Swift Q & A


How do I create custom error types in Swift?

In Swift, creating custom error types is a fundamental aspect of robust error handling. Custom error types allow you to define specific error conditions that can occur in your code, making it easier to handle errors in a structured and meaningful way. Here’s a step-by-step guide on how to create custom error types in Swift:


  1. Define an Enum for Your Errors:

   To create custom error types, you typically use Swift’s `enum` type. Each case of the `enum` represents a specific error condition. For example:

enum NetworkError: Error {
    case noInternetConnection
    case serverError
    case requestTimeout

   In this example, we’ve defined a custom error type called `NetworkError` with three possible error cases related to network operations.


  1. Conform to the `Error` Protocol:

   To make your `enum` usable as an error type, it should conform to the built-in `Error` protocol. Swift’s `Error` protocol doesn’t require any specific methods or properties, so you only need to declare conformance:

enum NetworkError: Error {
    // Error cases

  1. Throwing Custom Errors:

   To indicate that a function can throw custom errors, you need to declare the `throws` keyword in the function signature and use the `throw` keyword to throw instances of your custom error type when an error condition is encountered. For example:

func fetchData() throws {
    if !isConnectedToInternet() {
        throw NetworkError.noInternetConnection
    // Fetch data

   Here, the `fetchData` function throws a `NetworkError.noInternetConnection` error if there’s no internet connection.


  1. Handling Custom Errors:

   To handle custom errors, you use a `do-try-catch` block. In the `catch` block, you can pattern match against your custom error cases to handle them appropriately:

do {
    try fetchData()
} catch NetworkError.noInternetConnection {
    print("No internet connection. Please check your network.")
} catch NetworkError.serverError {
    print("Server error. Please try again later.")
} catch NetworkError.requestTimeout {
    print("Request timed out. Please retry the request.")
} catch {
    print("An unknown error occurred.")

   In this example, we catch specific instances of `NetworkError` and provide user-friendly error messages.


Custom error types in Swift enhance the clarity and maintainability of your code by allowing you to handle errors in a structured manner. They enable you to define and communicate error conditions specific to your application domain, making it easier to write robust error-handling code.


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Experienced iOS Engineer with 7+ years mastering Swift. Created fintech solutions, enhanced biopharma apps, and transformed retail experiences.