In C# what is the difference between ToUpper() and ToUpperInvariant()?

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ToUpper uses the current culture. ToUpperInvariant uses the invariant culture.

The canonical example is Turkey, where the upper case of “i” isn’t “I”.

Sample code showing the difference:

using System;
using System.Drawing;
using System.Globalization;
using System.Threading;
using System.Windows.Forms;

public class Test
{
    [STAThread]
    static void Main()
    {
        string invariant = "iii".ToUpperInvariant();
        CultureInfo turkey = new CultureInfo("tr-TR");
        Thread.CurrentThread.CurrentCulture = turkey;
        string cultured = "iii".ToUpper();

        Font bigFont = new Font("Arial", 40);
        Form f = new Form {
            Controls = {
                new Label { Text = invariant, Location = new Point(20, 20),
                            Font = bigFont, AutoSize = true},
                new Label { Text = cultured, Location = new Point(20, 100),
                            Font = bigFont, AutoSize = true }
            }
        };        
        Application.Run(f);
    }
}

For more on Turkish, see this Turkey Test blog post.

I wouldn’t be surprised to hear that there are various other capitalization issues around elided characters etc. This is just one example I know off the top of my head… partly because it bit me years ago in Java, where I was upper-casing a string and comparing it with “MAIL”. That didn’t work so well in Turkey…

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