> So IConstraintProvider<User> will return for example StringIsNotNull<User>, so I cannot directly apply those to the result of
> IConstraintProvider<UserDTO> because when I try to evaluate IsValid I would get an invalid cast.

Needs a bit of work but the code looks a bit like this:

var rulesToApplyToTarget = new DomainRuleSet<TTargetOfRules>();

foreach (IPropertyMappingDefinition propertyMappingDefinition in propertyMappings)
{
    var ruleDefinitionsForProperty = rulesToMap.RulesFor(propertyMappingDefinition.FromProperty);

    ruleDefinitionsForProperty.ForEach(ruleDefinition =>
                                {
                                    var newRuleDefinition =
                                            ruleDefinition.CloneForUseOn<TTargetOfRules>(propertyMappingDefinition.ToProperty);

                                    rulesToApplyToTarget.Add(newRuleDefinition);
                                });
}

Worth a bit of explanation, which probably indicates I should refactor, but essentially propertyMappings describes the mappings between the domain and DTO, each knows which property it is mapping to/from (ignore value objects for now). So I work out the rules for each property, and as you can see above for each of them we can create a new rule definition by calling CloneForUseOn:

public IReflectionBasedRuleDefinition<TTarget> CloneForUseOn<TTarget>(PropertyInfo property)
{
      return new ReflectionBasedRuleDefinition<TTarget, TProperty>(property, this.Rule);
}

public IDomainRule<TProperty> Rule { get; set; }

So you've got a new RelectionBasedRuleDefinition but setup for the property on the target, at run-time its executed like this against the target (the DTO):

public bool IsSatisfiedBy(TContainingProperty model)
{
       TProperty toValidate = (TProperty)this.PropertyRuleRelatesTo.GetValue(model, null);

       return this.Rule.IsSatisfiedBy(toValidate);
}

That cast can ofcourse fail. Anyway I'd fully accept its a bit convoluted, though actually using it is easy and all this is hidden away. Having said that its not been put through its paces on a big project yet and obviously there might be a better solution, and perhaps after trying it for a while I'll decide that just duplicating the rules on the DTO is fine.