Because you're concatenating literals, the compiler will translate it into string a = "abcd" ;) Strings are outwardly immutable, so concatenating any two strin

Because you're concatenating literals, the compiler will translate it into string a = "abcd" ;) Strings are outwardly immutable, so concatenating any two strings will create an entirely new String object, along with the memory allocation needed for that. However, there are some internal methods that allow String and other classes in the same corelib assembly (such as StringBuilder) to mutate them. `string a = x + y + z` will be translated into `string a = String.Concat(x, y, z)`. Because it knows the final length it can use the internal String methods to allocate a new String long enough to fit all the arguments and then copy the characters into that without any intermediate representations. [1] In a loop it doesn't know how big the final result will be, and so has to create a new String object on each iteration. If you're doing 16000000 concatenations as in this test that's a lot of intermediate strings that are largely being wasted. StringBuilders have a private String object and use the internal String methods to mutate that when you call Append. When you call ToString it returns a copy of the private String object. Again, this avoids creating lots of new intermediate String objects, but does have the overhead of having to guess how large the initial String should be, and growing it if you append too much (similar to how Lists work). [1] I believe in Java it's translated into using StringBuffer or StringBuilder, which has a small overhead of creating the StringBuffer/StringBuilder object. [Edit: Corrected how String.Concat and StringBuilders work based on the Mono codebase]