FluentValidation rule for null object

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

EDIT 2022-07-19
As some commenters have pointed out, please check out answer https://stackoverflow.com/a/52784357/1943 for a newer implementation. I haven't personally vetted, but it's worth a try to give that a go first.

If you're using an older version, or you enjoy nostalgia, my original answer below is from 2013.

---

You should be able to override the Validate method in your CustomerValidator class.

public class CustomerValidator: AbstractValidator<Customer> 
{
    // constructor...
    
    public override ValidationResult Validate(Customer instance)
    {
        return instance == null 
            ? new ValidationResult(new [] { new ValidationFailure("Customer", "Customer cannot be null") }) 
            : base.Validate(instance);
    }
}

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";
    }
}

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.
     });
}