Strings

A string in C# is a sequence of characters, and it's immutable — once created, its contents never change. Every method that appears to "modify" a string is actually handing you back a brand-new one.

Building strings: concatenation vs. interpolation

You can glue strings together with +, but once numbers get mixed in, it gets noisy fast. String interpolation — a $ before the opening quote, with expressions inside { } — reads much closer to the sentence you're actually trying to produce:

string name = "Diego";
int orders = 4;

string message1 = "Hello, " + name + "! You have " + orders + " orders.";
string message2 = $"Hello, {name}! You have {orders} orders.";

Console.WriteLine(message1);
Console.WriteLine(message2);

Hello, Diego! You have 4 orders.
Hello, Diego! You have 4 orders.

Both lines print identical output. message2 just took less effort to write and, more importantly, less effort to read back later — there's no hunting for a missing space between quote marks.

Useful string methods

Because strings are immutable, methods like Trim and ToUpper don't change the original — they return a new string that you need to capture (or chain straight onto the next call):

string title = "  the great gatsby  ";

Console.WriteLine(title.Trim());
Console.WriteLine(title.Trim().ToUpper());
Console.WriteLine(title.Length);

the great gatsby
THE GREAT GATSBY
20

Note that title.Length is 20, counting the original leading and trailing spaces — Trim() never touched title itself, it only affected the new string returned to Console.WriteLine on the earlier lines.

Searching, slicing, replacing

string email = "maria@example.com";

Console.WriteLine(email.Contains("@"));
Console.WriteLine(email.Substring(0, 5));
Console.WriteLine(email.Replace("example", "mystore"));

True
maria
maria@mystore.com

Substring(0, 5) means "starting at index 0, take 5 characters." Replace swaps every occurrence of the first argument with the second, returning the new full string — again, email itself is untouched after this runs.