[c#] Replace first occurrence of pattern in a string

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How do I replace the first instance of a string in .NET?

Let's say I have the string:

string s = "Hello world.";

how can I replace the first o in the word Hello for let's say Foo?

In other words I want to end up with:

"HellFoo world."

I know how to replace all the o's but I want to replace just the first one

public string ReplaceFirst(string text, string search, string replace)
{
  int pos = text.IndexOf(search);
  if (pos < 0)
  {
    return text;
  }
  return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
}

here is an Extension Method that could also work as well per VoidKing request

public static class StringExtensionMethods
{
    public static string ReplaceFirst(this string text, string search, string replace)
    {
      int pos = text.IndexOf(search);
      if (pos < 0)
      {
        return text;
      }
      return text.Substring(0, pos) + replace + text.Substring(pos + search.Length);
    }
}

There are a number of ways that you could do this, but the fastest might be to use IndexOf to find the index position of the letter you want to replace and then substring out the text before and after what you want to replace.