How to Replace Only the First Instance of a String in .NET?

First instance of string replacement in .NET

.NET provides several ways to replace the first occurrence in a string. The most straightforward approach is to use a combination of the IndexOf method to find the index of the first match, and then use the string's Substring method to build the replaced string. Here is sample code to implement this functionality:

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;
}

Example:

string str = "The brown brown fox jumps over the lazy dog";
str = ReplaceFirst(str, "brown", "quick"); // str 现在是 "The quick brown fox jumps over the lazy dog"

Additional notes:

• As @itsmatt mentioned, you can also use the Regex.Replace(String, String, Int32) method, but it may not be as efficient as the custom method provided here.
• For scenarios with high performance requirements, you can create an extension method to simplify the replacement operation.

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