Finding all positions of substring in a larger string in C#

Here’s an example extension method for it:

public static List<int> AllIndexesOf(this string str, string value) {
    if (String.IsNullOrEmpty(value))
        throw new ArgumentException("the string to find may not be empty", "value");
    List<int> indexes = new List<int>();
    for (int index = 0;; index += value.Length) {
        index = str.IndexOf(value, index);
        if (index == -1)
            return indexes;
        indexes.Add(index);
    }
}

If you put this into a static class and import the namespace with using, it appears as a method on any string, and you can just do:

List<int> indexes = "fooStringfooBar".AllIndexesOf("foo");

For more information on extension methods, http://msdn.microsoft.com/en-us/library/bb383977.aspx

Also the same using an iterator:

public static IEnumerable<int> AllIndexesOf(this string str, string value) {
    if (String.IsNullOrEmpty(value))
        throw new ArgumentException("the string to find may not be empty", "value");
    for (int index = 0;; index += value.Length) {
        index = str.IndexOf(value, index);
        if (index == -1)
            break;
        yield return index;
    }
}