C# Dictionary(IEqualityComparer?comparer) constructor example

Question

I have tried to study the below mentioned constructor example of C# in Microsoft Documentation, however I am not able to understand it:

Dictionary<string,string>(IEqualityComparer<string>?comparer)

Can anyone explain me what can be a good example where we can use this constructor along with some sample code. Thanks in advance...

What if you wanted your string key to be case insensitive? You would pass in a comparer that supported that (for example `StringComparer.InvariantCultureIgnoreCase`) — DavidG, Jul 18 '22 at 11:53
To know when to use this constructor, one needs to know what `IEqualityComparer` does and how it is used by `Dictionary`. Have you researched that aspect? There are some [questions on Stack Overflow](https://stackoverflow.com/q/6694508/215552) [about it...](https://stackoverflow.com/q/7143948/215552) — Heretic Monkey, Jul 18 '22 at 11:55
You may elaborate on what you don't understand. If you say you read the documentation and presumably have seen the examples what is the part of the examples you don't understand? We might fill the holes then or actually come up with a different example. Currently i could just repeat what is in the docs. — Ralf, Jul 18 '22 at 12:08

Guru Stron · Answer 1 · 2022-07-18T12:01:24.213

I would say that documentation contains good example for this Dictionary<TKey,TValue> constructor:

The following code example creates a Dictionary<TKey,TValue> with a case-insensitive equality comparer for the current culture. The example adds four elements, some with lower-case keys and some with upper-case keys. The example then attempts to add an element with a key that differs from an existing key only by case, catches the resulting exception, and displays an error message. Finally, the example displays the elements in the dictionary.

Dictionary<string, string> openWith =
    new Dictionary<string, string>(StringComparer.CurrentCultureIgnoreCase);

openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("DIB", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

// Try to add a fifth element with a key that is the same
// except for case; this would be allowed with the default
// comparer.
try
{
    openWith.Add("BMP", "paint.exe");
}
catch (ArgumentException)
{
    Console.WriteLine("\nBMP is already in the dictionary.");
}

// List the contents of the sorted dictionary.
Console.WriteLine();
foreach( KeyValuePair<string, string> kvp in openWith )
{
    Console.WriteLine("Key = {0}, Value = {1}", kvp.Key,
        kvp.Value);
}

i.e. this dictionary performs case-insensitive hashcode calculation and comparison using current culture (for example "bmp", "BMP" and "bMp" will be matched into the same entry in the dictionary).

More concise example can look as simple as:

var caseSensitive = new Dictionary<string, string>();
var caseInsensitive = new Dictionary<string, string>(StringComparer.InvariantCultureIgnoreCase);
caseSensitive.Add("test", string.Empty);
caseInsensitive.Add("test", string.Empty);
Console.WriteLine(caseSensitive.ContainsKey("test"));    // prints True 
Console.WriteLine(caseSensitive.ContainsKey("TEST"));    // prints False
Console.WriteLine(caseInsensitive.ContainsKey("TEST"));  // prints True
Console.WriteLine(caseInsensitive.ContainsKey("TEST"));  // prints True

C# Dictionary(IEqualityComparer?comparer) constructor example

1 Answers1