bool aEqualsB = a == b; // true
bool aEqualsC = a == c; // false
bool aEqualsBEq = a.Equals(b); // true
bool aEqualsCEq = a.Equals(c); // true

In this example, a and b contain the same characters, so using either == or String.Equals to compare them will return true. However, c is a copy of a, so while c contains the same characters as a, == will return false because c and a are not the same object. String.Equals, on the other hand, will return true because it compares the contents of the two strings.

When to Use Which Option

In general, it's best to use String.Equals when comparing strings for equality, as it compares the contents of the strings rather than their references. This is particularly important if the strings are obtained from user input or external sources, as they may not be interned (i.e., stored in the same memory location) even if they contain the same characters.

However, there are situations where using == is more appropriate. For example, when checking if a string is null or empty, using == is simpler and more readable than using String.Equals. Here's an example:

string str = null;

bool isNullOrEmpty = (str == null) || (str == ""); // true
bool isNullOrEmptyEq = string.IsNullOrEmpty(str); // true

In this example, we use the == operator to check if str is null or empty. This is a straightforward and common pattern in C# code. However, if we want to check if a string is null or empty using String.Equals, we would have to write something like this:

string str = null;

bool isNullOrEmpty = (str == null) || (str.Equals("")); // true

While this achieves the same result, it's less readable and more verbose than using ==.

Conclusion

In conclusion, both String.Equals and the == operator have their uses when comparing string values in C#. However, in general, it's best to use String.Equals when comparing for equality, as it compares the contents of the strings rather than their references. When checking if a string is null or empty, using == is simpler and more readable than using String.Equals.