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

Find and replace the first occurrence of a string in .NET

Challenge:

Replacing the first occurrence of a string in a given text is a common task in programming. This article explores how to do this efficiently in .NET.

Solution:

In order to replace the first occurrence of a string in .NET we can use the following method:

<code class="language-csharp">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);
}</code>

This method takes three parameters: the original text, the string to search for, and the replacement string. It first uses IndexOf to locate the index of the first occurrence of the search string in the text. If no occurrence is found (i.e. pos is negative), the original text is returned unchanged.

Next, the method works by concatenating the substring before the first occurrence of the search string (from index 0 to pos), the replacement string, and the substring after the search string (from index pos search.Length to end of text) to construct a new string.

Example:

Consider the following example:

<code class="language-csharp">string str = "The brown brown fox jumps over the lazy dog";

str = ReplaceFirst(str, "brown", "quick");</code>

After this code runs, the value of str will become "The quick brown fox jumps over the lazy dog". The first occurrence of "brown" has been replaced with "quick".

Additional notes:

The
• Regex.Replace(String, String, Int32) method can also perform this task, but it may be computationally more expensive than the custom method presented here.
• For frequent string replacement operations, consider extending this method into a reusable extension method of the String class.

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