dotnet-patterns

Files (7)

• SKILL.md
• cqrs-mediatr.md · references/
• dotnet-cqrs.md · references/
• dotnet-event-sourcing.md · references/
• dotnet-pipeline-behaviors.md · references/
• dotnet-result-pattern.md · references/
• ef-core-patterns.md · references/

SKILL.md (first lines)

# .NET Patterns

## Purpose
Implement production-grade .NET patterns — MediatR pipeline behaviors, CQRS with separate models, Result pattern for explicit error handling, EF Core performance patterns, and event sourcing.

## Agent Protocol

### Trigger
User request includes: `dotnet MediatR pattern`, `dotnet CQRS`, `dotnet Result pattern`, `dotnet EF Core`, `dotnet pipeline behavior`, `dotnet FluentValidation`, `dotnet AutoMapper`.

### Input Context
- Architecture (Clean Architecture, Vertical Slices)
- ORM (EF Core, Dapper)
- Patterns needed (CQRS, Event Sourcing, Outbox, Saga)
- Testing approach (xUnit, NUnit)

### Output Artifact
Code examples for requested patterns — pipeline behavior, CQRS handler, Result type, EF optimization.

### Response Format
Code-first output. Pattern name, problem statement, implementation. No preamble, no postamble.

### Completion Criteria
- MediatR pipeline behaviors ordered
- CQRS with separate Command/Query handlers
- Result type with implicit conversions
- EF Core query optimization applied
- Unit of Work pattern with SaveChangesAsync

### Max Response Length
4096 tokens

## Architecture Decision Trees

### Pipeline Behavior Order

| Order | Behavior | Purpose |
|-------|----------|---------|
| 1 | LoggingBehavior | Log request start/end with timing |
| 2 | ValidationBehavior | Validate before processing |
| 3 | PerformanceBehavior | Warn if > threshold (500ms) |
| 4 | UnhandledExceptionBehavior | Catch and wrap unhandled |
| 5 | TransactionBehavior | Wrap in DB transaction |

Pipeline behaviors run in registration order (first added = outer wrapper, runs first).

### FluentValidation vs Data Annotations

| Criterion | FluentValidation | Data Annotations |
|-----------|-----------------|------------------|
| Separation | External to model | On model properties |
| Conditional rules | Yes (When/Unless) | Limited |
| Cross-property | Yes (Must) | No |
| DI support | Yes (inject services) | No |
| Testability | Easy (validator per model) | Requires model instantiation |

Decision: Complex validation rules → FluentValidation. Simple non-null/range checks → Data Annotations.

## Workflow

### Step 1: MediatR Pipeline Behaviors

```csharp
// Application/Common/Behaviors/PerformanceBehavior.cs
public class PerformanceBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly ILogger<PerformanceBehavior<TRequest, TResponse>> _logger;

    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken ct)
    {
        const int threshold = 500;
        var sw = Stopwatch.StartNew();
        var response = await next();
        sw.Stop();

        if (sw.ElapsedMilliseconds > threshold)
        {
            _logger.LogWarning("Long running request: {Name} ({Elapsed}ms)",
                typeof(TRequest).Name, sw.ElapsedMilliseconds);
        }

        return response;
    }
}

// Application/Common/Behaviors/TransactionBehavior.cs
public class TransactionBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
    where TRequest : IRequest<TResponse>
{
    private readonly AppDbContext _context;

    public async Task<TResponse> Handle(TRequest request, RequestHandlerDelegate<TResponse> next, CancellationToken ct)
    {
        if (request is IQuery<TResponse>) return await next();

        await using var transaction = await _context.Database.BeginTransactionAsync(ct);
        try
        {
            var response = await next();
            await transaction.CommitAsync(ct);
            return response;
        }
        catch
        {
            await transaction.RollbackAsync(ct);
            throw;
        }
    }
}
```

### Step 2: Result Pattern

```csharp
// Application/Common/Models/Result.cs
public class Result<T>
{
    private readonly T? _value;

From j4flmao/agent-skills, MIT.

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