C# String comparisons: Difference between CurrentCultureIgnoreCase and InvariantCultureIgnoreCase

Question

When doing a string comparison in C#, what is the difference between doing a

string test = "testvalue";
test.Equals("TESTVALUE", StringComparison.CurrentCultureIgnoreCase);

and

string test = "testvalue";
test.Equals("TESTVALUE", StringComparison.InvariantCultureIgnoreCase);

... and is it important to include that extra parameter, anyway?

Jon Skeet · Answer 1 · 2022-02-21T07:09:32.207

39

The other posts have given good advice, but I thought it might be nice to show an example of where it definitely makes a difference:

using System;
using System.Globalization;
using System.Threading;

class Test
{
    static void Main()
    {
        CultureInfo turkish = CultureInfo.CreateSpecificCulture("tr");
        Thread.CurrentThread.CurrentCulture = turkish;                

        // In Turkey, "i" does odd things
        string lower = "i";
        string upper = "I";
        
        // Prints False
        Console.WriteLine(lower.Equals(upper, 
            StringComparison.CurrentCultureIgnoreCase));
        // Prints True
        Console.WriteLine(lower.Equals(upper, 
            StringComparison.InvariantCultureIgnoreCase));
    }
}

(There are no doubt many other cases - this was just the first one I thought of.)

edited Feb 21 '22 at 07:09

answered Jan 06 '09 at 20:50

Jon Skeet

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8

Yes, the turkish i is a special case. They have a lowercase dotless "ı" with an uppercase "I", and a lowercase "i" with an uppercase "İ". It is considered the canonical case for culture differences. – configurator Jan 07 '09 at 00:54
3

For more on the nature of Turkish as the "canonical case" mentioned by configurator, see: http://www.moserware.com/2008/02/does-your-code-pass-turkey-test.html – JeffH Mar 27 '09 at 14:06
1

I met the same thing in Vietnamese `String.Compare("logid", "logId", StringComparison.CurrentCultureIgnoreCase) == 1`. But `String.Compare("id", "Id", StringComparison.CurrentCultureIgnoreCase) == 0` :O – Nhat Hong Ly Nov 21 '16 at 06:09

score 29 · Accepted Answer · edited Mar 21 '13 at 19:57

Microsoft gives some decent guidance for when to use the InvariantCulture property:

MSDN: CultureInfo.InvariantCulture Property

... an application should use the invariant culture only for processes that require culture-independent results, such as formatting and parsing data that is persisted to a file. In other cases, it produces results that might be linguistically incorrect or culturally inappropriate.

Security Considerations

If a security decision will be made based on the result of a string comparison or case change, your application should use an ordinal comparison that ignores case instead of using InvariantCulture. [...]

String Operations

If your application needs to perform a culture-sensitive string operation that is not affected by the value of CurrentCulture, it should use a method that accepts a CultureInfo parameter. [...]

Persisting Data

The InvariantCulture property is useful for storing data that will not be displayed directly to users. Storing data in a culture-independent format guarantees a known format that does not change. When users from different cultures access the data, it can be formatted appropriately based on specific user. [...]

C# String comparisons: Difference between CurrentCultureIgnoreCase and InvariantCultureIgnoreCase

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