Chương 2-Bài 12. Tạo phương thức/hàm trong C#

A method is a group of statements that together perform a task. Every C# program has at least one class with a method named Main. To use a method, you need to −

• Define the method
• Call the method

Defining Methods in C#

When you define a method, you basically declare the elements of its structure. The syntax for defining a method in C# is as follows −

<Access Specifier> <Return Type> <Method Name>(Parameter List) {
   Method Body
}

Following are the various elements of a method −

• Access Specifier − This determines the visibility of a variable or a method from another class.
• Return type − A method may return a value. The return type is the data type of the value the method returns. If the method is not returning any values, then the return type is void.
• Method name − Method name is a unique identifier and it is case sensitive. It cannot be same as any other identifier declared in the class.
• Parameter list − Enclosed between parentheses, the parameters are used to pass and receive data from a method. The parameter list refers to the type, order, and number of the parameters of a method. Parameters are optional; that is, a method may contain no parameters.
• Method body − This contains the set of instructions needed to complete the required activity.

Example

Following code snippet shows a function FindMax that takes two integer values and returns the larger of the two. It has public access specifier, so it can be accessed from outside the class using an instance of the class.

class NumberManipulator {

   public int FindMax(int num1, int num2) {
      /* local variable declaration */
      int result;

      if (num1 > num2)
         result = num1;
      else
         result = num2;

      return result;
   }
   ...
}

Calling Methods in C#

You can call a method using the name of the method. The following example illustrates this −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public int FindMax(int num1, int num2) {
         /* local variable declaration */
         int result;
         
         if (num1 > num2)
            result = num1;
         else
            result = num2;
         return result;
      }
      
      static void Main(string[] args) {
         /* local variable definition */
         int a = 100;
         int b = 200;
         int ret;
         NumberManipulator n = new NumberManipulator();

         //calling the FindMax method
         ret = n.FindMax(a, b);
         Console.WriteLine("Max value is : {0}", ret );
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Max value is : 200

You can also call public method from other classes by using the instance of the class. For example, the method FindMax belongs to the NumberManipulatorclass, you can call it from another class Test.

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public int FindMax(int num1, int num2) {
         /* local variable declaration */
         int result;
         
         if(num1 > num2)
            result = num1;
         else
            result = num2;
         
         return result;
      }
   }
   class Test {
      static void Main(string[] args) {
         /* local variable definition */
         int a = 100;
         int b = 200;
         int ret;
         NumberManipulator n = new NumberManipulator();
         
         //calling the FindMax method
         ret = n.FindMax(a, b);
         Console.WriteLine("Max value is : {0}", ret );
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Max value is : 200

Recursive Method Call

A method can call itself. This is known as recursion. Following is an example that calculates factorial for a given number using a recursive function −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public int factorial(int num) {
         /* local variable declaration */
         int result;
         if (num == 1) {
            return 1;
         } else {
            result = factorial(num - 1) * num;
            return result;
         }
      }
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         //calling the factorial method {0}", n.factorial(6));
         Console.WriteLine("Factorial of 7 is : {0}", n.factorial(7));
         Console.WriteLine("Factorial of 8 is : {0}", n.factorial(8));
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Factorial of 6 is: 720
Factorial of 7 is: 5040
Factorial of 8 is: 40320

Passing Parameters to a Method

When method with parameters is called, you need to pass the parameters to the method. There are three ways that parameters can be passed to a method −

Sr.No.	Mechanism & Description
1	Value parametersThis method copies the actual value of an argument into the formal parameter of the function. In this case, changes made to the parameter inside the function have no effect on the argument.
2	Reference parametersThis method copies the reference to the memory location of an argument into the formal parameter. This means that changes made to the parameter affect the argument.
3	Output parametersThis method helps in returning more than one value.

1. Value parameters

This is the default mechanism for passing parameters to a method. In this mechanism, when a method is called, a new storage location is created for each value parameter. The values of the actual parameters are copied into them. Hence, the changes made to the parameter inside the method have no effect on the argument. The following example demonstrates the concept −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public void swap(int x, int y) {
         int temp;
         
         temp = x; /* save the value of x */
         x = y;    /* put y into x */
         y = temp; /* put temp into y */
      }
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         
         /* local variable definition */
         int a = 100;
         int b = 200;
         
         Console.WriteLine("Before swap, value of a : {0}", a);
         Console.WriteLine("Before swap, value of b : {0}", b);
         
         /* calling a function to swap the values */
         n.swap(a, b);
         
         Console.WriteLine("After swap, value of a : {0}", a);
         Console.WriteLine("After swap, value of b : {0}", b);
         
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Before swap, value of a :100
Before swap, value of b :200
After swap, value of a :100
After swap, value of b :200

It shows that there is no change in the values though they had changed inside the function.

2. Reference parameters

A reference parameter is a reference to a memory location of a variable. When you pass parameters by reference, unlike value parameters, a new storage location is not created for these parameters. The reference parameters represent the same memory location as the actual parameters that are supplied to the method. You can declare the reference parameters using the ref keyword. The following example demonstrates this −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public void swap(ref int x, ref int y) {
         int temp;

         temp = x; /* save the value of x */
         x = y;    /* put y into x */
         y = temp; /* put temp into y */
      }
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         
         /* local variable definition */
         int a = 100;
         int b = 200;

         Console.WriteLine("Before swap, value of a : {0}", a);
         Console.WriteLine("Before swap, value of b : {0}", b);

         /* calling a function to swap the values */
         n.swap(ref a, ref b);

         Console.WriteLine("After swap, value of a : {0}", a);
         Console.WriteLine("After swap, value of b : {0}", b);
 
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Before swap, value of a : 100
Before swap, value of b : 200
After swap, value of a : 200
After swap, value of b : 100

It shows that the values have changed inside the swap function and this change reflects in the Main function.

3. Output parameters

A return statement can be used for returning only one value from a function. However, using output parameters, you can return two values from a function. Output parameters are similar to reference parameters, except that they transfer data out of the method rather than into it. The following example illustrates this −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public void getValue(out int x ) {
         int temp = 5;
         x = temp;
      }
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         
         /* local variable definition */
         int a = 100;
         
         Console.WriteLine("Before method call, value of a : {0}", a);
         
         /* calling a function to get the value */
         n.getValue(out a);

         Console.WriteLine("After method call, value of a : {0}", a);
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Before method call, value of a : 100
After method call, value of a : 5

The variable supplied for the output parameter need not be assigned a value. Output parameters are particularly useful when you need to return values from a method through the parameters without assigning an initial value to the parameter. Go through the following example, to understand this −

using System;

namespace CalculatorApplication {
   class NumberManipulator {
      public void getValues(out int x, out int y ) {
          Console.WriteLine("Enter the first value: ");
          x = Convert.ToInt32(Console.ReadLine());
          
          Console.WriteLine("Enter the second value: ");
          y = Convert.ToInt32(Console.ReadLine());
      }
      static void Main(string[] args) {
         NumberManipulator n = new NumberManipulator();
         
         /* local variable definition */
         int a , b;
         
         /* calling a function to get the values */
         n.getValues(out a, out b);
         
         Console.WriteLine("After method call, value of a : {0}", a);
         Console.WriteLine("After method call, value of b : {0}", b);
         Console.ReadLine();
      }
   }
}

When the above code is compiled and executed, it produces the following result −

Enter the first value:
7
Enter the second value:
8
After method call, value of a : 7
After method call, value of b : 8