Make HashSet<string> case-insensitive

Question

I have method with HashSet parameter. And I need to do case-insensitive Contains within it:

public void DoSomething(HashSet<string> set, string item) {     var x = set.Contains(item);     ...  } 

Is it any way to make existing HashSet case-insensitive (do not create new one)?

I'm looking for solution with best perfomance.

Edit

Contains can be called multiple times. So IEnumerable extensions are not acceptable for me due to lower perfomance than native HashSet Contains method.

Solution

Since, answer to my question is NO, it is impossible, I've created and used following method:

public HashSet<string> EnsureCaseInsensitive(HashSet<string> set) {     return set.Comparer == StringComparer.OrdinalIgnoreCase            ? set            : new HashSet<string>(set, StringComparer.OrdinalIgnoreCase); } 

Answer 1

The HashSet<T> constructor has an overload that lets you pass in a custom IEqualityComparer<string>. There are a few of these defined for you already in the static StringComparer class, a few of which ignore case. For example:

var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase); set.Add("john"); Debug.Assert(set.Contains("JohN")); 

You'll have to make this change at the time of constructing the HashSet<T>. Once one exists, you can't change the IEqualityComparer<T> it's using.

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Just so you know, by default (if you don't pass in any IEqualityComparer<T> to the HashSet<T> constructor), it uses EqualityComparer<T>.Default instead.

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Edit

The question appears to have changed after I posted my answer. If you have to do a case insensitive search in an existing case sensitive HashSet<string>, you will have to do a linear search:

set.Any(s => string.Equals(s, item, StringComparison.OrdinalIgnoreCase)); 

There's no way around this.

Answer 2

You can not magically make case-sensetive HashSet (or Dictionary) to behave in case-insensitive way.

You have to recreate one inside your function if you can not rely on incoming HashSet to be case-insensitive.

Most compact code - use constructor from existing set:

var insensitive = new HashSet<string>(    set, StringComparer.InvariantCultureIgnoreCase); 

Note that copying HashSet is as expensive as walking through all items, so if your function does just on search it would be cheaper (O(n)) to iterate through all items. If your function called multiple times to make single case-insensitive search you should try to pass proper HashSet to it instead.