FluentValidation rule for null object

I can't really test that right now, but you can either try to override Validate, or include the rules in the When block:

public CustomerValidator()
{
     When(x => x != null, () => {
         RuleFor(x => x.Surname).NotEmpty();
         //etc.
     });
}

This is an older post, but want to update the answers to include the following from the FluentValidation documentation:

Using PreValidate

If you need to run specific code every time a validator is invoked, you can do this by overriding the PreValidate method. This method takes a ValidationContext as well as a ValidationResult, which you can use to customise the validation process.

public class MyValidator : AbstractValidator<Person> {
  public MyValidator() {
    RuleFor(x => x.Name).NotNull();
  }

  protected override bool PreValidate(ValidationContext<Person> context, ValidationResult result) {
    if (context.InstanceToValidate == null) {
      result.Errors.Add(new ValidationFailure("", "Please ensure a model was supplied."));
      return false;
    }
    return true;
  }
}

https://docs.fluentvalidation.net/en/latest/advanced.html?#prevalidate

For those using version >6.2.1 you need to override this signature instead, in order to achieve the same as @chrispr:

public override ValidationResult Validate(ValidationContext<T> context)
{
    return (context.InstanceToValidate == null) 
        ? new ValidationResult(new[] { new ValidationFailure("Property", "Error Message") })
        : base.Validate(context);       
}

/// EXAMPLE FOR NETCORE-3.1
/// fluentvalidator-9.5.0

public class Organisation
{ 
    public string Name { get; set; }
}

public class OrganisationValidator : AbstractValidator<Organisation>
{
    public OrganisationValidator()
    {
        RuleFor(x => x.Name).NotNull().MaximumLength(50);
    }

    protected override bool PreValidate(ValidationContext<Organisation> context, ValidationResult result)
    {
        if (context.InstanceToValidate == null) {
            result.Errors.Add(new ValidationFailure("", "org is null"));
            return false;
        }
        return base.PreValidate(context, result);
    }
}


[TestClass]
public class UnitTest1
{
    [TestMethod]
    public void ValidateWithNull()
    {
        var validator = new OrganisationValidator();
        Organisation organisation = null;
        var result = validator.Validate(organisation);
        // result.Errors[0].ErrorMessage == "org is null";
    }
}

As the above solutions didn't work for me (FluentValidation, Version=6.2.1.0 for Net45), I am posting what I did. This is just a simple replacement/wrapper for ValidateAndThrow extension method.

public static class ValidatorExtensions
{
    public static void ValidateAndThrowNotNull<T>(this IValidator<T> validator, T instance)
    {
        if (instance == null)
        {
            var validationResult = new ValidationResult(new[] { new ValidationFailure("", "Instance cannot be null") });
            throw new ValidationException(validationResult.Errors);
        }
        validator.ValidateAndThrow(instance);
    }
}

Override EnsureInstanceNotNull as below

protected override void EnsureInstanceNotNull(object instanceToValidate)
{
    if(instanceToValidate==null)
      throw new ValidationException("Customer can not be null");
}

By means of Custom(). It can be also very helpful when validation of another field is based on validation of your current field.

ruleBuilder.Custom((obj, context) =>
        {
            if (obj != null)
            {
                var propertyName = <field where should be validation>;
                context.AddFailure(propertyName, "'Your field name' Your validation message.");
            }
        });

Use the Cascade mode.

Here is the example from the documentation.

RuleFor(x => x.Surname).Cascade(CascadeMode.StopOnFirstFailure).NotNull().NotEqual("foo");

Also from the documentation:

If the NotNull validator fails then the NotEqual validator will not be executed. This is particularly useful if you have a complex chain where each validator depends on the previous validator to succeed.

I inherited from the fluent AbstractValidator and created a NullReferenceAbstractValidator class instead:

public class NullReferenceAbstractValidator<T> : AbstractValidator<T>
{
    public override ValidationResult Validate(T instance)
    {
        return instance == null
            ? new ValidationResult(new[] { new ValidationFailure(instance.ToString(), "response cannot be null","Error") })
            : base.Validate(instance);
    }
}

and then inherited from that class with each validator that needed a null reference check:

public class UserValidator : NullReferenceAbstractValidator<User>

You can override a virtual method called EnsureInstanceNotNull as the author recommends here

public class CustomerValidator: AbstractValidator<Customer> 
{
    public CustomerValidator() 
    {
        // Rule to check the customer instance is not null.
        RuleFor(c => c).NotNull();
        // Don't continue validating.

        RuleFor(c => c.Surname).NotEmpty();
    }

    protected override void EnsureInstanceNotNull(object instance) { }
}

The commonly accepted PreValidate answer will not work in this instance, as per here:

If you use SetValidator with an AbstractValidator derivative, then it will not be run if the property value is null. This is intentional behaviour as AbstractValidator derivatives are designed to validate the properties of a complex type, which cannot be done if the instance is null. AbstractValidator is not designed for use with simple/primative types or object. Now, if you want to check for null here, then you can precede the SetValidator call with a NotNull rule (although it doesn't seem like this is what you want in this case).

For me the only thing that worked is (my example):

RuleForEach(command => command.ProductDto.ProductInfos).NotNull().WithMessage("Custom Message").SetValidator(new ProductInfoValidator());

Without the "NotNull()", null values will be skipped.