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

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);
}

Example:

string str = "The brown brown fox jumps over the lazy dog";

str = ReplaceFirst(str, "brown", "quick");

EDIT: As @itsmatt mentioned, there's also Regex.Replace(String, String, Int32), which can do the same, but is probably more expensive at runtime, since it's utilizing a full featured parser where my method does one find and three string concatenations.

EDIT2: If this is a common task, you might want to make the method an extension method:

public static class StringExtension
{
  public static string ReplaceFirst(this string text, string search, string replace)
  {
     // ...same as above...
  }
}

Using the above example it's now possible to write:

str = str.ReplaceFirst("brown", "quick");

As itsmatt said Regex.Replace is a good choice for this however to make his answer more complete I will fill it in with a code sample:

using System.Text.RegularExpressions;
...
Regex regex = new Regex("foo");
string result = regex.Replace("foo1 foo2 foo3 foo4", "bar", 1);             
// result = "bar1 foo2 foo3 foo4"

The third parameter, set to 1 in this case, is the number of occurrences of the regex pattern that you want to replace in the input string from the beginning of the string.

I was hoping this could be done with a static Regex.Replace overload but unfortunately it appears you need a Regex instance to accomplish it.

Take a look at Regex.Replace.

Taking the "first only" into account, perhaps:

int index = input.IndexOf("AA");
if (index >= 0) output = input.Substring(0, index) + "XQ" +
     input.Substring(index + 2);

?

Or more generally:

public static string ReplaceFirstInstance(this string source,
    string find, string replace)
{
    int index = source.IndexOf(find);
    return index < 0 ? source : source.Substring(0, index) + replace +
         source.Substring(index + find.Length);
}

Then:

string output = input.ReplaceFirstInstance("AA", "XQ");

In C# syntax:

int loc = original.IndexOf(oldValue);
if( loc < 0 ) {
    return original;
}
return original.Remove(loc, oldValue.Length).Insert(loc, newValue);

C# extension method that will do this:

public static class StringExt
{
    public static string ReplaceFirstOccurrence(this string s, string oldValue, string newValue)
    {
         int i = s.IndexOf(oldValue);
         return s.Remove(i, oldValue.Length).Insert(i, newValue);    
    } 
}

Enjoy

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.

And because there is also VB.NET to consider, I would like to offer up:

Private Function ReplaceFirst(ByVal text As String, ByVal search As String, ByVal replace As String) As String
    Dim pos As Integer = text.IndexOf(search)
    If pos >= 0 Then
        Return text.Substring(0, pos) + replace + text.Substring(pos + search.Length)
    End If
    Return text 
End Function

One of the overloads of Regex.Replace takes an int for "The maximum number of times the replacement can occur". Obviously, using Regex.Replace for plain text replacement may seem like overkill, but it's certainly concise:

string output = (new Regex("AA")).Replace(input, "XQ", 1);

Regex.Replace, especially RegEx.Replace(string, string, int), is probably what you're looking for. That or String.IndexOf which will give you the index and then you can cut and rebuild the string with the new text you want.

An example demonstrating the latter (as first demonstrated by @David Humpohl):

string str = "Hello WorldWorld";

str = ReplaceFirst(str, "World", "StackOverflow ");

...

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

string abc = "AAAAX1";

            if(abc.IndexOf("AA") == 0)
            {
                abc.Remove(0, 2);
                abc = "XQ" + abc;
            }

using System.Text.RegularExpressions;

RegEx MyRegEx = new RegEx("F");
string result = MyRegex.Replace(InputString, "R", 1);

will find first F in InputString and replace it with R.