# Adding Validation

Add input validation to your commands and queries using FluentValidation.

## Why Validation?

Validation ensures:
- ✅ Data integrity - Only valid data enters your system
- ✅ Security - Prevent injection attacks and malformed input
- ✅ User experience - Clear, structured error messages
- ✅ Business rules - Enforce domain constraints

## Install FluentValidation

Add the required packages:

```bash
dotnet add package Svrnty.CQRS.FluentValidation
dotnet add package FluentValidation
```

## Step 1: Create a Validator

Let's add validation to the `CreateUserCommand` from the previous guide.

Create `Validators/CreateUserCommandValidator.cs`:

```csharp
using FluentValidation;
using MyApp.Commands;

namespace MyApp.Validators;

public class CreateUserCommandValidator : AbstractValidator<CreateUserCommand>
{
    public CreateUserCommandValidator()
    {
        RuleFor(x => x.Name)
            .NotEmpty()
            .WithMessage("Name is required")
            .MaximumLength(100)
            .WithMessage("Name must not exceed 100 characters");

        RuleFor(x => x.Email)
            .NotEmpty()
            .WithMessage("Email is required")
            .EmailAddress()
            .WithMessage("Email must be a valid email address");
    }
}
```

## Step 2: Register the Validator

### Option 1: Register Command with Validator

```csharp
// Program.cs
builder.Services.AddCommand<CreateUserCommand, int, CreateUserCommandHandler, CreateUserCommandValidator>();
```

This single line registers:
1. The command handler
2. The validator
3. The metadata for discovery

### Option 2: Register Separately

```csharp
// Register command
builder.Services.AddCommand<CreateUserCommand, int, CreateUserCommandHandler>();

// Register validator
builder.Services.AddTransient<IValidator<CreateUserCommand>, CreateUserCommandValidator>();
```

## Step 3: Test Validation

### Valid Request

```bash
curl -X POST http://localhost:5000/api/command/createUser \
  -H "Content-Type: application/json" \
  -d '{
    "name": "Alice Smith",
    "email": "alice@example.com"
  }'
```

**Response (200 OK):**

```json
123
```

### Invalid Request

```bash
curl -X POST http://localhost:5000/api/command/createUser \
  -H "Content-Type: application/json" \
  -d '{
    "name": "",
    "email": "invalid-email"
  }'
```

**HTTP Response (400 Bad Request):**

```json
{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "Name": [
      "Name is required"
    ],
    "Email": [
      "Email must be a valid email address"
    ]
  }
}
```

This follows **RFC 7807** (Problem Details for HTTP APIs).

## Validation Rules

FluentValidation provides many built-in validators:

### Required Fields

```csharp
RuleFor(x => x.Name)
    .NotEmpty()
    .WithMessage("Name is required");
```

### String Length

```csharp
RuleFor(x => x.Name)
    .MinimumLength(3)
    .WithMessage("Name must be at least 3 characters")
    .MaximumLength(100)
    .WithMessage("Name must not exceed 100 characters");
```

### Email Validation

```csharp
RuleFor(x => x.Email)
    .EmailAddress()
    .WithMessage("Email must be a valid email address");
```

### Numeric Range

```csharp
RuleFor(x => x.Age)
    .GreaterThan(0)
    .WithMessage("Age must be greater than 0")
    .LessThanOrEqualTo(120)
    .WithMessage("Age must be less than or equal to 120");
```

### Regular Expression

```csharp
RuleFor(x => x.PhoneNumber)
    .Matches(@"^\d{3}-\d{3}-\d{4}$")
    .WithMessage("Phone number must be in format: 123-456-7890");
```

### Must (Custom Rule)

```csharp
RuleFor(x => x.StartDate)
    .Must(BeAFutureDate)
    .WithMessage("Start date must be in the future");

private bool BeAFutureDate(DateTime date)
{
    return date > DateTime.UtcNow;
}
```

