Are `string.Equals()` and the `==` Operator Always Equivalent for String Comparison?

String comparison: Differences between string.Equals() method and == operator

In programming, people often think that the string.Equals() method and the == operator can be used interchangeably in string comparison. However, an interesting puzzle arises that challenges this preconceived notion.

Problem Analysis

Consider the following code:

<code class="language-csharp">string s = "Category";
TreeViewItem tvi = new TreeViewItem();
tvi.Header = "Category";

Console.WriteLine(s == tvi.Header); // false
Console.WriteLine(s.Equals(tvi.Header)); // true
Console.WriteLine(s == tvi.Header.ToString()); // true</code>

Unexpectedly, the == operator returns false on the first comparison, indicating that s and tvi.Header are not considered equal. In contrast, the Equals() method returns true, indicating equality.

Key Differences

The difference in results results from two significant differences between the two comparison mechanisms:

• Polymorphism: The Equals() method is polymorphic, it takes an implementation of the target object's class. The == operator, on the other hand, relies on compile-time type information. For example, if s is declared as StringBuilder and later converted to a string, == will still compare it to tvi.Header as a StringBuilder object.
• NullException: Calling Equals() on a null object will result in NullReferenceException, while the == operator will simply return false. To avoid this problem, use the object.Equals() method even if one of the objects is empty.

Conclusion

While the string.Equals() and == operators can generally produce similar results, they are different comparison mechanisms with unique advantages and limitations. Understanding these differences is critical to writing robust code that handles string comparisons correctly.

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