C# Professional - Basics & OOP - Exercises

talent-agile

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01 Conditional Logic

02 Exception Handling

03 Exception Handling - Exercises

04 Equality - Exercises

05 ValueTypes - Exercises

06 Casting - Exercises

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Exception Handling - Mini-Project

This mini project will help you understand how to handle exceptions from real code.

Logging exceptions

The basic need is the simple one.

Change the code to make sure that all exceptions are logged, then rethrown to the calling code.

Catching a generic exception

// {
using System;
using System.Linq;
namespace Exceptions
{
    public class UserReport
    {
        private readonly IUserRepository _userRepository;
        private readonly IFileSystem _fileSystem;
        public UserReport(ILog logger, IUserRepository userRepository, IFileSystem fileSystem)
        {
            _logger = logger;
            _userRepository = userRepository;
            _fileSystem = fileSystem;
        }
// }
        private readonly ILog _logger;
        public bool SaveListOfUsers(string filePath)
        {
            var allUsers = _userRepository.GetAllUsers();
            var exportData = string.Join(Environment.NewLine, allUsers.Select(user => $"{user.FirstName} {user.LastName}"));
            if (_fileSystem.FileExists(filePath))
            {
                _fileSystem.Delete(filePath);
            }
            _fileSystem.WriteAllLines(filePath, exportData);
            return true;
        }
// {
    }
}
// }

Handling file I/O exceptions

As the code is writing to a file for the export, it may fail if the user running the code has insufficient rights.

We need to handle this case by adding two rules:

Catch exceptions of type UnauthorizedAccessException
Log these exceptions
Make sure the method will not throw an exception, but should return false to indicate that the export was not done

Handling exceptions

// {
using System;
using System.Linq;
namespace Exceptions
{
    public class UserReport
    {
        private readonly IUserRepository _userRepository;
        private readonly IFileSystem _fileSystem;
        public UserReport(ILog logger, IUserRepository userRepository, IFileSystem fileSystem)
        {
            _logger = logger;
            _userRepository = userRepository;
            _fileSystem = fileSystem;
        }
// }
        private readonly ILog _logger;
        public bool SaveListOfUsers(string filePath)
        {
            var allUsers = _userRepository.GetAllUsers();
            var exportData = string.Join(Environment.NewLine, allUsers.Select(user => $"{user.FirstName} {user.LastName}"));
            if (_fileSystem.FileExists(filePath))
            {
                _fileSystem.Delete(filePath);
            }
            _fileSystem.WriteAllLines(filePath, exportData);
            return true;
        }
// {
    }
}
// }

Wrap other exceptions

If a NullReferenceException is raised, mainly for technical issues, we will catch it and wrap it in an InvalidOperationException with a proper exception message indicating that the export was not done.

Implement these new rules:

Catch exceptions of type NullReferenceException
Log the exception
Make the method throw a new exception of type InvalidOperationException
- The new exception must have a message "Could not create the report, check logs for more details"
- The new exception must have a reference to the caught one

Wrapping in a new exception

// {
using System;
using System.Linq;
namespace Exceptions
{
    public class UserReport
    {
        private readonly IUserRepository _userRepository;
        private readonly IFileSystem _fileSystem;
        public UserReport(ILog logger, IUserRepository userRepository, IFileSystem fileSystem)
        {
            _logger = logger;
            _userRepository = userRepository;
            _fileSystem = fileSystem;
        }
// }
        private readonly ILog _logger;
        public bool SaveListOfUsers(string filePath)
        {
            var allUsers = _userRepository.GetAllUsers();
            var exportData = string.Join(Environment.NewLine, allUsers.Select(user => $"{user.FirstName} {user.LastName}"));
            if (_fileSystem.FileExists(filePath))
            {
                _fileSystem.Delete(filePath);
            }
            _fileSystem.WriteAllLines(filePath, exportData);
            return true;
        }
// {
    }
}
// }

All combined

Before pushing our code to production, make sure that all three cases are handled correctly at the same time.

Wrapping in a new exception

// {
using System;
using System.Linq;
namespace Exceptions
{
    public class UserReport
    {
        private readonly IUserRepository _userRepository;
        private readonly IFileSystem _fileSystem;
        public UserReport(ILog logger, IUserRepository userRepository, IFileSystem fileSystem)
        {
            _logger = logger;
            _userRepository = userRepository;
            _fileSystem = fileSystem;
        }
// }
        private readonly ILog _logger;
        public bool SaveListOfUsers(string filePath)
        {
            var allUsers = _userRepository.GetAllUsers();
            var exportData = string.Join(Environment.NewLine, allUsers.Select(user => $"{user.FirstName} {user.LastName}"));
            if (_fileSystem.FileExists(filePath))
            {
                _fileSystem.Delete(filePath);
            }
            _fileSystem.WriteAllLines(filePath, exportData);
            return true;
        }
// {
    }
}
// }