Design patterns are general solutions to common problems that occur in software design. They provide a way to structure code in a reusable and maintainable manner.

There are several design patterns that can be applied to C# code. Some of the commonly used ones are:

1. Singleton: This pattern ensures that only one instance of a class is created and provides a global point of access to the instance.

2. Factory: This pattern is used to create objects without specifying the exact class of the object to be created. It provides a way to create instances of a class without exposing the instantiation logic.

3. Observer: This pattern defines one-to-many dependencies between objects so that when one object changes state, all its dependents are notified and updated automatically.

4. Adapter: This pattern allows objects with incompatible interfaces to work together by creating an adapter class that acts as a bridge between the two.

5. Strategy: This pattern allows the behavior of an object to be changed at runtime by encapsulating different algorithms in separate classes and allowing the client to choose which algorithm to use.

6. Decorator: This pattern allows behavior to be added to an individual object dynamically, without affecting the behavior of other objects from the same class.

7. Proxy: This pattern provides a surrogate or placeholder for another object to control access to it.

8. Template Method: This pattern defines the skeleton of an algorithm in a base class, allowing subclasses to provide the implementation details.

These are just a few examples of the many design patterns that can be applied in C#. Each pattern is designed to solve a specific problem and provides a proven solution that can be reused across different scenarios.