DSoftStudio Mediator

FluentValidation Integration

The companion package DSoftStudio.Mediator.FluentValidation provides automatic request validation via FluentValidation — a single pipeline behavior that resolves all IValidator<TRequest> instances from DI and short-circuits on failure.

dotnet add package DSoftStudio.Mediator.FluentValidation

Registration

services
    .AddMediator()
    .RegisterMediatorHandlers()
    .AddMediatorFluentValidation()    // ← registers ValidationBehavior<,>
    .PrecompilePipelines()
    .PrecompileNotifications()
    .PrecompileStreams();

// Register your validators
services.AddTransient<IValidator<CreateUser>, CreateUserValidator>();

Note: Register AddMediatorFluentValidation() before PrecompilePipelines() so the validation behavior is included in the precompiled pipeline chain.

Define a Validator

Use standard FluentValidation rules — validators are resolved from DI and can have injected dependencies:

public record CreateUser(string Name, string Email) : ICommand<Guid>;

public class CreateUserValidator : AbstractValidator<CreateUser>
{
    public CreateUserValidator()
    {
        RuleFor(x => x.Name).NotEmpty().WithMessage("Name is required.");
        RuleFor(x => x.Email).NotEmpty().EmailAddress();
    }
}

What Happens on Failure

When validation fails, a MediatorValidationException is thrown before the handler executes. The exception contains:

Failures — IReadOnlyList<ValidationFailure> with all errors from all validators
ErrorsByProperty — IReadOnlyDictionary<string, string[]> grouped by property name, ready for ValidationProblemDetails

try
{
    await mediator.Send(new CreateUser("", "bad"));
}
catch (MediatorValidationException ex)
{
    // ex.Failures → [{ PropertyName: "Name", ... }, { PropertyName: "Email", ... }]
    // ex.ErrorsByProperty → { "Name": ["Name is required."], "Email": ["..."] }
}

Multiple Validators per Request

Multiple validators for the same request type are supported — all are executed and their failures are aggregated:

services.AddTransient<IValidator<TransferMoney>, TransferMoneyAccountValidator>();
services.AddTransient<IValidator<TransferMoney>, TransferMoneyAmountValidator>();

Mapping to ProblemDetails

In ASP.NET Core, map MediatorValidationException to a 400 response:

app.UseExceptionHandler(error => error.Run(async context =>
{
    var ex = context.Features.Get<IExceptionHandlerFeature>()?.Error;

    if (ex is MediatorValidationException validationEx)
    {
        context.Response.StatusCode = 400;
        await context.Response.WriteAsJsonAsync(new ValidationProblemDetails(
            validationEx.ErrorsByProperty.ToDictionary(
                kvp => kvp.Key,
                kvp => kvp.Value)));
    }
}));

Behavior Summary

Scenario	Result
No validators registered for request	Pass-through — handler executes normally
All validators pass	Handler executes normally
One or more validators fail	`MediatorValidationException` thrown, handler not invoked
Validator has DI dependencies	Fully supported — validators are resolved from the DI container