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How does the Generic Host's dependency validation (ValidateOnBuild and ValidateScopes) help catch DI configuration errors early?

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Answer

When ValidateOnBuild is enabled (on by default in Development environment via CreateApplicationBuilder/CreateDefaultBuilder), calling host.Build() eagerly validates that every registered service's dependencies can actually be resolved, throwing an immediate, clear exception at startup for missing registrations rather than a confusing failure later when a specific code path first tries to resolve that broken service. ValidateScopes (also on by default in Development) enables the captive dependency detection discussed earlier, catching Singleton-depends-on-Scoped configuration errors immediately.

var builder = Host.CreateApplicationBuilder(args);
// In Development, these validations run automatically at Build() time:
// - ValidateOnBuild: catches missing service registrations immediately
// - ValidateScopes: catches captive dependency (Singleton -> Scoped) issues

var host = builder.Build();  // throws here if misconfigured, not later at runtime

Real-world example A missing service registration that would have caused a runtime crash only when a specific rarely-hit code path executed is instead caught immediately at application startup during local development, thanks to ValidateOnBuild eagerly checking the entire dependency graph.

Common follow-ups: Why are these validations typically disabled in Production for performance reasons?;How do you explicitly enable them in Production if you want the extra safety despite the cost?

Dependency Injection;Diagnostics & Performance