What is the difference between string and StringBuilder in C#?

In C#, both string and StringBuilder are classes used for manipulating strings, but they have some important differences.

1. Immutability: Strings are immutable, meaning they cannot be changed once created. Any modification to a string will create a new string object. However, StringBuilder is mutable and allows modifications to be made to the existing object without creating a new one.
2. Performance: frequent modifications to strings may lead to a decrease in performance due to their immutability, as each modification creates a new object. In contrast, StringBuilder performs better when handling frequent string operations by avoiding the creation of multiple objects.
3. Memory allocation: Due to immutability, modifying a string will result in frequent memory allocation and garbage collection. StringBuilder allocates less memory when performing string operations because it can modify within the existing buffer.
4. Thread safety: Strings are thread-safe because they are immutable. Multiple threads can access the same string object simultaneously without encountering race conditions. On the other hand, StringBuilder is not thread-safe; if multiple threads attempt to modify the same StringBuilder object simultaneously, unexpected results may occur.

In summary, if extensive string manipulation is needed and thread safety is not a concern, it is recommended to use StringBuilder. If the string is immutable and thread safety is required, then use String.