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

Question

Accepted Answer

**string** — immutable. **StringBuilder** — mutable.

**string:**
- Immutable (cannot change).
- Each modification creates new object.
- Memory overhead for concatenation.

```csharp
string s = "Hello";
s += " World";  // Creates NEW string
s += "!";       // Creates another NEW string
// 3 string objects created!
```

**StringBuilder:**
- Mutable.
- Modifies existing buffer.
- Efficient for multiple modifications.

```csharp
var sb = new StringBuilder();
sb.Append("Hello");
sb.Append(" World");
sb.Append("!");
string result = sb.ToString();
// Only 1 final string created
```

**Performance comparison:**
- **string** — good for few concatenations.
- **StringBuilder** — good for many concatenations.

```csharp
// Bad
string s = "";
for (int i = 0; i < 1000; i++)
    s += i;

// Good
var sb = new StringBuilder();
for (int i = 0; i < 1000; i++)
    sb.Append(i);
string s = sb.ToString();
```

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