How Can I Efficiently Find All Occurrences of a Substring in a C# String?

Finding All Positions of a Substring in a Larger String in C#

Finding the occurrences of a substring within a larger string is a common programming task. In C#, the string.IndexOf() method provides a convenient way to find the first occurrence of a substring, but it does not provide a straightforward way to find all occurrences.

To find all occurrences of a substring, you can use a loop that iterates through the larger string while using the string.IndexOf() method to locate each occurrence. However, this approach can be inefficient if the larger string is large and the substring is found multiple times.

A more efficient approach is to use an extension method, which allows you to add custom methods to existing classes. Here's an example of an extension method that finds all occurrences of a substring in a string:

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);
  }
}

With this extension method, you can find all occurrences of a substring in a string using the following syntax:

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

Alternatively, you can also use an iterator to find all occurrences of a substring:

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;
  }
}

This iterator allows you to iterate through the occurrences of the substring using the foreach statement:

foreach (int index in "fooStringfooBar".AllIndexesOf("foo"))
{
  // do something with the index
}

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