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Tuesday, October 11, 2011

Optimal use of case insensitive Dictionary in .Net

A question arose at the office today: What is faster? Using a Dictionary<string,object> with a StringComparer.OrdinalCaseInsensitive constructor parameter or using a normal constructor call and using ToLower on the key before using it. The quick answer: using ToLower on the key.

The longer answer is that StringComparer.OrdinalCaseInsensitive implements IEqualityComparer<string> by using a native code function for GetHasCode(), which is very efficient. Unfortunately, it must use the case insensitive string comparison on both input key and stored keys, while calling ToLower on the keys before using them makes the comparison only once.

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