What is the difference between string and StringBuilder? When would you use each?

String:

• Immutable - once created, cannot be changed
• Any modification creates a new string object in memory
• Thread-safe by nature (due to immutability)
• Use when: Few modifications, constant strings, or passing data between methods

Example of immutability:

string str = "Hello";
str = str + " World";  // Creates a NEW string object, original "Hello" is garbage collected

StringBuilder:

• Mutable - can be modified without creating new objects
• More efficient for multiple string manipulations
• Not thread-safe (use synchronization if needed)
• Use when: Multiple modifications, loops, or building large strings

Performance comparison:

// BAD PRACTICE - Creates 1000 new string objects
string result = "";
for (int i = 0; i < 1000; i++)
{
    result += i.ToString();  // Very inefficient!
}

// GOOD PRACTICE - Modifies same StringBuilder object
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 1000; i++)
{
    sb.Append(i.ToString());  // Efficient!
}
string result = sb.ToString();

When to use each:

• String: 1-5 concatenations, constant values, simple operations
• StringBuilder: Loops, multiple operations, building complex strings, performance-critical code