C# Academy · Lesson

Custom exceptions; error design

Create small custom exceptions, pick correct built-in types (ArgumentException, InvalidOperationException), and add context with InnerException.

Purpose & goals

Aim: Make errors clear and catchable.

Use a specific type for a specific failure
Good message + InnerException for context
Prefer built-ins when they fit

Custom exception type

Derive from Exception, end the name with Exception, and add the standard constructors. Carry small, useful data.

using System;

// Custom, specific to our domain
public sealed class ConfigNotFoundException : Exception
{
  public string Key { get; private set; }

  public ConfigNotFoundException(string key)
    : base("Config key not found: " + key)
  {
    Key = key;
  }

  public ConfigNotFoundException(string key, Exception inner)
    : base("Config key not found: " + key, inner)
  {
    Key = key;
  }
}

public class Program
{
  public static void Main(string[] args)
  {
    try
    {
      throw new ConfigNotFoundException("ApiUrl");
    }
    catch (ConfigNotFoundException ex)
    {
      Console.WriteLine(ex.Message);
    }
  }
}

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