TIL: Always check for missing configuration values inside constructors

This post is part of my Advent of Code 2022.

This is a lesson I learned after trying to use a shared NuGet package in one of my client’s projects and getting an ArgumentNullException. I had no clue that I needed some configuration values in my appsettings.json file. This is what I learned.

Always check for missing configuration values inside constructors. In case they’re not set, throw a human-friendly exception message showing the name of the expected configuration value. For example: ‘Missing Section:Subsection:Value in config file’.

A missing configuration value

This is what happened. I needed to import a feature from a shared Nuget package. It had a method to register its dependencies. Something like services.AddFeature().

When calling an API endpoint that used that feature, I got an ArgumentNullException: “Value cannot be null. (Parameter ‘uriString’).” It seemed that I was missing a URI. But what URI?

Without any XML docstrings on the AddFeature() method, I had no other solution than to decompile that DLL. I found a service like this one,

public class SomeService : ISomeService
{
    private readonly Uri _anyUri;

    public SomeService(IOptions<AnyConfigOptions> options, OtherParam otherParam)
    {
        _anyUri = new Uri(options.Value.AnyConfigValue);
        //                ^^^^^^^
        // System.ArgumentNullException: Value cannot be null. (Parameter 'uriString')
    }

    public async Task DoSomethingAsync()
    {
        // Beep, beep, boop...
        // Doing something here...
    }
}

There it was! The service used the IOptions pattern to read configuration values. And I needed an URL inside a section in the appsettings.json file. How was I supposed to know?

A better exception message

Then I realized that a validation inside the constructor with a human-friendly message would have saved me (and any other future developer using that NuGet package) some time. And it would have pointed me in the right direction. I mean having something like,

public class SomeService : ISomeService
{
    private readonly Uri _anyUri;

    public SomeService(IOptions<AnyConfigOptions> options, OtherParam otherParam)
    {
        //  vvvvvvv
        if (string.IsNullOrEmpty(options?.Value?.AnyConfigValue))
        {
            throw new ArgumentNullException("Missing 'AnyConfigOptions:AnyConfigValue' in config file.");
            //                              ^^^^^^^^
            // I think this would be a better message
        }

        _anyUri = new Uri(options.Value.AnyConfigValue);
    }

    public async Task DoSomethingAsync()
    {
        // Beep, beep, boop...
        // Doing something here again...
    }
}

Even better, what if the AddFeature() method had an overload that receives the expected configuration value? Something like AddFeature(AnyConfigOptions options). This way, the client of that package could decide the source of those options. Either read them from a configuration file or hardcode them.

The book “Growing Object-Oriented Software Guided by Tests” suggests having a StupidProgrammerMistakeException or a specific exception for this type of scenario: missing configuration values. This would be a good use case for that exception type.

Voilà! That’s what I learned today: always validate configuration values inside constructors and use explicit error messages when implementing the Options pattern. It reminded me of “The given key was not present in the dictionary” and other obscure error messages. Do you write friendly and clear error messages?

To read more content about ASP.NET Core, check how to add caching with Redis and how to read configuration values.

Happy coding!