# Microsoft.Extensions.Options.OptionsValidationException: A configuration validation error occurred. DataAnnotation validation failed for 'MyOptions' members: 'ApiKey' is required.

- **ID:** `dotnet/aspnetcore-options-validation-failure`
- **Domain:** dotnet
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 90%

## Root Cause

ASP.NET Core's options validation using DataAnnotations (e.g., [Required]) is triggered at startup via ValidateOnStart(), but the required configuration key is missing or empty in appsettings.json or environment variables.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| .NET 6.0 | active | — | — |
| .NET 7.0 | active | — | — |
| .NET 8.0 | active | — | — |
| ASP.NET Core 6.0+ | active | — | — |

## Workarounds

1. **Add the missing configuration key to appsettings.json or environment variables with the correct path matching the options class hierarchy.** (95% success)
   ```
   Add the missing configuration key to appsettings.json or environment variables with the correct path matching the options class hierarchy.
   ```
2. **If the value is optional, remove the [Required] attribute from the property and handle null checks in code.** (80% success)
   ```
   If the value is optional, remove the [Required] attribute from the property and handle null checks in code.
   ```

## Dead Ends

- **Adding a default value in the options class constructor to avoid null** — Validation is based on DataAnnotations attributes, not null checks; a default value does not satisfy [Required] if the config key is missing. (60% fail)
- **Removing ValidateOnStart() to defer validation until first use** — The error moves to runtime, causing unexpected failures later; the root issue (missing config) remains. (40% fail)