Is there an easy way to create ordinals in C#?

This page gives you a complete listing of all custom numerical formatting rules:

Custom numeric format strings

As you can see, there is nothing in there about ordinals, so it can't be done using String.Format. However its not really that hard to write a function to do it.

public static string AddOrdinal(int num)
{
    if( num <= 0 ) return num.ToString();

    switch(num % 100)
    {
        case 11:
        case 12:
        case 13:
            return num + "th";
    }
    
    switch(num % 10)
    {
        case 1:
            return num + "st";
        case 2:
            return num + "nd";
        case 3:
            return num + "rd";
        default:
            return num + "th";
    }
}

Update: Technically Ordinals don't exist for <= 0, so I've updated the code above. Also removed the redundant ToString() methods.

Also note, this is not internationalized. I've no idea what ordinals look like in other languages.

Remember internationalisation!

The solutions here only work for English. Things get a lot more complex if you need to support other languages.

For example, in Spanish "1st" would be written as "1.o", "1.a", "1.os" or "1.as" depending on whether the thing you're counting is masculine, feminine or plural!

So if your software needs to support different languages, try to avoid ordinals.

My version of Jesse's version of Stu's and samjudson's versions :)

Included unit test to show that the accepted answer is incorrect when number < 1

/// <summary>
/// Get the ordinal value of positive integers.
/// </summary>
/// <remarks>
/// Only works for english-based cultures.
/// Code from: http://stackoverflow.com/questions/20156/is-there-a-quick-way-to-create-ordinals-in-c/31066#31066
/// With help: http://www.wisegeek.com/what-is-an-ordinal-number.htm
/// </remarks>
/// <param name="number">The number.</param>
/// <returns>Ordinal value of positive integers, or <see cref="int.ToString"/> if less than 1.</returns>
public static string Ordinal(this int number)
{
    const string TH = "th";
    string s = number.ToString();

    // Negative and zero have no ordinal representation
    if (number < 1)
    {
        return s;
    }

    number %= 100;
    if ((number >= 11) && (number <= 13))
    {
        return s + TH;
    }

    switch (number % 10)
    {
        case 1: return s + "st";
        case 2: return s + "nd";
        case 3: return s + "rd";
        default: return s + TH;
    }
}

[Test]
public void Ordinal_ReturnsExpectedResults()
{
    Assert.AreEqual("-1", (1-2).Ordinal());
    Assert.AreEqual("0", 0.Ordinal());
    Assert.AreEqual("1st", 1.Ordinal());
    Assert.AreEqual("2nd", 2.Ordinal());
    Assert.AreEqual("3rd", 3.Ordinal());
    Assert.AreEqual("4th", 4.Ordinal());
    Assert.AreEqual("5th", 5.Ordinal());
    Assert.AreEqual("6th", 6.Ordinal());
    Assert.AreEqual("7th", 7.Ordinal());
    Assert.AreEqual("8th", 8.Ordinal());
    Assert.AreEqual("9th", 9.Ordinal());
    Assert.AreEqual("10th", 10.Ordinal());
    Assert.AreEqual("11th", 11.Ordinal());
    Assert.AreEqual("12th", 12.Ordinal());
    Assert.AreEqual("13th", 13.Ordinal());
    Assert.AreEqual("14th", 14.Ordinal());
    Assert.AreEqual("20th", 20.Ordinal());
    Assert.AreEqual("21st", 21.Ordinal());
    Assert.AreEqual("22nd", 22.Ordinal());
    Assert.AreEqual("23rd", 23.Ordinal());
    Assert.AreEqual("24th", 24.Ordinal());
    Assert.AreEqual("100th", 100.Ordinal());
    Assert.AreEqual("101st", 101.Ordinal());
    Assert.AreEqual("102nd", 102.Ordinal());
    Assert.AreEqual("103rd", 103.Ordinal());
    Assert.AreEqual("104th", 104.Ordinal());
    Assert.AreEqual("110th", 110.Ordinal());
    Assert.AreEqual("111th", 111.Ordinal());
    Assert.AreEqual("112th", 112.Ordinal());
    Assert.AreEqual("113th", 113.Ordinal());
    Assert.AreEqual("114th", 114.Ordinal());
    Assert.AreEqual("120th", 120.Ordinal());
    Assert.AreEqual("121st", 121.Ordinal());
    Assert.AreEqual("122nd", 122.Ordinal());
    Assert.AreEqual("123rd", 123.Ordinal());
    Assert.AreEqual("124th", 124.Ordinal());
}