Why Doesn't `String.Replace` Modify the Original String in C#?

Understanding C#'s Immutable Strings and String.Replace

In C#, strings are immutable. This means that once a string object is created, its value cannot be changed. The String.Replace method, therefore, doesn't modify the original string; instead, it returns a new string with the replacements made.

This often leads to confusion. Simply assigning the result of String.Replace back to the original variable appears to do nothing, because it's creating a new string object and assigning its reference to the original variable name.

Correctly Using String.Replace

To achieve the desired modification, you must explicitly reassign the returned string to the original variable:

<code class="language-csharp">someTestString = someTestString.Replace(someID.ToString(), sessionID);</code>

This overwrites the original string variable's reference with the reference to the newly created, modified string.

Alternatively, you can assign the result to a new variable:

<code class="language-csharp">var modifiedString = someTestString.Replace(someID.ToString(), sessionID);</code>

This preserves the original string and allows you to work with the modified version separately.

This behavior is consistent across all C# string manipulation methods that return new strings rather than modifying in place, including Remove, Insert, Trim, and various substring methods. Remember that strings are immutable; any operation that appears to modify a string actually creates a new one.

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