C# Return Void

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C# Void Data Type
  • The void keyword can be used as a return data type in a method.
  • void tells users the method won't return a value.
  • return can be used in the void method without any data type following the return statement.
  • void can be used in unsafe code which will be introduced in our advanced lessons.
  • void is an alias of the System.Void type in the .NET Framework.
Note Note
void cannot be used as a data type in the safe mode in C#.

Example 01-44-01

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using System;
class TestVoid
{
    public int score;

    static void Main(string[] args)
    {
        TestVoid t = new TestVoid();
        t.score = 80;
        t.print();
        t.result();
		
        t.score = -10;
        t.print();
        t.result();
        Console.Read();
    }

    public void print(){
        Console.Write("Print the result: ");
    }

    public void result(){
        if ( score < 0 )
        {
            return;
        }
        else
        {
            if ( score < 60 )
                Console.WriteLine("Failed.");
            else
                Console.WriteLine("Passed.");
        }
    }
}

Output

Print the result: Passed.
Print the result:

Explanation

  • Line 2: Define a class TestVoid.
  • Line 4: Declare a public int field score.
  • Line 6: Create an instance of class Test and assign it to the variable t.
  • Line 19-21: Define a void method print(). The method does not need to return a value because of void.
  • Line 23-35: Define another void method result().
  • Line 24: if statement.
  • Line 26: return statement without data. The statement makes the control go to the method calling statement.
  • Line 30-33: if score < 60, output "Failed" otherwise output "Passed".
  • Line 6-17: Main method of the class.
  • Line 8: Instantiate the class TestVoid.
  • Line 9: Set the field score value.
  • Line 10-11: Invoke the 2 methods of the class.
  • Line 13: Assign -10 to the field score.
  • Line 14: Run the method print() to output a string. The control returns to this statement after calling.
  • Line 15: Run result(). Because -10 < 0 then run return statement in line 26 to terminate the method call. The control goes back to this line.
return Statement
  • The return statement is used to terminate execution of the method and transfer the control to the method calling statement.
  • If the return type of the method is void, the return statement can be omitted.
  • The return statement is obligatory for the method with non-void return type and the return type must be compatible with the type defined at the beginning of the method.

For example, you will get compile-time error for the method below.

public float test()
{
    return 55.5;
}

Because the default type of the above literal 55.5 is double type. To fix the problem, you can change the return statement as follows.

    return 55.5F;

55.5F is the float type literal which meets the return type. Another solution is to cast to float explicitly.

    return (float)55.5;

Example 01-44-02

In the following example, we will show you how to calculate the area of a circle and how to return an object.

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using System;

namespace TestReturn
{
    class Circle
    {
        public const double PI = 3.14;
        public double radius = 1.0;
		
        public double getArea()
        {
            return radius * radius * PI;
        }

        public Circle getCircle()
        {
            return new Circle();
        }
    }
	
    class TestCircle
    {
        static void Main(string[] args)
        {
            Circle c = new Circle();
            c.radius = 20;
			
            Console.WriteLine("The area of the circle is {0}", c.getArea());
            Console.WriteLine("The default area is {0}", c.getCircle().getArea());
            Console.Read();
        }
    }
}

Output

The area of the circle is 1256
The default area is 3.14

Explanation

  • Line 5-19: Define a class Circle.
  • Line 7: Define a public constant PI with double type.
  • Line 8: Declare a double type variable radius.
  • Line 10-13: Get the area of the circle method. It defines a returned double type.
  • Line 12: Based on the formula to calculate the area of the circle and return the result.
  • Line 15-18: Define another method getCircle. The returned type is an object of the class Circle.
  • Line 17: Return statement. It returns an object by using new keyword to create the instance.
  • Line 21-32: Define a TestCircle class.
  • Line 25: Create a Circle instance and assign it to the variable c.
  • Line 26: Set the field value of the object.
  • Line 28: Output the area of the circle by calling the method getArea() of the class.
  • Line 29: Output the default area. c.getCircle() returns a new instance and the default value of the field radius is 1.0 which is defined in line 10. Then run getArea method of the instance and output the result.
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