Automatically generates the registration code for IOptions and can validate them on startup using DataAnnotations or fluent validation.

1. Install the NuGet package into your project.

Install-Package IeuanWalker.AppSettings

2. Inherit the IAppSettings interface onto the class that models your IOptions

public class ConfirmationEmailSettings : IAppSettings
{
	public required string Subject { get; set; }
}

3. Register the app settings

   Once IAppSettings has been added to a class in your project, several extension methods will automatically be created. The extension method name convention is AddAppSettingsFrom + your assembly name, and the namespace is your assembly name.

   3.1 An extension method for IServiceCollection is created for all project types by default

   builder.Services.AddAppSettingsFromApiProjectNestedClassLibrary(builder.Configuration);

   3.2 If your project is a backend/blazor project, then it will also have an extension method for IHostApplicationBuilder, allowing you to easily chain the registration in your progam.cs

   builder.AddAppSettingsFromApiProject();

   3.3 If it's a MAUI project, it will also have an extension method for MauiAppBuilder, allowing you to easily chain the registration in your MauiProgam.cs

   builder.AddAppSettingsFromMauiProject();

By default, it maps the IOptions model to the section name based on the name of the model. For example, the following model -

public class ConfirmationEmailSettings : IAppSettings
{
	public required string Subject { get; set; }
}

Would automatically map to the following app setting section -

{
  "ConfirmationEmailSettings": {
    "Subject": "Test subject"
  }
}

If your model name and configuration section don't match or you want to bind a nested configuration, you can override this within your model by using the SectionName attribute

[SectionName("ConfirmationEmail")]
public class ConfirmationEmailSettings : IAppSettings
{
    public required string Subject { get; set; }
}

[SectionName("NestedConfiguration:ConfirmationEmail")]
public class ConfirmationEmailSettings : IAppSettings
{
    public required string Subject { get; set; }
}

You can perform validation on startup using DataAnnotations or FluentValidation.

All you need to do is add a DataAnnotation attribute onto any property

public class ConfirmationEmailSettings : IAppSettings
{
	[MinLength(5)]
	public required string Subject { get; set; }
}

To use FluentValidation you need to create an AbstractValidator for your app settings model and add that validator to the IAppSettings inheritance.

public class ConfirmationEmailSettings : IAppSettings<ConfirmationEmailSettingsValidator>
{
	public required string Subject { get; set; }
}

sealed class ConfirmationEmailSettingsValidator : AbstractValidator<ConfirmationEmailSettings>
{
	public ConfirmationEmailSettingsValidator()
	{
		RuleFor(x => x.Subject)
			.NotEmpty()
			.MinimumLength(5);
	}
}

You can use FluentValidation without the source generator by not inheriting from IAppSettings and using the extension method

public class ConfirmationEmailSettings
{
	public required string Subject { get; set; }
}

sealed class ConfirmationEmailSettingsValidator : AbstractValidator<ConfirmationEmailSettings>
{
	public ConfirmationEmailSettingsValidator()
	{
		RuleFor(x => x.Subject)
			.NotEmpty()
			.MinimumLength(5);
	}
}

In your startup -

services.AddScoped<IValidator<ConfirmationEmailSettings>, ConfirmationEmailSettingsValidator>();
services.AddOptions<ConfirmationEmailSettings>()
    .Configure(options => configuration.GetSection("FluentValidationWithValidationButNoSectionNameSettings").Bind(options))
    .ValidateFluentValidation()
    .ValidateOnStart();

If something fails validation as the application starts up, you will get an exception explaining the exact issue -

The generated code is just standard C#/ .NET APIs -

Left is the AppSettings model, Right is the generated code

I do not recommend adding validation to a MAUI project as it can/ will slow startup. To prevent validation, add the DontValidate attribute above your class.

[DontValidate]
public class MobileAppSettings : IAppSettings
{
	public required string Subject { get; set; }
}