string.Length / IndexOf()

// Returns the length (number of characters) of a string.
string.Length

// Returns the index of the first occurrence of the specified value. Returns -1 if not found.
string.IndexOf(string value)
string.IndexOf(string value, int startIndex)

// Returns the index of the last occurrence of the specified value. Returns -1 if not found.
string.LastIndexOf(string value)

using System;

string greeting = "Hello, World!";

// Use Length to get the character count.
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

// 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); // 8
Console.WriteLine(next);  // 25

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

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().