The System.Text.StringBuilder class represents a mutable string of characters.

This class represents a string-like object whose value is a mutable sequence of characters. The value is said to be mutable because it can be modified once it has been created by appending, removing, replacing, or inserting characters.

the AppendFormat method Appends a formatted string, which contains zero or more format specifications, to this instance. Each format specification is replaced by the string representation of a corresponding object argument.