What are extension methods in C#?

Question

Accepted Answer

**Extension methods** — add methods to existing types without inheritance.

**Definition:**
```csharp
public static class StringExtensions
{
    public static bool IsNullOrEmpty(this string str)
    {
        return string.IsNullOrEmpty(str);
    }

public static string Truncate(this string str, int maxLength)
    {
        if (str.Length <= maxLength)
            return str;
        return str[..maxLength] + "...";
    }
}
```

**Usage:**
```csharp
using MyApp.Extensions;  // Required!

string name = "Alice";
if (name.IsNullOrEmpty()) { }
string truncated = name.Truncate(3);  // "Ali..."
```

**Rules:**
- Static class.
- Static method.
- 'this' keyword on first parameter.
- First parameter = type extended.

**LINQ is extension methods:**
```csharp
list.Where(x => x > 5)
    .Select(x => x * 2)
    .ToList();
```

C#: What are extension methods in C#?