Understanding delegates with C#

A delegate is similar to a function pointer in C or C++ except that delegates are type-safe. The term type-safe means that code is specified in a well-defined manner that can be recognized by a compiler. In this case it means that an incorrect use of a delegate is a compile-time error. This is quite different than in C++, where an incorrect use of a function pointer may not cause an error until the program is running.

Delegates allow you to write code that can dynamically change the methods that it calls.

A delegate contains a reference to a method rather than the method name.

By using delegates, you can invoke a method without knowing its name. Calling the delegate will actually execute the method referenced by the delegate.

To use a delegate, you must follow these steps:

  1. First, define it using the reserved keyword delegate.
  2. Second, instantiate it.
  3. Third, write the implementation with the same return value and signature of your delegate.
Fig 1. Steps to use a delegate.

A delegate is similar to an interface. It specifies a contract between a caller and an implementer.

The following code shows how to define, create and call delegates, It creates an array of delegates with instances of delegates that refers to the methods that represent each arithmetic operation.
We execute each delegate with a foreach keyword that iterates through this array.

Fig 2. Sample using delegates.

Using delegates is a solution much simpler than using function pointer.

Fig 3. Testing the program.

Fig 4. Another test.

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C# Recipe 1: How to calculate a leap year.

The Gregorian Calendar is the most widely used calendar in the world today.It is a reform of the Julian calendar, proposed by Aloysius Lilius, and decreed by Pope Gregory XIII, from whom it was named, on 24 February 1582 by papal bull Inter gravissimas

The changes made by Gregory also corrected the drift in the civil calendar which arose because the mean Julian calendar year was slightly too long, causing the vernal equinox, and consequently the date on which Easter was being celebrated, to slowly drift forward in relation to the civil calendar and the seasons. The Gregorian Calendar system dealt with these problems by dropping 10 days to bring the calendar back into synchronization with the seasons, and adopting the following leap year rule:

Every year that is exactly divisible by four is a leap year, except for years that are exactly divisible by 100; the centurial years that are exactly divisible by 400 are still leap years. For example, the year 1800 is not a leap year; the year 1984 is a leap year and the year 2000 too.

Fig 1 The following example calculates a leap year.

Fig 2 Running the sample, output 1

Fig 3 Running the sample, output 2

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Overriding Methods in C#

In addition to producing a new class based on an old one by adding additional features, you can modify existing behavior of the parent class.

If a method is defined in a derived class so that the name, return type, and argument list match exactly those of a method in the parent class, then the new method is said to override the old one.

The keyword virtual

In C#, a class can declare virtual methods, properties, and indexers, and derived classes can override the implementation of these function members.

The keyword virtual allows programmers to specify methods that a derived class can override, C# methods are non-virtual by default and must be explicitly declared as virtual.

The implementation of a non-virtual method is invariant: The implementation is the same whether the method is invoked on an instance of the class in which it is declared or an instance of derived class. In contrast, the implementation of a virtual method can be changed by derived classes.

The keyword override

To override a base-class method definition, a derived class must specify that the derived-class method overrides the base-class method with keyword override in the method header.

If the override modifier is not used, the new member hides the inherited member, and a compiler warning occurs. If a derived class attempts to override a non-virtual inherited member, a compiler error will occur.
The following examples illustrate the using of virtual and override keywords.

Fig 1. Class diagram for Employee and Manager using Inheritance.

Listing 1: Sample code for Employee class

Listing 2: Sample code for Manager class

Consider these sample methods in the Employee and Manager classes:
Fig 2. The GetDetails method of the Employee class.

Fig 3. The GetDetails method of the Manager class.

The Manager class has a GetDetails method by definition because it inherits one from the Employee class. However, the original method has been replaced, or overridden, by the derived class’s version.

Listing 3 A simple program to demonstrate how method overriding works.

Fig 4 The output of executing this program is the following.

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Understanding abstract classes with C#

The C# programming language enables a class designer to specify that a base class declares a method that does not supply an implementation. This is called an abstract method. The implementation of this method is supplied by the derived classes. Any class with one or more abstract method is called an abstract class.

Fig 1 UML model of the abstract class.

The purpose of an abstract class is primarily to provide an appropriate base class from which other classes can inherit and thus share a common design.
However, abstract classes can have data attributes, concrete methods, and constructors. For example, the Vehicle class might include load and maxLoad attributes and a constructor to initialize them.
Not all inheritance hierarchies contain abstract classes. However, programmers often write client code that uses only abstract base class types to reduce the client code’s dependencies on a range of specific derived class types.
The C# compiler always prevents you form instantiating an abstract class, for example the following statement is illegal.

