c# - 如何替换表达式树中的属性类型及其值

Question

我有一个带有 Nullable DateTime 属性的 PersonDTO 类:

public class PersonDTO
{
    public virtual long Id { get; set; }
    public virtual string Name { get; set; }
    // YYYYMMDD format
    public virtual Nullable<int> Birthday { get; set; }
}

还有表示层的一个类:

public class PersonViewModel
{
    public virtual long Id { get; set; }
    public virtual string Name { get; set; }
    public virtual Nullable<DateTime> Birthday { get; set; }
}

在我的表单上，我有两个方法负责创建 Expression<Func<PersonViewModel, bool>>对象:

    private Expression<Func<PersonViewModel, bool>> GetFilterExpression()
    {             
        Expression condition = null;
        ParameterExpression pePerson = Expression.Parameter(typeof(PersonViewModel), "person");
        //...
        if (dtpBirth.Format != DateTimePickerFormat.Custom)
        {
            Expression target = Expression.Property(pePerson, pePerson.Type.GetProperty("Birthday", typeof(DateTime?)));
            UnaryExpression date = Expression.Convert(Expression.Constant(dtpBirth.Value.Date), typeof (DateTime?));
            condition = (condition == null)
                    ? Expression.GreaterThan(target, date)
                    : Expression.And(condition, Expression.GreaterThan(target, date));
        }
        // Формируем лямбду с условием и возвращаем результат сформированного фильтра
        return condition != null ? Expression.Lambda<Func<PersonViewModel, bool>>(condition, pePerson) : null;
    }

我也在使用 AutoMapper？转换一个 Expression<Func<PersonViewModel, bool>>至 Expression<Func<PersonDTO, bool>> .转换代码如下所示:

// ...
Mapper.CreateMap<PersonViewModel, PersonDTO>()
              .ForMember(dto => dto.Birthday, opt => opt.MapFrom(model => model.BirthdaySingle.NullDateTimeToNullInt("yyyyMMdd")));
// ...
public static class DataTypesExtensions
{
    public static DateTime? NullIntToNullDateTime(this int? input, string format)
    {
        if (input.HasValue)
        {
            DateTime result;
            if (DateTime.TryParseExact(input.Value.ToString(), format, CultureInfo.InvariantCulture, DateTimeStyles.None, out result))
            {
                return result;
            }
        }
        return null;
    }

    //...
}

我的表达式转换器看起来像:

    public static Expression<Func<TDestination, TResult>> RemapForType<TSource, TDestination, TResult>(
        this Expression<Func<TSource, TResult>> expression)
    {
        var newParameter = Expression.Parameter(typeof(TDestination));

        var visitor = new AutoMapVisitor<TSource, TDestination>(newParameter);
        var remappedBody = visitor.Visit(expression.Body);
        if (remappedBody == null)
        {
            throw new InvalidOperationException("Unable to remap expression");
        }

        return Expression.Lambda<Func<TDestination, TResult>>(remappedBody, newParameter);
    }

public class AutoMapVisitor<TSource, TDestination> : ExpressionVisitor
{
    private readonly ParameterExpression _newParameter;
    private readonly TypeMap _typeMap = Mapper.FindTypeMapFor<TSource, TDestination>();

    public AutoMapVisitor(ParameterExpression newParameter)
    {
        _newParameter = newParameter;
    }

    protected override Expression VisitMember(MemberExpression node)
    {
        var propertyMaps = _typeMap.GetPropertyMaps();

        // Find any mapping for this member
        // Here I think is a problem, because if it comes (person.Birthday => Convert(16.11.2016 00:00:00)) it can't find it.
        var propertyMap = propertyMaps.SingleOrDefault(map => map.SourceMember == node.Member);
        if (propertyMap == null)
        {
            return base.VisitMember(node);
        }

        var destinationProperty = propertyMap.DestinationProperty;
        var destinationMember = destinationProperty.MemberInfo;

        // Check the new member is a property too
        var property = destinationMember as PropertyInfo;
        if (property == null)
        {
            return base.VisitMember(node);
        }

        // Access the new property
        var newPropertyAccess = Expression.Property(_newParameter, property);
        return base.VisitMember(newPropertyAccess);
    }
}

我需要以某种方式转换部分 lambda 表达式:person => person.Birthday > Convert(15.11.2016 00:00) (在这种情况下，person 是 PersonViewModel 和 DateTime 类型的 Birthday？)看起来像:person => person.Birthday > 20161115 (在这种情况下 person 是 PersonDTO 和 int 类型的 Birthday？)。如果没有这个问题，一切都会正常映射和工作。我知道我需要更深入地研究树并进行一些操作，但我不明白我应该如何以及在何处执行此操作。

Answer 1

我会使用 sg 调整二进制表达式的日期时间值:

class AutoMapVisitor<TSource, TDestination>: ExpressionVisitor
{
    // your stuff
    protected override Expression VisitBinary(BinaryExpression node)
    {
        var memberNode = IsBirthdayNode(node.Left)
            ? node.Left
            : IsBirthdayNode(node.Right)
                ? node.Right
                : null;
        if (memberNode != null)
        {
            var valueNode = memberNode == node.Left
                ? node.Right
                : node.Left;
            // get the value
            var valueToChange = (int?)getValueFromNode(valueNode);
            var leftIsMember = memberNode == node.Left;
            var newValue = Expression.Constant(DataTypesExtensions.NullIntToNullDateTime(valueToChange, /*insert your format here */ ""));
            var newMember = Visit(memberNode);
            return Expression.MakeBinary(node.NodeType, leftIsMember ? newMember : newValue, leftIsMember ? newValue : newMember); // extend this if you have a special comparer or sg
        }
        return base.VisitBinary(node);
    }

    private bool IsBirthdayNode(Expression ex)
    {
        var memberEx = ex as MemberExpression;
        return memberEx != null && memberEx.Member.Name == "Birthday" && memberEx.Member.DeclaringType == typeof(PersonViewModel);
    }

    private object getValueFromNode(Expression ex)
    {
        var constant = ex as ConstantExpression;
        if (constant != null)
            return constant.Value;
        var cast = ex as UnaryExpression;
        if (cast != null && ex.NodeType == ExpressionType.Convert)
            return getValueFromNode(cast.Operand);
        // here you can add more shortcuts to improve the performance of the worst case scenario, which is:
        return Expression.Lambda(ex).Compile().DynamicInvoke(); // this will throw an exception, if you have references to other parameters in your ex
    }

}

它非常具体，但你明白了，你可以让它更适合你的用例。

但我认为您对属性的映射是错误的。在 sql 中你想使用 int 比较。以上为您完成。当 automapper 更改您的属性时，它应该只用新生日(更改类型)替换旧生日，而不调用 NullDateTimeToNullInt。上面的代码将处理用于比较的类型更改。如果您在匿名选择或其他地方有成员，我相信您仍然会有问题...