What is StringBuilder? What is inside it? How does memory allocation work?

StringBuilder is a class in C# designed for efficient construction and modification of strings. Unlike regular strings (string), which are immutable, StringBuilder allows changing the content without creating new objects for each operation.

Inside StringBuilder, there is a character array (char[]), initially allocated with some extra capacity. When new characters or strings are added, if the current array cannot hold the new data, a new larger array is allocated, old data is copied, and new data is added.

The memory allocation mechanism typically doubles the size of the internal buffer upon overflow, reducing the number of copies and increasing performance during multiple concatenation operations.

Example usage:

var sb = new StringBuilder();
sb.Append("Hello");
sb.Append(" World");
string result = sb.ToString(); // "Hello World"

Thus, StringBuilder is optimal for scenarios with many string modifications, where creating new strings would be costly.