What is this pattern called?

Question

string s = new string("Hello World").Replace(" ","_").ToLower().ToUpper();

So you basically return from each method the modified object so can you call new methods on it.

Answer 1

Method chaining. (Wikipedia)

Answer 2

Equivenlant to:

string s = new string("Hello World");
s = s.Replace(" ","_");
s = s.ToLower();
s = s.ToUpper();

Answer 3

Or Fluent Interface

Answer 4

The answer is provided by Boldewyn, I am just writing this as a suggestion.

When chaining methods like this, try to write it as follows -

string s = new string("Hello World")
               .Replace(" ","_")
               .ToLower()
               .ToUpper();

This improves code readability.