A list of all frequently asked C# Interview Questions and Answers ,covers C# Classes and Structs.
1 Structs are largely redundant in C++. Why does C# have them?
In C++, a struct and a class are pretty much the same thing. The only difference is the default visibility level (public for structs, private for classes). However, in C# structs and classes are very different. In C#, structs are value types (instances stored directly on the stack, or inline within heap-based objects), whereas classes are reference types (instances stored on the heap, accessed indirectly via a reference). Also structs cannot inherit from structs or classes, though they can implement interfaces. Structs cannot have destructors. A C# struct is much more like a C struct than a C++ struct.
2 Does C# support multiple inheritance (MI)?
No, though it does support implementation of multiple interfaces on a single class or struct.
3 Is a C# interface the same as a C++ abstract class?
No, not quite. An abstract class in C++ cannot be instantiated, but it can (and often does) contain implementation code and/or data members. A C# interface cannot contain any implementation code or data members - it is simply a group of method names & signatures. A C# interface is more like a COM interface than a C++ abstract class.
4 Are C# constructors the same as C++ constructors?
Very similar, but there are some significant differences. First, C# supports constructor chaining. This means one constructor can call another:
class Person
{
public Person( string name, int age ) { ... }
public Person( string name ) : this( name, 0 ) {}
public Person() : this( "", 0 ) {}
}
Another difference is that virtual method calls within a constructor are routed to the most derived implementation - see Can I Call a virtual method from a constructor.
Error handling is also somewhat different. If an exception occurs during construction of a C# object, the destuctor (finalizer) will still be called. This is unlike C++ where the destructor is not called if construction is not completed. (Thanks to Jon Jagger for pointing this out.)
Finally, C# has static constructors. The static constructor for a class runs before the first instance of the class is created.
Also note that (like C++) some C# developers prefer the factory method pattern over constructors. See Brad Wilson's article.
6 If C# destructors are so different to C++ destructors, why did MS use the same syntax?
Presumably they wanted C++ programmers to feel at home. I think they made a mistake.
7 Are all methods virtual in C#?
No. Like C++, methods are non-virtual by default, but can be marked as virtual.
8 How do I declare a pure virtual function in C#?
Use the abstract modifier on the method. The class must also be marked as abstract (naturally). Note that abstract methods cannot have an implementation (unlike pure virtual C++ methods).
9 Can I call a virtual method from a constructor/destructor?
Yes, but it's generally not a good idea. The mechanics of object construction in .NET are quite different from C++, and this affects virtual method calls in constructors.
C++ constructs objects from base to derived, so when the base constructor is executing the object is effectively a base object, and virtual method calls are routed to the base class implementation. By contrast, in .NET the derived constructor is executed first, which means the object is always a derived object and virtual method calls are always routed to the derived implementation. (Note that the C# compiler inserts a call to the base class constructor at the start of the derived constructor, thus preserving standard OO semantics by creating the illusion that the base constructor is executed first.)
The same issue arises when calling virtual methods from C# destructors. A virtual method call in a base destructor will be routed to the derived implementation.
10 Should I make my destructor virtual?
A C# destructor is really just an override of the System.Object Finalize method, and so is virtual by definition.
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