### Nested Object Validation

```csharp
public record CreateOrderCommand
{
    public AddressDto ShippingAddress { get; init; } = null!;
    public List<OrderItemDto> Items { get; init; } = new();
}

public class CreateOrderCommandValidator : AbstractValidator<CreateOrderCommand>
{
    public CreateOrderCommandValidator()
    {
        RuleFor(x => x.ShippingAddress)
            .NotNull()
            .SetValidator(new AddressValidator());

        RuleForEach(x => x.Items)
            .SetValidator(new OrderItemValidator());

        RuleFor(x => x.Items)
            .NotEmpty()
            .WithMessage("Order must contain at least one item");
    }
}

public class AddressValidator : AbstractValidator<AddressDto>
{
    public AddressValidator()
    {
        RuleFor(x => x.Street).NotEmpty();
        RuleFor(x => x.City).NotEmpty();
        RuleFor(x => x.ZipCode).Matches(@"^\d{5}$");
    }
}
```

## Complete Validation Example

Here's a comprehensive validator:

```csharp
using FluentValidation;

namespace MyApp.Validators;

public class CreateUserCommandValidator : AbstractValidator<CreateUserCommand>
{
    private readonly IUserRepository _userRepository;

    public CreateUserCommandValidator(IUserRepository userRepository)
    {
        _userRepository = userRepository;

        // Required fields
        RuleFor(x => x.Name)
            .NotEmpty().WithMessage("Name is required")
            .MaximumLength(100).WithMessage("Name must not exceed 100 characters");

        RuleFor(x => x.Email)
            .NotEmpty().WithMessage("Email is required")
            .EmailAddress().WithMessage("Email must be a valid email address")
            .MustAsync(BeUniqueEmail).WithMessage("Email already exists");

        RuleFor(x => x.Age)
            .GreaterThan(0).WithMessage("Age must be greater than 0")
            .LessThanOrEqualTo(120).WithMessage("Age must be realistic");

        RuleFor(x => x.PhoneNumber)
            .Matches(@"^\d{3}-\d{3}-\d{4}$")
            .When(x => !string.IsNullOrEmpty(x.PhoneNumber))
            .WithMessage("Phone number must be in format: 123-456-7890");
    }

    private async Task<bool> BeUniqueEmail(string email, CancellationToken cancellationToken)
    {
        var existingUser = await _userRepository.GetByEmailAsync(email, cancellationToken);
        return existingUser == null;
    }
}
```

## Async Validation

For validation that requires database access or external API calls:

```csharp
RuleFor(x => x.Email)
    .MustAsync(BeUniqueEmail)
    .WithMessage("Email already exists");

private async Task<bool> BeUniqueEmail(string email, CancellationToken cancellationToken)
{
    var exists = await _userRepository.EmailExistsAsync(email, cancellationToken);
    return !exists;
}
```

## Conditional Validation

Validate only when certain conditions are met:

```csharp
// Validate only when property is not null
RuleFor(x => x.ShippingAddress)
    .SetValidator(new AddressValidator())
    .When(x => x.ShippingAddress != null);

// Validate based on other property
RuleFor(x => x.CreditCardNumber)
    .NotEmpty()
    .When(x => x.PaymentMethod == "CreditCard")
    .WithMessage("Credit card number is required for credit card payments");
```

## Validating Queries

Queries can also be validated:

```csharp
// Query
public record SearchUsersQuery
{
    public string Keyword { get; init; } = string.Empty;
    public int Page { get; init; } = 1;
    public int PageSize { get; init; } = 10;
}

// Validator
public class SearchUsersQueryValidator : AbstractValidator<SearchUsersQuery>
{
    public SearchUsersQueryValidator()
    {
        RuleFor(x => x.Keyword)
            .MinimumLength(3)
            .When(x => !string.IsNullOrEmpty(x.Keyword))
            .WithMessage("Keyword must be at least 3 characters");

        RuleFor(x => x.Page)
            .GreaterThan(0)
            .WithMessage("Page must be greater than 0");

        RuleFor(x => x.PageSize)
            .InclusiveBetween(1, 100)
            .WithMessage("Page size must be between 1 and 100");
    }
}

// Registration
builder.Services.AddQuery<SearchUsersQuery, List<UserDto>, SearchUsersQueryHandler, SearchUsersQueryValidator>();
```

