Overview

Custom exceptions in C# allow developers to create specific error types tailored to their application's requirements, enhancing error handling and readability. By defining exceptions that clearly indicate the nature of an error, developers can more effectively manage and respond to exceptional conditions in their code.

Key Concepts

1. Creating Custom Exceptions: Understanding how to define and implement custom exceptions by extending the System.Exception class.
2. Using Custom Exceptions: Knowing when and how to throw custom exceptions to signal specific error conditions.
3. Best Practices: Strategies for designing meaningful exception classes and leveraging them to improve error handling within an application.

Common Interview Questions

Basic Level

1. What is a custom exception, and why would you use one?
2. How do you create a custom exception in C#?

Intermediate Level

3. What are some best practices for designing and using custom exceptions?

Advanced Level

4. Can you discuss a scenario where custom exceptions provided a significant advantage in your code? How did you implement it?

Detailed Answers

1. What is a custom exception, and why would you use one?

Answer: A custom exception is a user-defined exception class that extends the System.Exception class. Custom exceptions are used to represent specific error conditions that are not adequately covered by the predefined exceptions in the .NET Framework. They make it easier for developers to understand and handle errors by providing more contextual information about the error itself.

Key Points:
- Custom exceptions offer a way to convey specific error messages and error codes.
- They enhance the readability and maintainability of error handling code.
- Custom exceptions can carry additional information pertinent to the error condition.

Example:

public class UserNotFoundException : Exception
{
    public UserNotFoundException() { }

    public UserNotFoundException(string message) : base(message) { }

    public UserNotFoundException(string message, Exception inner) : base(message, inner) { }
}

2. How do you create a custom exception in C#?

Answer: To create a custom exception in C#, you define a class that inherits from the System.Exception class. This custom exception class can then be extended with additional properties and constructors as needed to convey specific error details.

Key Points:
- Custom exceptions should inherit from Exception or a more specific exception class.
- It's good practice to provide constructors that mirror those in the base Exception class.
- Custom exceptions can include additional properties and methods to capture error-specific information.

Example:

public class ValidationErrorException : Exception
{
    public int ErrorCode { get; }

    public ValidationErrorException() { }

    public ValidationErrorException(string message) : base(message) { }

    public ValidationErrorException(string message, Exception inner) : base(message, inner) { }

    public ValidationErrorException(string message, int errorCode) : base(message)
    {
        ErrorCode = errorCode;
    }
}

3. What are some best practices for designing and using custom exceptions?

Answer: Best practices for custom exceptions include providing meaningful error messages, including additional context relevant to the error, and ensuring that custom exceptions are used consistently throughout the application. It's also important to inherit from the appropriate level in the exception hierarchy and to not overuse custom exceptions for conditions that could be handled by existing exceptions.

Key Points:
- Custom exceptions should extend from a specific exception type that closely relates to the error condition they represent.
- They should not be used for control flow in an application.
- Custom exceptions should include additional information (e.g., error codes, failed operations) to help diagnose issues.

Example:

public class InsufficientFundsException : ApplicationException
{
    public decimal AccountBalance { get; }
    public decimal WithdrawalAmount { get; }

    public InsufficientFundsException(string message, decimal accountBalance, decimal withdrawalAmount) : base(message)
    {
        AccountBalance = accountBalance;
        WithdrawalAmount = withdrawalAmount;
    }
}

4. Can you discuss a scenario where custom exceptions provided a significant advantage in your code? How did you implement it?

Answer: In a financial application, custom exceptions were instrumental in handling various error conditions related to transactions, such as insufficient funds, invalid transactions, and exceeding withdrawal limits. Implementing specific exceptions for these conditions allowed for clear and concise error handling and reporting, making it easier to diagnose issues and communicate them to users.

Key Points:
- Custom exceptions enabled precise error handling tailored to the financial domain.
- They facilitated better user communication by allowing specific error messages and remediation steps to be associated with each exception type.
- Custom exceptions also simplified logging and debugging by providing a clear distinction between different types of transaction errors.

Example:

public class TransactionException : Exception
{
    public TransactionException() { }

    public TransactionException(string message) : base(message) { }

    public TransactionException(string message, Exception inner) : base(message, inner) { }
}

public class InsufficientFundsException : TransactionException
{
    public decimal MissingAmount { get; }

    public InsufficientFundsException(decimal missingAmount)
        : base($"Insufficient funds. Missing amount: {missingAmount}")
    {
        MissingAmount = missingAmount;
    }
}

Utilizing these custom exceptions made it straightforward to catch and handle specific transaction-related errors, improving the robustness and user experience of the financial application.