String Comparison in C#: When Are `String.Equals()` and `==` Interchangeable?

C# string comparison confusion: Are the String.Equals() method and the == operator interchangeable?

In C#, string comparisons can sometimes produce unexpected results. A common question is whether the String.Equals() method and the == equality operator behave exactly the same.

Consider the following code snippet:

<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>

Although both s and tvi.Header contain the same value "Category", the == operator returns false and the String.Equals() method returns true. This raises the question: Why do these two comparison methods produce different results?

Key Differences between

String.Equals() and ==

There are two fundamental differences between the

String.Equals() and == operators:

1. type-specific implementation: The == operator compares based on the compile-time type of the object, while String.Equals() is polymorphic, meaning its implementation depends on the runtime of the compared object type.
2. Null value handling: The == operator will not throw an exception when comparing null references, while the String.Equals() operator will throw a NullReferenceException exception if either argument is null.

Example demonstrating the difference

To illustrate these differences, consider the following code:

<code class="language-csharp">object x = new StringBuilder("hello").ToString();
object y = new StringBuilder("hello").ToString();

if (x.Equals(y)) // True

if (x == y) // False</code>

Although x and y have the same value, the == operator returns false because it compares based on the object's compile-time type (object), which are different. In order to get a correct comparison, the object must be explicitly cast to the appropriate type (string in this case).

<code class="language-csharp">string xs = (string)x;
string ys = (string)y;

if (xs == ys) // True</code>

Conclusion

While the String.Equals() and == operators are often used interchangeably, it is important to understand the subtle differences in their implementation and null value handling. To ensure reliable and consistent string comparisons, it is generally recommended to use the String.Equals() method, especially when comparing objects of different types or when dealing with null references.

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