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

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...

Jon's answer is perfect. I just wanted to add that ToUpperInvariant is the same as calling ToUpper(CultureInfo.InvariantCulture).

That makes Jon's example a little simpler:

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".ToUpper(CultureInfo.InvariantCulture);
        string cultured = "iii".ToUpper(new CultureInfo("tr-TR"));

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

I also used New Times Roman because it's a cooler font.

I also set the Form's Font property instead of the two Label controls because the Font property is inherited.

And I reduced a few other lines just because I like compact (example, not production) code.

I really had nothing better to do at the moment.

Start with MSDN

http://msdn.microsoft.com/en-us/library/system.string.toupperinvariant.aspx

The ToUpperInvariant method is equivalent to ToUpper(CultureInfo.InvariantCulture)

Just because a capital i is 'I' in English, doesn't always make it so.