C# Case-Insensitive Dictionary

Case, dictionary. A case-insensitive Dictionary is ideal in some programs.

It helps with comparing file names in Windows, which ignore case. Sometimes user names also are case-insensitive. We see an implementation of a case-insensitive string Dictionary.

Tip: With "case-insensitive," we mean "Python" and "PYTHON" are equal. Uppercase and lowercase are the same.

Example. To start, the Dictionary class in the .NET Framework has several overloaded constructors. The most important ones for this solution accept a parameter of type IEqualityComparer(string).

IEqualityComparer

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

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());
	}
    }
}

Output

2
4
Contains dog.
[Cat, 2]
[Python, 4]
[DOG, 6]

This example constructs a case-insensitive Dictionary. The instance called caseInsensitiveDictionary uses the Dictionary constructor that accepts an IEqualityComparer. We specify StringComparer.OrdinalIgnoreCase as the comparer.

StringComparer.OrdinalIgnoreCase implements (at minimum) two methods, Equals and GetHashCode. These two methods are used internally by the Dictionary to compute hash keys and compare buckets.

Info: The word "ordinal" means that the characters are treated by their numeric values.

Using the Dictionary. You can add keys with either case using the Add method. The keys retain their original case internally in the Dictionary buckets. When you enumerate the KeyValuePairs, the original cases are retained.

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

Using ContainsKey and indexers. In this example, the ContainsKey method (along with TryGetValue) will accept keys of either case. For example, "PYTHON" will return the same value as "Python".

ContainsKeyTryGetValue

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. This is sometimes better if you do not want to preserve the mixed cases.

ToLower

However: It introduces an inefficiency because keys must be copied in the ToLower method.

File names. In Windows, file names are case-insensitive. But when users copy files, they expect them to retain their original case. This Dictionary is ideal here. It will mimic the behavior of Windows NTFS filesystems.

Summary. We implemented a case-insensitive string Dictionary in the C# language. You do not need to write your own IEqualityComparer, although if your requirements are slightly unusual you can do so.

Review: We used the Dictionary overloaded constructor and the StringComparer.OrdinalIgnoreCase implementation.

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