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?

Answer 1

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. [...]

Answer 2

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";

        Console.WriteLine(lower.Equals(upper, 
            StringComparison.CurrentCultureIgnoreCase));
        Console.WriteLine(lower.Equals(upper, 
            StringComparison.InvariantCultureIgnoreCase));
    }
}

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