From the documentation (bold text bolded by me, not in the source)
A string is a sequential collection of Unicode characters that is used to represent text. A String object is a sequential collection of System.Char objects that represent a string. The value of the String object is the content of the sequential collection, and that value is immutable (that is, it is read-only). For more information about the immutability of strings, see the Immutability and the StringBuilder class section later in this topic. The maximum size of a String object in memory is 2 GB, or about 1 billion characters.