# BaseExceptionFilterContext

**Kind:** Class

**Source:** [`packages/core/exceptions/base-exception-filter-context.ts`](https://github.com/nestjs/nest/blob/master/packages/core/exceptions/base-exception-filter-context.ts#L11)

**Part of:** [Core](subsystem-packages-core)

`BaseExceptionFilterContext` resolves exception filter metadata into executable `ExceptionFilter` instances. It supports both direct filter objects and dependency-injected filter classes, using module context and reflection metadata to locate registered providers.

**Extends:** `ContextCreator`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `createConcreteContext` | `createConcreteContext(metadata: T, contextId: undefined, inquirerId: string)` | `R` |
| `getFilterInstance` | `getFilterInstance(filter: Function | ExceptionFilter, contextId: undefined, inquirerId: string)` | `ExceptionFilter | null` |
| `getInstanceByMetatype` | `getInstanceByMetatype(metatype: Type<unknown>)` | `InstanceWrapper | undefined` |
| `reflectCatchExceptions` | `reflectCatchExceptions(instance: ExceptionFilter)` | `Type<any>[]` |

## Properties

| Property | Type |
|---|---|
| `moduleContext` | `string` |

## Where it refuses work

- `BaseExceptionFilterContext` stops the work with an early return when `isEmpty(metadata)`.
- `BaseExceptionFilterContext` stops the work with an early return when `isObject`.
- `BaseExceptionFilterContext` stops the work with an early return when `!instanceWrapper`.
- `BaseExceptionFilterContext` stops the work with an early return when `!this.moduleContext`.
- `BaseExceptionFilterContext` stops the work with an early return when `!moduleRef`.

## Diagram

```mermaid
graph LR
  A[Exception filter metadata] --> B[BaseExceptionFilterContext]
  B --> C{Filter is an object?}
  C -->|Yes| D[Use filter instance]
  C -->|No| E[Find InstanceWrapper in module injectables]
  E --> F[Resolve instance for context ID]
  D --> G[Validate catch() method]
  F --> G
  G --> H[Concrete ExceptionFilter instances]
```

## Usage

```ts
import {
  ArgumentsHost,
  Catch,
  ExceptionFilter,
} from '@nestjs/common';
import { BaseExceptionFilterContext } from '@nestjs/core/exceptions';

// A filter object can be resolved without looking up a DI provider.
@Catch(Error)
class LogErrorFilter implements ExceptionFilter {
  catch(exception: Error, host: ArgumentsHost) {
    console.error('Request failed:', exception.message);
  }
}

// In framework integration code, `container` is Nest's NestContainer instance.
const filterContext = new BaseExceptionFilterContext(container);

const filters = filterContext.createConcreteContext<
  ExceptionFilter[],
  ExceptionFilter[]
>([new LogErrorFilter()]);

// `filters` contains only valid objects that implement catch().
filters.forEach(filter => filter.catch(new Error('Example'), host));
```

## AI Coding Instructions

- Resolve filter classes through `getFilterInstance()` so request-scoped and dependency-injected providers use the correct context ID.
- Ensure every resolved filter implements a callable `catch()` method; invalid or unresolved filters are excluded from the concrete context.
- Set the active module context before resolving class-based filters, since `getInstanceByMetatype()` searches that module’s injectable providers.
- Use `reflectCatchExceptions()` to read `@Catch()` metadata rather than manually inspecting decorator metadata keys.

## Relationships

- IMPORTS → `FILTER_CATCH_EXCEPTIONS`
- IMPORTS → `Type`
- IMPORTS → `ExceptionFilter`
- IMPORTS → `isEmpty`
- IMPORTS → `isFunction`

## Used by

2 references from 2 files. Each is a place in this repository where the symbol is actually used — go read one rather than trusting an example.

### Imported by (2)

- `ExceptionFiltersContext` — `packages/microservices/context/exception-filters-context.ts`:16
- `ExceptionFiltersContext` — `packages/websockets/context/exception-filters-context.ts`:10