C# throw

Summary: in this tutorial, you’ll learn how to use the C# throw keyword to raise an exception in your application.

Introduction to the C# throw keyword

The throw keyword allows you to raise an exception. Here’s the syntax of how to use the throw keyword:

throw exception;Code language: C# (cs)

In this syntax, the exception is an object that represents the exception that you want to raise. You can use any built-in exception class that is derived from the System.Exception class. Also, you can raise a custom exception, which will cover in the next tutorial.

Typically, you use the throw keyword with an if statement to throw an exception once a certain condition is met:

if (condition)
{
    throw exception;
}Code language: C# (cs)

Using the C# throw keyword to raise an exception example

The following program demonstrates how to use the throw keyword to throw an ArgumentOutOfRangeException exception:

using static System.Console;

public class Circle
{
    public double Radius
    {
        get; set;
    }
    public Circle(double radius)
    {
        if (radius <= 0)
        {
            throw new ArgumentOutOfRangeException(
                 nameof(radius),
                "The radius should be positive"
             );
        }
        Radius = radius;
    }
    public double GetArea() => Math.PI * Radius * Radius;

}

class Program
{
    public static void Main(string[] args)
    {
        WriteLine("Enter a radius:");
        var input = ReadLine();
        if (input != null)
        {
            var radius = double.Parse(input);
            try
            {
                var circle = new Circle(radius);
                WriteLine($"The area is {circle.GetArea():F2}");
            }
            catch (ArgumentOutOfRangeException ex)
            {
                WriteLine(ex.Message);
            }

        }
    }
}Code language: C# (cs)

How it works.

First, define a class called Circle that represents a circle with a given radius.

In the Circle class, we define a constructor that takes a double value representing the radius of the circle. If the value of the radius is less than or equal to zero, we throw an ArgumentOutOfRangeException with a custom error message using the throw keyword.

In the Circle class, we also define a method GetArea() that returns the area of the circle using the formula πr².

Second, prompt the user to enter a radius in the Main method of the program.

The Main() method:

• Reads the input from the console
• Parses it to a double
• Creates an instance of the Circle class with the entered radius value
• Calls the GetArea() method to calculate the area of the circle.

If the entered radius is not positive, the Main method catches the ArgumentOutOfRangeException exception raised by the constructor and displays the error message on the console.

The throw Expression

Starting from C# 7.0, you can use the throw keyword as an expression. This allows you to throw an exception inline, without having to define a separate variable. The syntax of the throw expression is as follows:

throw expression;Code language: JavaScript (javascript)

In this syntax, the expression evaluates to an object derived from the System.Exception class. For example, you can change the Circle constructor in the previous example to the following:

public Circle(double radius)
{
    Radius = radius > 0
                ? radius
                : throw new ArgumentOutOfRangeException(
                    nameof(radius),
                    "The radius should be positive"
                  );
}Code language: C# (cs)

In this example, if the radius is greater than zero, we assign it to the Radius property. Otherwise, we throw the ArgumentOutOfRangeException exception. This has the same effect as the previous example but is more concise.

Summary

• Use the C# throw keyword to raise an exception explicitly.