 Vehicle v = new Vehicle(); // this is illegal and the compiler generates an error.

Although we cannot instantiate objects of abstract base classes, we will see that we can use abstract base classes to declare variables that can hold references to objects of any concrete classes derived from those abstract classes.

The employee abstract class: A example using Polymorphism through inheritance

What is polymorphism?

The term polymorphism refers to the ability of two or more objects belonging to different classes to respond to exactly the same message (method call) in different class-specific ways, thus polymorphism enables us to “program in the general” rather than “program in the specific”.

Polymorphism allows:

  • Different behavoirs from the same type.
  • Runtime polymorphism is done via overriding.
  • Compile time polymorphism is done via method overloading.
  • Invoke methods of a derived class through a base class.

I think the best way to learn about this concept is through an example, so I wrote a set of examples using an inheritance hierarchy containing types of employees in a company.

Fig 2 Employee hierarchy UML class diagram.

We use abstract class Employee to represent the general concept of an employee. The classes that inherit from Employee are: CommissionEmployee, PieceEmployee and HourlyEmployee.

1. I use abstract class Employee to represent the general concept of an employee. The classes that inherit from Employee are: CommissionEmployee, PieceEmployee and HourlyEmployee.

Listing 1. Abstract base class Employee

In this example, class Employee will provide two methods Earnings and Details, and properties that manipulate an Employee’s instance variables, the method Earnings certainly applies generally to all employee’s, but each earnings calculation depends on the derived employee’s class. So these two methods must be abstract in the Employee class because an implementation does not make sense there, so each derived class must override those methods with an appropriate implementation.

2. I wrote three concrete classes: HourlyWorker, CommissionWorker and PieceWorker. Such classes provide implementations of the abstracts methods declared in our abstract class, because abstract base classes are too general to create real objects.

Listing 2. Concrete derived class HourlyEmployee

Listing 3. Concrete derived class CommissionEmployee

Listing 4. Concrete derived class PieceWorker

To test our example, the following program creates an array of our abstract base class Employee, then assigns the four instances of our concretes classes to the array variable. Finally, the program iterates through array Employee and polymorphically invokes methods Earnings and Details to display the output of each of these methods.

Listing 5. Test program for our example.

Fig 3. The output for our example.

Note: Those examples were based on the examples included in: Deitel’s developer series C# 2010 for programmers.

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Understanding Bitwise Logical Operators with C#

Bit manipulation operations, including logical and shift operations, perform low-level operations directly on the binary representations used in integers. The ability to operate directly on binary might save large amounts of memory, might enable certain computations to be performed very efficiently, and can greatly simplify operations on collections of bits, such as data read form or written to parallel I/O ports.

The C# programming language supports bitwise operations on both numeric and boolean data types. These are represented as the operators ~, &, ^, and | for the bitwise operations of NOT (bitwise complement), bitwise AND, bitwise XOR, and bitwise OR, respectively.
The operators work in parallel on all bits of the operands and never cause overflow, even in a checked context.

Unsigned integers are normally used with the bitwise operators.

Bitwise NOT (complement)

The bitwise complement or NOT is used to flip the bits of a value, this operator is unary and inverts the bit value. Since this operator is a unary operator it cannot be combined with the = sign.

Fig 1 Example of bitwise NOT operator

Bitwise XOR

When combining two values with the logical XOR bitwise operator, you get the following result: if both bits are the same, the result is 0 else if 1 bit is 0 and the other is 1, the result is 1

Fig 2 Example of bitwise XOR Operator

Bitwise OR

When combining 2 byte values results in the following: if both bits are 0, the result is 0. If either or both bits are 1, the result is 1.

Fig 3 Example of bitwise OR operator

Bitwise AND

The bitwise AND operator produces a one in the output bit if both input bits are one; otherwise it produces a zero.

Fig 4 Example of bitwise AND operator

When using the bitwise operator, it’s useful to display values in binary to show the precise effects of these operators.
The following example demonstrate the use of the bitwise operators, it uses the method PrintBits (implemented in class Util) to print the unsigned int values.

The full sample is shown below:

Listing 1 Source code for Util class

Listing 2 Source code for the main program class

Fig 5 Running the sample

Fig 6 Output of the complete example

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Uso de Custom Exceptions (excepciones personalizadas) en .NET parte II

En la entrada anterior del blog, me olvide de publicar el archivo App.config donde se encuentra la cadena de conexión (ConnectionString) y se accede a su valor mediante la siguiente línea en la aplicación y se asigna a la variable connStr.

En caso de ser necesario y de tener algún incoveniente en construir la solución aqui dejo el enlace para descargar el código fuente.

  Descarga el código fuente