C# Case Insensitive DictionaryCreate a case-insensitive Dictionary with StringComparer.OrdinalIgnoreCase.
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Case, Dictionary. A case-insensitive Dictionary is ideal in some programs. It helps with comparing file names in Windows, which ignore case.


Notes, upper and lower. Sometimes user names are case-insensitive—upper and lower letters are treated as equal. We implement a case-insensitive string Dictionary.

An example. The Dictionary class has several overloaded constructors. Here we use one that accepts a parameter of type IEqualityComparer(string).

IEqualityComparer This object implements an interface and is used by Dictionary to acquire hash codes and compare keys.


New We use a Dictionary constructor that accepts an IEqualityComparer. We specify StringComparer.OrdinalIgnoreCase as the comparer.

Methods StringComparer.OrdinalIgnoreCase implements (at minimum) 2 methods, Equals and GetHashCode.

StringComparison, StringComparer

Info These 2 methods are used internally by the Dictionary to compute hash keys and compare buckets.

C# program that uses case-insensitive Dictionary
using System; using System.Collections.Generic; class Program { static void Main() { // Create case insensitive string Dictionary. var caseInsensitiveDictionary = new Dictionary<string, int>( StringComparer.OrdinalIgnoreCase); caseInsensitiveDictionary.Add("Cat", 2); caseInsensitiveDictionary.Add("Python", 4); caseInsensitiveDictionary.Add("DOG", 6); // Write value of "cat". Console.WriteLine(caseInsensitiveDictionary["CAT"]); // 2 // Write value of "PYTHON". Console.WriteLine(caseInsensitiveDictionary["PYTHON"]); // 4 // See if "dog" exists. if (caseInsensitiveDictionary.ContainsKey("dog")) { Console.WriteLine("Contains dog."); // <-- Displayed } // Enumerate all KeyValuePairs. foreach (var pair in caseInsensitiveDictionary) { Console.WriteLine(pair.ToString()); } } }
2 4 Contains dog. [Cat, 2] [Python, 4] [DOG, 6]

Notes, usage. You can add keys with either case using the Add method. The keys retain their original case internally in the Dictionary buckets.

And When you enumerate the KeyValuePairs (in foreach), the original cases are retained.


Caution When you add the strings "Cat" and "CAT", you will get an exception. The Dictionary considers the 2 strings to be equivalent.

Note, ContainsKey. ContainsKey (along with TryGetValue) will accept keys of either case. So "PYTHON" will return the same value as "Python".



A discussion. Another approach you can use to normalize string data in your Dictionary is always calling ToLower on the keys and before looking up keys.


File names In Windows, file names are case-insensitive. But when users copy files, they expect them to retain their original case.

Thus This Dictionary is ideal here. It will mimic the behavior of Windows NTFS filesystems.

A summary. We implemented a case-insensitive string Dictionary. You do not need to write your own IEqualityComparer, although if your requirements are slightly unusual you can do so.

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