c sharp index of substring

// To find the index of the first substring in a string use
// 'IndexOf()'
string str = "Hello World!"
str.IndexOf("o"); // Output: 4

// You can give a starting index and a length to search at
str.IndexOf("o", 6, 4); // Output: 7

// To find the last instance of a substring use 'LastIndexOf()'
str.LastIndexOf("o"); // Output: 7