C# - Add A Substring Before The First Occurrence Of A String

C#: Insert a substring before the first occurrence of a string

Last update on January 14 2026 12:30:42 (UTC/GMT +8 hours)

Write a program in C# Sharp to insert a substring before the first occurrence of a string.

Sample Solution:-

C# Sharp Code:

using System; // Define the Exercise20 class public class Exercise20 { // Main method - entry point of the program public static void Main() { // Declare variables to store user input and index string str1; string findstring; string insertstring; int i; // Prompt the user to input the original string Console.Write("\n\nInsert a substring before the first occurrence of a string :\n"); Console.Write("--------------------------------------------------------------\n"); Console.Write("Input the original string : "); str1 = Console.ReadLine(); // Prompt the user to input the string to be searched for Console.Write("Input the string to be searched for : "); findstring = Console.ReadLine(); // Prompt the user to input the string to be inserted Console.Write("Input the string to be inserted : "); insertstring = Console.ReadLine(); // Locate the position of the first occurrence of the string to be found i = str1.IndexOf(findstring); // Modify the insert string for formatting purposes insertstring = " " + insertstring.Trim() + " "; // Insert the insert string before the first occurrence of the found string str1 = str1.Insert(i, insertstring); // Display the modified string Console.Write("The modified string is : {0}\n\n", str1); } }

Sample Output:

Insert a substing before the first occurence of a string : -------------------------------------------------------------- Input the original string : The string is str Input the string to be searched for : string Input the string to be inserted : original The modified string is : The original string is str

Flowchart:

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