Learn Error Handling | Data Structures & File Handling

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


              1234567
            
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


              123456789101112131415161718192021
            
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:

index.cs


              12345678910111213141516171819202122232425
            
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");
        }
    }
}

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Section 1. Chapter 9

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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.

Following is the syntax of the try-catch block:

index.cs


              1234567
            
try {
    // code to try
} 
catch (Exception errorVar) 
{
    // code to handle error
}

Here Exception is a keyword which represents the datatype Exception.

Example

index.cs


              1234567891011121314151617
            
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


              123456789101112131415161718
            
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


              1234567891011121314151617
            
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


              123456789101112131415161718192021
            
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:

index.cs


              12345678910111213141516171819202122232425
            
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");
        }
    }
}

Everything was clear?

Thanks for your feedback!

Section 1. Chapter 9

Error Handling

Example

Division By Zero

Invalid Index of an Array or a List

Key Not Found (for Dictionaries):

The finally Block

1. What will be the output of the following program?

2. In C#, what is the purpose of the `finally` block?

3. Which of the following statements is true regarding the `try-catch` block in C#?

Awesome!

Error Handling

Example

Division By Zero

Invalid Index of an Array or a List

Key Not Found (for Dictionaries):

The finally Block

1. What will be the output of the following program?

2. In C#, what is the purpose of the `finally` block?

3. Which of the following statements is true regarding the `try-catch` block in C#?

Error Handling

Example

Division By Zero

Invalid Index of an Array or a List

Key Not Found (for Dictionaries):

The finally Block

1. What will be the output of the following program?

2. In C#, what is the purpose of the finally block?

3. Which of the following statements is true regarding the try-catch block in C#?

Awesome!

Error Handling

Example

Division By Zero

Invalid Index of an Array or a List

Key Not Found (for Dictionaries):

The finally Block

1. What will be the output of the following program?

2. In C#, what is the purpose of the finally block?

3. Which of the following statements is true regarding the try-catch block in C#?

2. In C#, what is the purpose of the `finally` block?

3. Which of the following statements is true regarding the `try-catch` block in C#?

2. In C#, what is the purpose of the `finally` block?

3. Which of the following statements is true regarding the `try-catch` block in C#?