dotnet-result-pattern

# Result Pattern Implementation

## Overview

The Result pattern provides explicit error handling without exceptions:

- **No exceptions for business errors** - Exceptions for truly exceptional cases only
- **Explicit success/failure** - Compiler forces handling of both cases
- **Composable errors** - Chain operations, fail fast
- **Self-documenting** - Method signatures show possible outcomes

## Quick Reference

| Type | Purpose | Usage |
|------|---------|-------|
| `Result` | Operation without return value | Update, Delete operations |
| `Result<T>` | Operation with return value | Create, Get operations |
| `Error` | Error information | Code + Description |

---

## Implementation Structure

```
/Domain/Abstractions/
├── Result.cs           # Result and Result<T>
├── Error.cs            # Error record
└── ValidationResult.cs # Multiple errors support
```

---

## Template: Core Result Types

```csharp
// src/{name}.domain/Abstractions/Error.cs
namespace {name}.domain.abstractions;

/// <summary>
/// Represents an error with a code and description
/// </summary>
public record Error(string Code, string Description)
{
    /// <summary>
    /// Represents no error (success state)
    /// </summary>
    public static readonly Error None = new(string.Empty, string.Empty);

    /// <summary>
    /// Represents a null value error
    /// </summary>
    public static readonly Error NullValue = new(
        "Error.NullValue",
        "A null value was provided");

    /// <summary>
    /// Creates an error from an exception
    /// </summary>
    public static Error FromException(Exception exception) => new(
        "Error.Exception",
        exception.Message);

    /// <summary>
    /// Implicit conversion to string (returns Code)
    /// </summary>
    public static implicit operator string(Error error) => error.Code;

    public override string ToString() => Code;
}
```

```csharp
// src/{name}.domain/Abstractions/Result.cs
namespace {name}.domain.abstractions;

/// <summary>
/// Represents the outcome of an operation that doesn't return a value
/// </summary>
public class Result
{
    protected Result(bool isSuccess, Error error)
    {
        if (isSuccess && error != Error.None)
        {
            throw new InvalidOperationException(
                "Cannot create successful result with an error");
        }

        if (!isSuccess && error == Error.None)
        {
            throw new InvalidOperationException(
                "Cannot create failed result without an error");
        }

        IsSuccess = isSuccess;
        Error = error;
    }

    public bool IsSuccess { get; }

    public bool IsFailure => !IsSuccess;

    public Error Error { get; }

    // ═══════════════════════════════════════════════════════════════
    // FACTORY METHODS
    // ═══════════════════════════════════════════════════════════════

    /// <summary>
    /// Creates a successful result
    /// </summary>
    public static Result Success() => new(true, Error.None);

    /// <summary>
    /// Creates a failed result with the specified error
    /// </summary>
    public static Result Failure(Error error) => new(false, error);

    /// <summary>
    /// Creates a successful result with a value
    /// </summary>
    public static Result<TValue> Success<TValue>(TValue value) =>
        new(value, true, Error.None);

    /// <summary>
    /// Creates a failed result with the specified error
    /// </summary>
    public static Result<TValue> Failure<TValue>(Error error) =>
        new(default, false, error);

    /// <summary>
    /// Creates a result based on a condition
    /// </summary>
    public static Result Create(bool condition, Error error) =>
        condition ? Success() : Failure(error);

    /// <summary>
    /// Creates a result based on a condition with a value
    /// </summary>
    public static Result<TValue> Create<TValue>(TValue? value, Error error) =>
        value is not null ? Success(value) : Failure<TValue>(error);
}

/// <summary>
/// Represents the outcome of an operation that returns a value
/// </summary>
public class Result<TValue> : Result
{
    private readonly TValue? _value;

    protected internal Result(TValue? value, bool isSuccess, Error error)
        : base(isSuccess, error)
    {
        _value = value;
    }

    /// <summary>
    /// Gets the value if successful, throws if failed
    /// </summary>
    public TValue Value => IsSuccess
        ? _value!
        : throw new InvalidOperationException(
            $"Cannot access value of a failed result. Error: {Error.Code}");

    /// <summary>
    /// Implicit conversion from value to successful Result
    /// </summary>
    public static implicit operator Result<TValue>(TValue? value) =>
        value is not null ? Success(value) : Failure<TValue>(Error.NullValue);

    /// <summary>
    /// Implicit conversion from Error to failed Result
    /// </summary>
    public static implicit operator Result<TValue>(Error error) =>
        Failure<TValue>(error);
}
```

---

## Template: Result Extensions (Functional Operations)

```csharp
// src/{name}.domain/Abstractions/ResultExtensions.cs
namespace {name}.domain.abstractions;

public static class ResultExtensions
{
    // ═══════════════════════════════════════════════════════════════
    // MAP: Transform success value
    // ═══════════════════════════════════════════════════════════════

    /// <summary>
    /// Transforms the value if successful, preserves error if failed
    /// </summary>
    public static Result<TOut> Map<TIn, TOut>(
        this Result<TIn> result,
        Func<TIn, TOut> mapper)
    {
        return result.IsSuccess
            ? Result.Success(mapper(result.Value))
            : Result.Failure<TOut>(result.Error);
    }

    /// <summary>
    /// Async version of Map
    /// </summary>
    public static async Task<Result<TOut>> Map<TIn, TOut>(
        this Task<Result<TIn>> resultTask,
        Func<TIn, TOut> mapper)
    {
        var