How to Find Substring in C#: Simple Methods Explained

In C#, you can find a substring using the IndexOf method, which returns the position of the substring or -1 if not found. Alternatively, use Contains to check if the substring exists, returning a boolean value.

Syntax

The main methods to find a substring in C# are:

• int IndexOf(string value): Returns the zero-based index of the first occurrence of value or -1 if not found.
• bool Contains(string value): Returns true if value exists in the string, otherwise false.

csharp

string text = "Hello, world!";
int position = text.IndexOf("world");
bool exists = text.Contains("Hello");

Example

This example shows how to find the position of a substring and check if it exists in a string.

csharp

using System;

class Program
{
    static void Main()
    {
        string text = "Hello, world!";

        int index = text.IndexOf("world");
        if (index != -1)
        {
            Console.WriteLine($"Substring 'world' found at position: {index}");
        }
        else
        {
            Console.WriteLine("Substring not found.");
        }

        bool containsHello = text.Contains("Hello");
        Console.WriteLine($"Does the text contain 'Hello'? {containsHello}");
    }
}

Output

Substring 'world' found at position: 7
Does the text contain 'Hello'? True

Common Pitfalls

Common mistakes when finding substrings include:

• Not checking if IndexOf returns -1 before using the index.
• Ignoring case sensitivity; IndexOf and Contains are case-sensitive by default.
• Using Contains when you need the position of the substring.

To perform case-insensitive search, use IndexOf with StringComparison.OrdinalIgnoreCase.

csharp

string text = "Hello, World!";

// Wrong: assumes substring always found
int pos = text.IndexOf("world");
Console.WriteLine(pos + 1); // Will print 0 because pos is -1

// Right: check if found
int pos2 = text.IndexOf("world", StringComparison.OrdinalIgnoreCase);
if (pos2 != -1)
{
    Console.WriteLine($"Found at position {pos2}");
}
else
{
    Console.WriteLine("Not found");
}

Output

0
Found at position 7

Quick Reference

Method	Description	Return Type	Case Sensitivity
IndexOf(string value)	Finds position of substring or -1 if not found	int	Case-sensitive by default
IndexOf(string value, StringComparison)	Finds position with case options	int	Depends on StringComparison
Contains(string value)	Checks if substring exists	bool	Case-sensitive by default

Key Takeaways

Use IndexOf to get the position of a substring; it returns -1 if not found.

Use Contains to check if a substring exists, returning true or false.

Always check for -1 from IndexOf before using the index to avoid errors.

For case-insensitive search, use IndexOf with StringComparison.OrdinalIgnoreCase.

Contains does not provide position, only existence.