C# 0 The Complete Reference

IMyIF_A

and

IMyIF_B

. Thus, when 

MyClass

 implements both of these interfaces, it explicitly 

implements each one separately, fully qualifying its name in the process. Since the only way 

that an explicitly implemented method can be called is on an interface reference, 

MyClass

creates two such references, one for 

IMyIF_A

 and one for 

IMyIF_B

. It then calls two of its 

own methods, which call the interface methods, thereby removing the ambiguity.

Choosing Between an Interface and an Abstract Class

One of the more challenging parts of C# programming is knowing when to create an interface 

and when to use an abstract class in cases in which you want to describe functionality but 

not implementation. The general rule is this: When you can fully describe the concept in 

terms of “what it does” without needing to specify any “how it does it,” then you should 

use an interface. If you need to include some implementation details, then you will need to 

represent your concept in an abstract class.

The .NET Standard Interfaces

The .NET Framework defines a large number of interfaces that a C# program can use. For 

example,

System.IComparable

 defines the 

CompareTo( )

 method, which allows objects to be 

compared when an ordering relationship is required. Interfaces also form an important part 

of the Collections classes, which provide various types of storage (such as stacks and queues) 

for groups of objects. For example, 

System.Collections.ICollection

 defines the functionality 

of a collection. 

System.Collections.IEnumerator

 offers a way to sequence through the 

elements in a collection. These and many other interfaces are described in Part II.

Structures

As you know, classes are reference types. This means that class objects are accessed through 

a reference. This differs from the value types, which are accessed directly. However, sometimes 

it would be useful to be able to access an object directly, in the way that value types are. One 

reason for this is efficiency. Accessing class objects through a reference adds overhead onto 

every access. It also consumes space. For very small objects, this extra space might be 

significant. To address these concerns, C# offers the structure. A 

structure

 is similar to a 

class, but is a value type, rather than a reference type.

Structures are declared using the keyword 

struct

 and are syntactically similar to classes. 

Here is the general form of a 

struct

:

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