How can I check if a string has all different characters?

For example,

true if the string contains one or more repeated characters.

false if the string is composed of all different characters.

"A normal string with repeated characters" --> true
"Another" --> false
"abcdefghijklm" --> false
"aa" --> true

Think of the string as a sequence of characters. A sequence contain at least one duplicate if the count of the distinct elements is not equal to the overall count of the elements.

In other words:

bool containsDuplicates = str.Distinct().Count() != str.Length;

(This will be constructing a HashSet<char> behind the scenes, so it's basically equivalent to the answers which construct one explicitly and then use the Count property... I just find this approach slightly clearer, personally, with the explicit mention of distinctness.)

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