C# String IndexOf()

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.