C# String IndexOf()

The String IndexOf() method returns the index of the first occurrence of the specified character/substring within the string.

Example

using System;  
namespace CsharpString {  
  class Test {
    public static void Main(string [] args) {
     
      string str = "Ice cream";

// returns index of substring cream int result = str.IndexOf("cream");
Console.WriteLine(result); Console.ReadLine(); } } } // Output: 4

IndexOf() Syntax

The syntax of the string IndexOf() method is:

String.IndexOf(string value, int startindex, int count)

Here, IndexOf() is a method of class String.


IndexOf() Parameters

The IndexOf() method takes the following parameters:

  • value - string to search
  • startIndex - starting position of the search
  • count - number of character positions to examine

IndexOf() Return Value

The IndexOf() method returns:

  • index of the first occurrence of the specified character/string
  • -1 if the specified character/string is not found

Example 1: C# String IndexOf()

using System;  
namespace CsharpString {  
  class Test {
    public static void Main(string [] args) {
     
      string str = "Ice cream";
      int result;

// returns index of character 'I' result = str.IndexOf('I');
Console.WriteLine("Index of I: " + result);
// returns -1 result = str.IndexOf('P');
Console.WriteLine("Index of P: " + result); Console.ReadLine( ); } } }

Output

Index of I: 0
Index of P: -1

Here,

  • str.IndexOf('I') - returns 0 as 'I' is in index 0 of str
  • str.IndexOf('P') - returns -1 as 'P' is not in str

Example 2: IndexOf() With Start Index

using System;  
namespace CsharpString {  
  class Test {
    public static void Main(string [] args) {
     
      string str = "Ice cream";
      int result;

// returns index of char I result = str.IndexOf('I', 0);
Console.WriteLine("Index of I: " + result);
// returns -1 result = str.IndexOf('I', 2);
Console.WriteLine("Index of I: " + result); Console.ReadLine( ); } } }

Output

Index of I: 0
Index of I: -1

In this program,

  • str.IndexOf('I', 0) - starts the search from index 0
  • str.IndexOf('I', 2) - starts the search from index 2

As we can see, str.IndexOf('I', 2) returns -1 since 'I' cannot be found after index 2.


Example 3: IndexOf() With Start Index And Count

using System;  
namespace CsharpString {  
  class Test {
    public static void Main(string [] args) {
     
      string str = "Ice cream";
      int result;

// returns -1 result = str.IndexOf('m', 0, 1);
Console.WriteLine("Index of m: " + result);
// returns index of m result = str.IndexOf('m', 0, 9);
Console.WriteLine("Index of m: " + result); Console.ReadLine( ); } } }

Output

Index of m: -1
Index of m: 8

Here,

  • str.IndexOf('m', 0, 1) - performs search in 1 character from index 0
  • str.IndexOf('m', 0, 9) - performs search in 9 characters from index 0

As we can see, str.IndexOf('m', 0, 1) returns -1 since 'm' cannot be found within 1 character after index 0.

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