7.4. String Immutability

If an object cannot be changed, we say that it is immutable. Strings are immutable, which means we cannot change the individual characters within a given string. While we can access individual characters using bracket notation, attempting to change individual characters simply does not work.

Example

string nonprofit = "Launchcode";

Console.WriteLine(nonprofit);
nonprofit[6] = 'C';
Console.WriteLine(nonprofit);

Console Output

Launchcode
error CS0200: Property or indexer `string.this[int]' cannot be assigned to (it is read-only)

We attempted to change the value of the character at index 6 from 'c' to 'C', by using an assignment statement along with bracket notation on line 4 (perhaps to align with official LaunchCode branding guidelines). However, this change clearly did not take place. In many programming languages strings are immutable, and while trying to change a string in some languages results in an error, C# is not able to change this string as it is “read-only”.

It is important to notice that immutability applies to string values and not string variables.

Example

We can set a variable containing a string to a different value.

string nonprofit = "Launchcode";
nonprofit = "LaunchCode";

Console.WriteLine(nonprofit);

Console Output

LaunchCode

In this example, the change made on line 2 is carried out. The difference between this example and the one above is that here we are modifying the value that the variable is storing, and not the string itself. Using our visual analogy of a variable as a label that “points at” a value, the second example has the following effect:

When the value of a variable storing a string is changed, the variable then points to a new value, with the old value remaining unchanged.

7.4.1. Check Your Understanding

Question

Given string pet = "cat", why do the statements Console.WriteLine(pet + 's'); and pet += 's'; NOT violate the immutability of strings?