string.Length / IndexOf()

string.Length

string.IndexOf(string value)
string.IndexOf(string value, int startIndex)

string.LastIndexOf(string value)

Program.cs

using System;

string greeting = "Hello, World!";

Console.WriteLine(greeting.Length); // 13

// Use IndexOf() to find the position of a substring
int pos = greeting.IndexOf("World");
Console.WriteLine(pos); // 7

// Returns -1 if the substring is not found
int notFound = greeting.IndexOf("xyz");
Console.WriteLine(notFound); // -1

This produces the following output:

dotnet run
13
7
-1

Specifying startIndex in IndexOf() starts the search from that position. LastIndexOf() returns the position of the last occurrence.

SearchSample.cs

using System;

// Use startIndex to search from a specific position
string path = "C:/users/akiba/documents/akiba.txt";
int first = path.IndexOf("akiba");
int next = path.IndexOf("akiba", first + 1);
Console.WriteLine(first); // 9
Console.WriteLine(next); // 25

// Use LastIndexOf() to find the last occurrence
string text = "abcabcabc";
Console.WriteLine(text.LastIndexOf("a")); // 6

// Case-insensitive search
string msg = "Hello, C# World!";
int idx = msg.IndexOf("hello", StringComparison.OrdinalIgnoreCase);
Console.WriteLine(idx); // 0

This produces the following output:

dotnet run
9
25
6
0

This example uses IndexOf() and Substring() together to split an email address into a username and a domain.

EmailParse.cs

using System;

string email = "okabe@wp-p.info";
int atIndex = email.IndexOf("@");
if (atIndex >= 0) {
	string user = email.Substring(0, atIndex);
	string domain = email.Substring(atIndex + 1);
	Console.WriteLine($"User: {user}"); // User: okabe
	Console.WriteLine($"Domain: {domain}"); // Domain: wp-p.info
} else {
	Console.WriteLine("Invalid email address.");
}

This produces the following output:

dotnet run
User: okabe
Domain: wp-p.info

Common Mistake: Calling Substring() Without Checking the IndexOf() Return Value

If IndexOf() returns -1 (not found) and you pass that value directly to Substring(), an ArgumentOutOfRangeException will be thrown.

using System;

// NG: Calling Substring() when IndexOf() returns -1 throws an exception
string text = "Hello, World!";
int pos = text.IndexOf("C#"); // Returns -1
string part = text.Substring(pos); // ArgumentOutOfRangeException

This produces the following output:

dotnet run
Unhandled exception. System.ArgumentOutOfRangeException: StartIndex cannot be less than zero.

using System;

// OK: Always check for -1 first
string text = "Hello, World!";
int pos = text.IndexOf("C#");
if (pos >= 0) {
	string part = text.Substring(pos);
	Console.WriteLine(part);
} else {
	Console.WriteLine("Not found.");
}

This produces the following output:

dotnet run
Not found.

Length is a property of the string (not a method), so no parentheses are needed when accessing it. In C#, string indices are zero-based, and each character — including multi-byte characters — counts as one.

IndexOf() is case-sensitive. To perform a case-insensitive search, use IndexOf(value, StringComparison.OrdinalIgnoreCase).

To extract or remove part of a string, see Substring() / Remove(). To replace characters or check for containment, see Replace() / Contains().