尝试为linq中的日期创建大于，等于或大于动态过滤器

Question

我一直在尝试为Linq创建一个表达式树过滤器，该过滤器接受2个日期和一串可能的值{“ lessthan”，“ equals”，“ morethan”}。我希望将调用的格式设置为Query.Where（CompareDates（x => x.left，right，“小于”）））。

我有代码：

        public static IQueryable<TSource> CompareDates<TSource>(
            this IQueryable<TSource> source, 
            Expression<Func<TSource, DateTime?>> left, 
            DateTime? right, 
            string equality)
        {
            if (right == null || string.IsNullOrWhiteSpace(equality))
                return source;

            var p = left.Parameters.Single();
            Expression member = p;

            Expression leftExpression = Expression.Property(member, "left");
            Expression rightParameter = Expression.Constant(right, typeof(DateTime));

            BinaryExpression BExpression = null;
            switch (equality)
            {
                case "lessthan":
                    BExpression = Expression.LessThan(leftExpression, rightParameter);
                    break;
                case "equal":
                    BExpression = Expression.Equal(leftExpression, rightParameter);
                    break;
                case "morethan":
                    BExpression = Expression.GreaterThan(leftExpression, rightParameter);
                    break;
                default:
                    throw new Exception(String.Format("Equality {0} not recognised.", equality));
            }

            return source.Where(Expression.Lambda<Func<TSource, bool>>(BExpression, p));
        }

不幸的是，这会产生错误“ System.ArgumentException：在SARRestAPI.Extensions.Expressions.CompareDates处System.Linq.Expressions.Expression.Property（Expression expression，String propertyName）上没有为类型'Model定义实例属性'left' [TSource]（IQueryable 1 source, Expression1 src，提供了DateTime，字符串相等）“

有人知道为什么会这样吗？

Answer 1

开始了; 您想要做的是使用.Body传入选择器的，而不是寻找.left。这意味着，给定的输入选择器x => x.Foo.Bar.Blap，常量和比较，要构造类似x => x.Foo.Bar.Blap < someValue，通过复用两个x（参数，这你已经做）和身体（x.Foo.Bar.Blap）。

在代码中（请注意，这适用于任何TValue，而不仅仅是DateTime）：

public enum Comparison
{
    Equal,
    NotEqual,
    LessThan,
    LessThanOrEqual,
    GreaterThan,
    GreaterThanOrEqual
}
public static IQueryable<TSource> Compare<TSource, TValue>(
        this IQueryable<TSource> source,
        Expression<Func<TSource, TValue>> selector,
        TValue value,
        Comparison comparison)
{
    Expression left = selector.Body;
    Expression right = Expression.Constant(value, typeof(TValue));

    BinaryExpression body;
    switch (comparison)
    {
        case Comparison.LessThan:
            body = Expression.LessThan(left, right);
            break;
        case Comparison.LessThanOrEqual:
            body = Expression.LessThanOrEqual(left, right);
            break;
        case Comparison.Equal:
            body = Expression.Equal(left, right);
            break;
        case Comparison.NotEqual:
            body = Expression.NotEqual(left, right);
            break;
        case Comparison.GreaterThan:
            body = Expression.GreaterThan(left, right);
            break;
        case Comparison.GreaterThanOrEqual:
            body = Expression.GreaterThanOrEqual(left, right);
            break;
        default:
            throw new ArgumentOutOfRangeException(nameof(comparison));
    }

    return source.Where(Expression.Lambda<Func<TSource, bool>>(body, selector.Parameters));
}

用法示例（此处使用LINQ-to-Objects，但它也适用于其他LINQ后端）：

var arr = new[] { new { X = 11 }, new { X = 12 }, new { X = 13 }, new { X = 14 } };
var source = arr.AsQueryable();

var filtered = source.Compare(x => x.X, 12, Comparison.GreaterThan);
foreach (var item in filtered)
{
    Console.WriteLine(item.X); // 13 and 14
}

请注意，使用C＃vCurrent您可以执行以下操作：

var body = comparison switch
{
    Comparison.LessThan => Expression.LessThan(left, right),
    Comparison.LessThanOrEqual => Expression.LessThanOrEqual(left, right),
    Comparison.Equal => Expression.Equal(left, right),
    Comparison.NotEqual => Expression.NotEqual(left, right),
    Comparison.GreaterThan => Expression.GreaterThan(left, right),
    Comparison.GreaterThanOrEqual => Expression.GreaterThanOrEqual(left, right),
    _ => throw new ArgumentOutOfRangeException(nameof(comparison)),
};
return source.Where(Expression.Lambda<Func<TSource, bool>>(body, selector.Parameters));

您可能会更好。