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

Question

I want to replace the first occurrence in a given string.

How can I accomplish this in .NET?

Answer 1

Assumes that AA only needs to be replaced if it is at the very start of the string:

var newString;
if(myString.StartsWith("AA"))
{
  newString ="XQ" + myString.Substring(2);
}

If you need to replace the first occurrence of AA, whether the string starts with it or not, go with the solution from Marc.