Case insensitive access for generic dictionary

by

There’s no way to specify a StringComparer at the point where you try to get a value. If you think about it, "foo".GetHashCode() and "FOO".GetHashCode() are totally different so there’s no reasonable way you could implement a case-insensitive get on a case-sensitive hash map.

You can, however, create a case-insensitive dictionary in the first place using:-

var comparer = StringComparer.OrdinalIgnoreCase;
var caseInsensitiveDictionary = new Dictionary<string, int>(comparer);

Or create a new case-insensitive dictionary with the contents of an existing case-sensitive dictionary (if you’re sure there are no case collisions):-

var oldDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
var newDictionary = new Dictionary<string, int>(oldDictionary, comparer);

This new dictionary then uses the GetHashCode() implementation on StringComparer.OrdinalIgnoreCase so comparer.GetHashCode("foo") and comparer.GetHashcode("FOO") give you the same value.

Alternately, if there are only a few elements in the dictionary, and/or you only need to lookup once or twice, you can treat the original dictionary as an IEnumerable<KeyValuePair<TKey, TValue>> and just iterate over it:-

var myKey = ...;
var myDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
var value = myDictionary.FirstOrDefault(x => String.Equals(x.Key, myKey, comparer)).Value;

Or if you prefer, without the LINQ:-

var myKey = ...;
var myDictionary = ...;
var comparer = StringComparer.OrdinalIgnoreCase;
int? value;
foreach (var element in myDictionary)
{
  if (String.Equals(element.Key, myKey, comparer))
  {
    value = element.Value;
    break;
  }
}

This saves you the cost of creating a new data structure, but in return the cost of a lookup is O(n) instead of O(1).

More Related Contents:

  1. Static Generic Class as Dictionary
  2. SortedList, SortedDictionary and Dictionary
  3. The given key was not present in the dictionary. Which key?
  4. Filtering out values from a C# Generic Dictionary
  5. Hashtable to Dictionary syncroot .
  6. Collection was modified; enumeration operation may not execute
  7. Serialize Class containing Dictionary member
  8. Generics in C#, using type of a variable as parameter [duplicate]
  9. ASP.NET MVC Binding to a dictionary
  10. Get dictionary key by value
  11. C# use System.Type as Generic parameter
  12. getting type T from IEnumerable
  13. Why isn’t ArrayList marked [Obsolete]?
  14. Why cannot IEnumerable be cast as IEnumerable?
  15. Can I specify a generic type in XAML (pre .NET 4 Framework)?
  16. Generic Type in constructor
  17. C# 3.0 generic type inference – passing a delegate as a function parameter
  18. Binding DataGrid to ObservableCollection
  19. Sorting a Dictionary in place with respect to keys
  20. Does .NET have a built-in EventArgs?
  21. C#: cast to generic interface with base type
  22. Define a generic that implements the + operator [duplicate]
  23. Wildcard equivalent in C# generics
  24. Why does C# (4.0) not allow co- and contravariance in generic class types?
  25. C# – static types cannot be used as type arguments
  26. Which C# method overload is chosen?
  27. Calling generic static method in PowerShell
  28. Accessing a Dictionary.Keys Key through a numeric index
  29. The component does not have a resource identified by the uri
  30. How does Java’s use-site variance compare to C#’s declaration site variance?

Leave a Comment