The main difference between a Singleton and a Static Classes is that singletons can implement interfaces.
Static Class Benefits and FeaturesThe static class is useful for putting a bunch of functions together (Utils, Extensions, Math, etc) . So the class name just aggregates these functions.
- You cannot create the instance of static class.
- Static class loaded automatically by the .NET Framework Common Language Runtime (CLR) when the program or namespace containing the class is loaded.
- You cannot pass the static class to method.
- You cannot inherit Static class to another Static class in C#.
- The static class can have only static methods.
- Better performance because static methods are bonded on compile time.
- Static classes are stored in High Frequency Heap
Singlenton Benefits and FeaturesSingleton is more flexible than static classes and can maintain state. You can initialize it asynchronously when you need it.
- You can create one instance of the object and reuse it.
- Singleton instance is created for the first time when the user requested.
- You can create the object of singleton class and pass it to method.
- Singleton class does not say any restriction of Inheritance.
- We can dispose the objects of a singleton class but not of static class.
- Methods can be overridden.
- Can be lazy loaded when need (static classes are always loaded).
- We can implement interface(static class can not implement interface).
- Singleton objects are stored in Heap
- Singletons are easier to work with when unit testing a class
- Singleton can easily be serialized
Note: static class and singleton should contain the safe-thread realization. By the way, this kind of questions like to ask at interviews when applying for a job. I recommend reading my article with popular questions on .net interview.