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

Last update on August 25 2025 12:50:37 (UTC/GMT +8 hours)

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

C# Sharp Exercises: 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:

Go to:

PREV : Write a program in C# Sharp to find the number of times a substring appears in the given string.
NEXT : Write a C# Sharp program to compare (less than, greater than, equal to ) two substrings.

C# Sharp Code Editor:

Contribute your code and comments through Disqus.