ioptions-pattern-migration

Category: .NET Configuration
Complexity: Intermediate
Tags: #aspnetcore #ioptions #configuration #dependency-injection

When to Use

• Migrating from manual config.Bind() + TryAddSingleton() pattern to ASP.NET Core IOptions
• Adding configuration validation at startup
• Improving testability of configuration-dependent services

Prerequisites

• ASP.NET Core application with appsettings.json
• Settings classes (POCOs) with properties matching config sections
• Understanding of Dependency Injection

Steps

1. Update Program.cs Registration

Before:

var settings = new MySettings();
builder.Configuration.Bind("MySection", settings);
builder.Services.TryAddSingleton<IMySettings>(settings);

After:

builder.Services.Configure<MySettings>(builder.Configuration.GetSection("MySection"));
builder.Services.AddOptions<MySettings>()
    .ValidateDataAnnotations()
    .ValidateOnStart();

2. Update Consumer Constructors

Before:

public class MyService
{
    private readonly IMySettings _settings;
    
    public MyService(IMySettings settings)
    {
        _settings = settings;
    }
}

After:

using Microsoft.Extensions.Options;

public class MyService
{
    private readonly MySettings _settings;
    
    public MyService(IOptions<MySettings> settingsOptions)
    {
        _settings = settingsOptions.Value;
    }
}

3. Update Tests

Before:

var mockSettings = new Mock<IMySettings>();
mockSettings.Setup(s => s.Property).Returns("value");
var sut = new MyService(mockSettings.Object);

After:

using Microsoft.Extensions.Options;

var settings = new MySettings { Property = "value" };
var settingsOptions = Options.Create(settings);
var sut = new MyService(settingsOptions);

4. Add Validation (Optional but Recommended)

using System.ComponentModel.DataAnnotations;

public class MySettings
{
    [Required]
    [Url]
    public required string ApiBaseUrl { get; set; }
    
    [Range(1, 3600)]
    public int TimeoutSeconds { get; set; } = 30;
}

Common Pitfalls

❌ Don't: Use IOptions<T> in Razor Views

Razor views should inject IOptions<T> and unwrap in a code block:

@using Microsoft.Extensions.Options
@inject IOptions<Settings> SettingsOptions
@{
    var settings = SettingsOptions.Value;
}
<img src="@settings.StaticContentRootUrl/logo.png" />

❌ Don't: Call .Value in Constructor

If unwrapping in constructor, store the unwrapped value:

private readonly string _apiUrl;

public MyService(IOptions<Settings> options)
{
    _apiUrl = options.Value.ApiUrl;  // ✅ Unwrap once, store value
}

❌ Don't: Mix IOptions with Manual Binding

Pick one pattern per project for consistency.

When to Use IOptionsSnapshot

Use IOptionsSnapshot<T> instead of IOptions<T> if:

• Configuration can reload during runtime
• Service is scoped (not singleton)
• Need per-request configuration values

builder.Services.AddScoped<IMyService, MyService>();

public MyService(IOptionsSnapshot<MySettings> settings) { }

Validation Options

1. Data Annotations (Recommended for Simple Cases)

builder.Services.AddOptions<MySettings>()
    .ValidateDataAnnotations()
    .ValidateOnStart();

2. Custom Validation

builder.Services.AddOptions<MySettings>()
    .Validate(settings => !string.IsNullOrWhiteSpace(settings.ApiKey), 
              "ApiKey must be provided")
    .ValidateOnStart();

3. IValidateOptions<T> (Complex Validation)

public class MySettingsValidator : IValidateOptions<MySettings>
{
    public ValidateOptionsResult Validate(string? name, MySettings options)
    {
        if (string.IsNullOrWhiteSpace(options.ApiKey))
            return ValidateOptionsResult.Fail("ApiKey is required");
        
        return ValidateOptionsResult.Success;
    }
}

builder.Services.AddSingleton<IValidateOptions<MySettings>, MySettingsValidator>();

Benefits

✅ Early validation (fail-fast at startup)
✅ Better testability (Options.Create() for tests)
✅ Standard ASP.NET Core pattern
✅ Supports config reloading (with IOptionsSnapshot)
✅ Type-safe configuration access

Related

• Microsoft Docs: Options pattern in ASP.NET Core
• Skill: CancellationToken Propagation
• Skill: OperationResult Error Handling