学ぶ Error Handling

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You might have come across the term runtime error in previous chapters. A runtime error is an error which occurs while the program is running - hence the name "runtime" error.

Usually when a runtime error occurs, the program crashes or stops responding.

The compiler can help you by pointing out most of the errors in our code but runtime errors are mostly unpredictable and often depend on uncertain parameters.

For example, in case the file path passed into StreamReader is invalid or does not exist, it will give a runtime error and the program will crash. Therefore, we can often put such dangerous code into try-catch blocks to try some code, and in case it fails, we catch and deal with the error instead of causing the program to crash.

Following is the syntax of the try-catch block:

index.cs


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try {
    // code to try
} 
catch (Exception errorVar) 
{
    // code to handle error
}

Here Exception is a keyword which represents the datatype Exception.

Example

index.cs


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using System;
using System.IO;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            new StreamWriter("C:/a/random/path/that/does/not/exist.txt");
        }
        catch(Exception error)
        {
            Console.WriteLine(error.Message);
        }
    }
}

Note

We can omit the (Exception error) part from the catch statement if we're not using error.

Following are some common cases where a runtime error can occur:

Division By Zero

index.cs


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using System;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            int a = 100;
            int b = 0;
            int result = a / b;
        }
        catch
        {
            Console.WriteLine("ERROR: Division by Zero.");
        }
    }
}

Invalid Index of an Array or a List

index.cs


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using System;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            var exampleArray = new int[10];
            Console.WriteLine(exampleArray[20]);
        }
        catch 
        {
            Console.WriteLine("ERROR: The array index is out of bounds.");
        }
    }
}

Key Not Found (for Dictionaries):

index.cs


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using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            Dictionary<string, string> myDict = new Dictionary<string, string>
            {
                { "key1", "value1" }
            };
            Console.WriteLine(myDict["key2"]);
        }
        catch
        {
            Console.WriteLine("Error: Key not found");
        }
    }
}

The finally Block

There's also an option code block called finally which is executed after the catch block is executed. The finally block will always be executed after the try and catch blocks, no matter if an exception was thrown or not. This guarantees that any code inside finally runs every time, making it useful for tasks like closing files or releasing resources.

index.cs


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using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        try
        {
            Dictionary<string, string> myDict = new Dictionary<string, string>
            {
                { "key1", "value1" }
            };
            Console.WriteLine(myDict["key2"]);
        }
        catch
        {
            Console.WriteLine("Error: Key not found");
        }
        finally
        {
            Console.WriteLine("This line will show after the error");
        }
    }
}