SOFTWARE INTERVIEW TIPS

3. Classes and Structs

3.1 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.

3.2 Does C# support multiple inheritance (MI)?

No, though it does support implementation of multiple interfaces on a single class or struct.

3.3 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.

3.4 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.

3.5 Are C# destructors the same as C++ destructors?

No. They look the same but they are very different. The C# destructor syntax (with the familiar ~ character) is just syntactic sugar for an override of the System.Object Finalize method. This Finalize method is called by the garbage collector when it determines that an object is no longer referenced, before it frees the memory associated with the object. So far this sounds like a C++ destructor. The difference is that the garbage collector makes no guarantees about when this procedure happens. Indeed, the algorithm employed by the CLR garbage collector means that it may be a long time after the application has finished with the object. This lack of certainty is often termed 'non-deterministic finalization', and it means that C# destructors are not suitable for releasing scarce resources such as database connections, file handles etc.
To achieve deterministic destruction, a class must offer a method to be used for the purpose. The standard approach is for the class to implement the IDisposable interface. The user of the object must call the Dispose() method when it has finished with the object. C# offers the 'using' construct to make this easier.

3.6 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.

3.7 Are all methods virtual in C#?

No. Like C++, methods are non-virtual by default, but can be marked as virtual.

3.8 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).

3.9 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.

3.10 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.

4. Exceptions

4.1 Can I use exceptions in C#?

Yes, in fact exceptions are the recommended error-handling mechanism in C# (and in .NET in general). Most of the .NET framework classes use exceptions to signal errors.

4.2 What types of object can I throw as exceptions?

Only instances of the System.Exception classes, or classes derived from System.Exception. This is in sharp contrast with C++ where instances of almost any type can be thrown.

4.3 Can I define my own exceptions?

Yes, just derive your exception class from System.Exception.
Note that if you want your exception to cross remoting boundaries you'll need to do some extra work - see http://www.thinktecture.com/Resources/RemotingFAQ/CustomExceptions.html for details.

4.4 Does the System.Exception class have any cool features?

Yes - the feature which stands out is the StackTrace property. This provides a call stack which records where the exception was thrown from. For example, the following code:
using System;

class CApp
{
public static void Main()
{
try
{
f();
}
catch( Exception e )
{
Console.WriteLine( "System.Exception stack trace = \n{0}", e.StackTrace );
}
}

static void f()
{
throw new Exception( "f went pear-shaped" );
}
}
produces this output:
System.Exception stack trace =
at CApp.f()
at CApp.Main()
Note, however, that this stack trace was produced from a debug build. A release build may optimise away some of the method calls which could mean that the call stack isn't quite what you expect.

4.5 When should I throw an exception?

This is the subject of some debate, and is partly a matter of taste. However, it is accepted by many that exceptions should be thrown only when an 'unexpected' error occurs. How do you decide if an error is expected or unexpected? This is a judgement call, but a straightforward example of an expected error is failing to read from a file because the seek pointer is at the end of the file, whereas an example of an unexpected error is failing to allocate memory from the heap.

4.6 Does C# have a 'throws' clause?

No, unlike Java, C# does not require (or even allow) the developer to specify the exceptions that a method can throw.