Using FluentValidation's WithMessage method with a list of named parameters

Question

I am using FluentValidation and I want to format a message with some of the object\'s properties value. The problem is I have very little experience with expressions and delegat

Answer 1

While KhalidAbuhakmeh's answer is very good and deep, I just want to share a simple solution to this problem. If you afraid of positional arguments, why not to encapsulate error creation mechanism with concatenation operator + and to take advantage of WithMessage overload, that takes Func. This CustomerValudator

public class CustomerValidator : AbstractValidator
{
    public CustomerValidator()
    {
        RuleFor(customer => customer.Name).NotEmpty().WithMessage("{0}", CreateErrorMessage);
    }

    private string CreateErrorMessage(Customer c)
    {
        return "The name " + c.Name + " is not valid for Id " + c.Id;
    }
}

Prints correct original error message in next code snippet:

var customer = new Customer() {Id = 1, Name = ""};
var result = new CustomerValidator().Validate(customer);

Console.WriteLine(result.Errors.First().ErrorMessage);

Alternatively, use an inline lambda:

public class CustomerValidator : AbstractValidator
{
    public CustomerValidator()
    {
        RuleFor(customer => customer.Name)
            .NotEmpty()
            .WithMessage("{0}", c => "The name " + c.Name + " is not valid for Id " + c.Id);
    }
}