## HTTP vs gRPC Validation

### HTTP (Minimal API)

Validation errors return **RFC 7807 Problem Details**:

```json
{
  "type": "https://tools.ietf.org/html/rfc7231#section-6.5.1",
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "Email": ["Email is required", "Email must be a valid email address"],
    "Age": ["Age must be greater than 0"]
  }
}
```

**HTTP Status:** `400 Bad Request`

### gRPC

Validation errors return **Google Rich Error Model**:

```protobuf
status {
  code: 3  // INVALID_ARGUMENT
  message: "Validation failed"
  details: [
    google.rpc.BadRequest {
      field_violations: [
        { field: "Email", description: "Email is required" },
        { field: "Email", description: "Email must be a valid email address" },
        { field: "Age", description: "Age must be greater than 0" }
      ]
    }
  ]
}
```

**gRPC Status Code:** `INVALID_ARGUMENT`

Both formats are automatically generated by Svrnty.CQRS!

## Validation Best Practices

### ✅ DO

- **Validate early** - At the API boundary
- **Use descriptive messages** - Help users fix errors
- **Validate business rules** - Not just data types
- **Use async validation** - For database checks
- **Return all errors** - Don't stop at first error
- **Validate commands AND queries** - Both need validation

### ❌ DON'T

- **Don't validate in handlers** - Use validators
- **Don't use exceptions** - Let FluentValidation handle it
- **Don't skip validation** - Even for "internal" commands
- **Don't return generic messages** - Be specific
- **Don't over-validate** - Balance security and usability

## Custom Validators

Create reusable validators:

```csharp
public static class CustomValidators
{
    public static IRuleBuilderOptions<T, string> MustBeValidUrl<T>(
        this IRuleBuilder<T, string> ruleBuilder)
    {
        return ruleBuilder
            .Must(url => Uri.TryCreate(url, UriKind.Absolute, out _))
            .WithMessage("'{PropertyName}' must be a valid URL");
    }

    public static IRuleBuilderOptions<T, string> MustBeStrongPassword<T>(
        this IRuleBuilder<T, string> ruleBuilder)
    {
        return ruleBuilder
            .MinimumLength(8).WithMessage("Password must be at least 8 characters")
            .Matches(@"[A-Z]").WithMessage("Password must contain uppercase letter")
            .Matches(@"[a-z]").WithMessage("Password must contain lowercase letter")
            .Matches(@"\d").WithMessage("Password must contain digit")
            .Matches(@"[^\w]").WithMessage("Password must contain special character");
    }
}

// Usage
RuleFor(x => x.Website)
    .MustBeValidUrl();

RuleFor(x => x.Password)
    .MustBeStrongPassword();
```

## Troubleshooting

### Validation Not Running

**Problem:** Requests succeed even with invalid data

**Solutions:**
1. Ensure you installed `Svrnty.CQRS.FluentValidation`
2. Verify validator is registered in DI
3. Check validator class inherits `AbstractValidator<T>`

### Validation Always Fails

**Problem:** All requests return 400 even with valid data

**Solutions:**
1. Check validator rules are correct
2. Verify async validators return correct boolean
3. Ensure property names match exactly

### Multiple Validators Registered

**Problem:** Conflicting validation rules

**Solutions:**
1. Only register one validator per command/query
2. Combine rules in a single validator
3. Use `RuleSet` for conditional validation

## What's Next?

Now you know how to add validation! Let's discuss when to use HTTP vs gRPC.

**Continue to [Choosing HTTP vs gRPC](06-choosing-http-or-grpc.md) →**

## See Also

- [HTTP Validation](../core-features/validation/http-validation.md) - RFC 7807 Problem Details
- [gRPC Validation](../core-features/validation/grpc-validation.md) - Google Rich Error Model
- [Custom Validation](../core-features/validation/custom-validation.md) - Advanced validation scenarios
- [FluentValidation Documentation](https://docs.fluentvalidation.net/) - Official FluentValidation docs