Config Checker

ValidateConfig.cs

using System.Reflection;

namespace ConfigChecker;

public class Validator
{
    private NullabilityInfoContext _nullabilityContext = new NullabilityInfoContext();
    
    public void ValidateConfig<T>(T config)
     {
        var props = config!.GetType().GetProperties();
        var invalid = props.Where(p => IsInvalid(config, p));

        if (invalid.Any())
        {
            throw new ArgumentException($"The following required properties had null values: [{string.Join(',', invalid.Select(i => i.Name))}]");
        }
    }

    private bool IsInvalid(object config, PropertyInfo property)
    {
        var value = property.GetValue(config);
        if (value != null) {
            return false;
        }

        if (property.PropertyType.IsValueType)
        {
            return Nullable.GetUnderlyingType(property.PropertyType) == null;
        }

        var nullabilityInfo = _nullabilityContext.Create(property);
        if (nullabilityInfo.WriteState is not NullabilityState.Nullable)
        {
            return true;
        }
        return false;
    }
}

public class SampleConfig {
    public string AwsKey { get; set; } = default!;
    public string? NServiceBusConfig { get; set; }
}

public class Tester {
    public static void Main(string[] args) {
        var validator = new Validator();

        // Should work, all props set
        validator.ValidateConfig(new SampleConfig
        {
            AwsKey = "asdf",
            NServiceBusConfig = "asdfasdf"
        });
        // Missing NServiseBusConfig, but that's optional
        validator.ValidateConfig(new SampleConfig
        {
            AwsKey = "asdf"
        });
        // Missing required prop, throws "System.ArgumentException: The following required properties had null values: [AwsKey]"
        validator.ValidateConfig(new SampleConfig());
    }
}

Throws an ArgumentException enumerating all non-nullable fields that are set to null after config has been hydrated.

Could be extended to use the built-in data annotations as well (e.g. [MinLength(1)] to cause an empty string to be considered invalid, as well.