String vs StringBuilder in C# - ablealias/asp.net GitHub Wiki

String

String is immutable. Immutable means once we create string object we cannot modify. Any operation like insert, replace or append happened to change string simply it will discard the old value and it will create new instance in memory to hold the new value.

• It’s an immutable
• Performance wise string is slow because every time it will create new instance
• In string we don’t have append keyword
• String belongs to System namespace

String Builder

String builder is mutable it means once we create string builder object we can perform any operation like insert, replace or append without creating new instance for every time.

• It’s mutable
• Performance wise stringbuilder is high because it will use same instance of object to perform any action
• In StringBuilder we can use append keyword
• Stringbuilder belongs to System.Text namespace