FluentValidation for generic unique values

Error.txt

Test Name:  Nao_deve_permitir_nome_duplicados
Test FullName:	Project.Test.FluentValidation.ModalidadeValidatorTest.Nao_deve_permitir_nome_duplicados
Test Source:	{path}\Project.Test\FluentValidation\ModalidadeValidatorTest.cs : line 28
Test Outcome:	Failed
Test Duration:	0:00:00,4946356

Result Message:	
Test method Project.Test.FluentValidation.ModalidadeValidatorTest.Nao_deve_permitir_nome_duplicados threw exception: 
System.ArgumentException: Expression of type 'System.Func`2[Project.Model.Modalidade,System.Int32]' cannot be used for return type 'System.Int32'
Result StackTrace:	
at System.Linq.Expressions.Expression.ValidateLambdaArgs(Type delegateType, Expression& body, ReadOnlyCollection`1 parameters)
   at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, String name, Boolean tailCall, IEnumerable`1 parameters)
   at System.Linq.Expressions.Expression.Lambda[TDelegate](Expression body, Boolean tailCall, IEnumerable`1 parameters)
   at Project.Web.Validacoes.UniqueValidator`1.IsValid(PropertyValidatorContext context) in {path}\Validacoes\UniqueValidator.cs:line 39
   at FluentValidation.Validators.PropertyValidator.Validate(PropertyValidatorContext context) in {path}\PropertyValidator.cs:line 63
   at FluentValidation.Internal.PropertyRule.<Validate>d__c.MoveNext() in {path}\Internal\PropertyRule.cs:line 216
   at System.Linq.Enumerable.<SelectManyIterator>d__14`2.MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at FluentValidation.AbstractValidator`1.Validate(ValidationContext`1 context) in {path}\AbstractValidator.cs:line 83
   at FluentValidation.TestHelper.ValidatorTester`2.ValidateError(T instanceToValidate) in {path}\ValidatorTester.cs:line 46
   at Project.Test.FluentValidation.ModalidadeValidatorTest.Nao_deve_permitir_nome_duplicados() in {path}\ModalidadeValidatorTest.cs:line 36

RuleBuilderExtensions.cs

public static IRuleBuilderOptions<T, TElement> MustBeUnique<T, TElement, TEntity>(this IRuleBuilder<T, TElement> ruleBuilder
  , Expression<Func<TEntity, int>> propertyId
	, IRepository<TEntity> repository
	)
	where TEntity : class, IEntity
{
	return ruleBuilder.SetValidator(new UniqueValidator<TEntity>(propertyId, repository));
}

UniqueValidator.cs

public class UniqueValidator<T> : PropertyValidator
  	where T : class, IEntity
	{
		private readonly Expression<Func<T, int>> PropertyId;
		private readonly IRepository<T> Repository;

		public UniqueValidator(Expression<Func<T, int>> propertyId, IRepository<T> repository)
			: base("{PropertyName} já existe!")
		{
			PropertyId = propertyId;
			Repository = repository;
		}

		protected override bool IsValid(PropertyValidatorContext context)
		{
			// TODO: Not Work!!  
			// Read: https://fluentvalidation.codeplex.com/discussions/353331#post1050039 and http://stackoverflow.com/questions/16765086/dynamic-expression-for-filter-iqueryable

			if (context.PropertyValue == null || string.IsNullOrWhiteSpace(context.PropertyValue.ToString()))
				return true;

			var currentId = Expression.Lambda<Func<int>>(PropertyId).Compile()(); // ERROR HERE

			// From: http://msdn.microsoft.com/en-us/library/vstudio/bb882637.aspx, http://msdn.microsoft.com/en-us/library/bb397951.aspx
			// Compose the expression tree that represents the parameter to the predicate.

			// p => p.Id != currentId
            ParameterExpression pId = Expression.Parameter(typeof(int), "Id");
			ConstantExpression cId = Expression.Constant(currentId, typeof(int));
			BinaryExpression notCurrent = Expression.NotEqual(pId, cId);
			Expression<Func<int, bool>> NotCurrentExpr =
                Expression.Lambda<Func<int, bool>>(
                    notCurrent,
                    new ParameterExpression[] { pId });
			
			// p.{PropertyName} == context.PropertyValue
			ParameterExpression pUnique = Expression.Parameter(typeof(string), context.PropertyName);
			ConstantExpression cUnique = Expression.Constant(context.PropertyValue, typeof(string));
			BinaryExpression checkUnique = Expression.Equal(pId, cId);
			Expression<Func<string, bool>> CheckUniqueExp =
				Expression.Lambda<Func<string, bool>>(
					checkUnique,
					new ParameterExpression[] { pUnique });

			var exp = Expression.And(NotCurrentExpr, CheckUniqueExp);

			throw new NotImplementedException();

			/*
			if (currentId > 0)
				return Repository.All().Provider.CreateQuery(exp); // y(p => p.Id != currentId && p.{HowToGetPropertyByExpression?} == context.PropertyValue)
			return Repository.All().Any(checkUnique);
			*/
		}
	}

XXValidator.cs

public class ModalidadeValidator : AbstractValidator<ModalidadeViewModel>
{
  public ModalidadeValidator(IXXRepository repositorio)
	{
		RuleFor(p => p.Name)
			.MustBeValidName()
			.MustBeUnique(p => p.Id, repositorio);
	}
}