What are the common design patterns in C#?

Question

Accepted Answer

**Design patterns** — reusable solutions to common problems.

**Singleton:**
```csharp
public sealed class Singleton
{
    private static Singleton _instance;
    public static Singleton Instance =>
        _instance ??= new Singleton();
}
// Use: Logger, Configuration
```

**Factory:**
```csharp
public interface IShape { double Area(); }
public class Circle : IShape { ... }
public class Rectangle : IShape { ... }

public static IShape Create(string type) => type switch
{
    "circle" => new Circle(),
    "rectangle" => new Rectangle(),
    _ => throw new ArgumentException()
};
```

**Observer:**
```csharp
public event EventHandler<DataEventArgs> DataChanged;

protected virtual void OnDataChanged(Data data)
{
    DataChanged?.Invoke(this, new DataEventArgs(data));
}
```

**Builder:**
```csharp
var query = new QueryBuilder()
    .Select("name", "age")
    .From("users")
    .Where("age > 18")
    .Build();
```

**Strategy:**
```csharp
public interface ISortStrategy { void Sort(int[] array); }
public class BubbleSort : ISortStrategy { ... }
public class QuickSort : ISortStrategy { ... }
```

C#: What are the common design patterns in C#?