String vs StringBuilder

Understand when to choose immutable strings or performance-friendly StringBuilder

Posted by Hüseyin Sekmenoğlu on December 23, 2020 Programming Fundamentals

In C#, both string and StringBuilder are used to handle sequences of characters, but they behave very differently behind the scenes. Choosing the right one affects performance, memory usage, and code clarity—especially in loops or high-frequency updates.

🧠 Key Differences

| Feature               | String                                     | StringBuilder                         |
| --------------------- | ------------------------------------------ | ------------------------------------- |
| Mutability            | ❌ Immutable                                | ✅ Mutable                             |
| Memory Allocation     | Each change creates a new object in memory | Same object is reused                 |
| Thread-Safety         | ✅ Safe                                     | ❌ Not thread-safe by default          |
| Speed in Loops        | ❌ Slow (recreates strings repeatedly)      | ✅ Fast (modifies buffer directly)     |
| Features & Simplicity | ✅ Simple to use, many built-in methods     | ❌ Fewer methods, more manual          |
| Use Case              | Small, rarely-changing texts               | Large, frequently updated texts       |
| Null Assignment       | ✅ Can be null                              | ❌ Cannot be null after initialization |

📦 Memory and Performance

  • string is stored in memory as an immutable object. Every time you change it, a new object is created. This leads to memory pressure and garbage collection overhead in intensive operations.

  • StringBuilder stores characters in a buffer. Appending or modifying text happens in-place without creating a new object each time.

📋 Example Comparison

Using string:

string colors;
colors += "red";
colors += "blue";
colors += "green";

Each += line creates a brand new string in memory. That means 3 new allocations.

Using StringBuilder:

StringBuilder sb = new StringBuilder();
sb.Append("red");
sb.Append("blue");
sb.Append("green");
string colors = sb.ToString();

All operations modify the same buffer, making it ideal inside loops or when building long strings.

🧪 When to Use What

Use string when:

  • Your text changes infrequently

  • You prefer simplicity over performance

  • You’re working with small text values

Use StringBuilder when:

  • You need to manipulate large text blocks

  • You’re appending strings in a loop

  • You want better performance with fewer memory allocations

Choosing the right tool for the job makes a huge difference in code quality and efficiency.