DivideByZeroException Exception finally int.Parse try

Catch, when

In catch we handle exceptions. An alternative flow, the try-catch pattern traps errors, separating them in a clear way. Programs written in C# become easier to read.

Catch features

The catch-block allows an optional variable. Multiple catch-blocks can be stacked to provide more control. And "when" can filter exceptions.

An example

This program shows 3 patterns of using try-catch blocks. Please notice the syntaxes used in the catch-blocks. We catch exceptions in different ways.

Here The program throws an exception 3 times. DivideByZero causes exceptions. Int.Parse prevents the compiler from spotting the error.

Then Three lines are written to screen. These numbers indicate what catch-blocks were executed in the control flow.

Detail After the catch-keyword, we use parentheses to declare an exception variable. This variable can optionally be named.

Note In the third example, we have more than one catch-block in a row. The most general catch comes last.

using System;

class Program
{
    static void Main()
    {
        // You can use an empty catch block.
        try
        {
            DivideByZero();
        }
        catch
        {
            Console.WriteLine("0");
        }
        // You can specify a variable in the catch.
        try
        {
            DivideByZero();
        }
        catch (Exception ex)
        {
            Console.WriteLine("1");
        }
        // You can use multiple catch blocks.
        try
        {
            DivideByZero();
        }
        catch (DivideByZeroException)
        {
            Console.WriteLine("2");
        }
        catch
        {
            Console.WriteLine("3");
        }
    }

    static int DivideByZero()
    {
        int value1 = 1;
        int value2 = int.Parse("0");
        return value1 / value2;
    }
}0
1
2

Filter, when

With the when-keyword, we filter exceptions. A condition (like one found in an if-statement) is the argument to when. This reduces complex logic in exception catch-blocks.

Tip We can stack "when" catch-clauses. In this program we catch the ArgumentException in multiple ways.

using System;

class Program
{
    static void Main()
    {
        try
        {
            throw new ArgumentException("Invalid x argument");
        }
        catch (ArgumentException ex) when (ex.Message == "Invalid x argument")
        {
            // This is reached.
            Console.WriteLine("X");
        }
        catch (ArgumentException ex) when (ex.Message == "Invalid y argument")
        {
            // This is not reached.
            // ... Change the string to have "y" and it will be reached.
            Console.WriteLine("Y");
        }
    }
}
X

Summary

In complex programs, exception handling is helpful. Catch is a key part of exception handling. By going outside of the control flow, we more easily trap unexpected conditions.