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_docs/02_document/modules/common_business_exception.md

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+# Module: Azaion.Common.BusinessException
+
+## Purpose
+Custom exception type for domain-level errors, paired with an `ExceptionEnum` catalog of all business error codes.
+
+## Public Interface
+
+### BusinessException
+| Member | Signature | Description |
+|--------|-----------|-------------|
+| Constructor | `BusinessException(ExceptionEnum exEnum)` | Creates exception with message from `ExceptionEnum`'s `[Description]` attribute |
+| `ExceptionEnum` | `ExceptionEnum ExceptionEnum { get; set; }` | The specific error code |
+| `GetMessage` | `static string GetMessage(ExceptionEnum exEnum)` | Looks up human-readable message for an error code |
+
+### ExceptionEnum
+| Value | Code | Description |
+|-------|------|-------------|
+| `NoEmailFound` | 10 | No such email found |
+| `EmailExists` | 20 | Email already exists |
+| `WrongPassword` | 30 | Passwords do not match |
+| `PasswordLengthIncorrect` | 32 | Password should be at least 8 characters |
+| `EmailLengthIncorrect` | 35 | Email is empty or invalid |
+| `WrongEmail` | 37 | (no description attribute) |
+| `HardwareIdMismatch` | 40 | Hardware mismatch — unauthorized hardware |
+| `BadHardware` | 45 | Hardware should be not empty |
+| `WrongResourceName` | 50 | Wrong resource file name |
+| `NoFileProvided` | 60 | No file provided |
+
+## Internal Logic
+Static constructor eagerly loads all `ExceptionEnum` descriptions into a dictionary via `EnumExtensions.GetDescriptions<ExceptionEnum>()`. Messages are retrieved by dictionary lookup with fallback to `ToString()`.
+
+## Dependencies
+- `EnumExtensions` — for `GetDescriptions<T>()`
+
+## Consumers
+- `BusinessExceptionHandler` — catches and serializes to HTTP 409 response
+- `UserService` — throws for email/password/hardware validation failures
+- `ResourcesService` — throws for missing file uploads
+- FluentValidation validators — reference `ExceptionEnum` codes in `.WithErrorCode()`
+
+## Data Models
+None.
+
+## Configuration
+None.
+
+## External Integrations
+None.
+
+## Security
+Error codes are returned to the client via `BusinessExceptionHandler`. Codes are numeric and messages are user-facing.
+
+## Tests
+None.