C#将表达式作为参数传递并使用PredicateBuilder

我想使用一个“criteria”对象动态构建LinqToSQL和EntityFramework查询子句，如果需要的话，还可以在一个简单的PredicateBuilder（）中利用C#

我只是想说清楚——我不知道如何创建C#表达式；o）

筛选器运算符和值定义为我的“查询条件”类的属性，如下所示：

public class QueryCriteria
{
    public DateTimeFilter DateTimeCompleted { get; set; }
}

“过滤器”定义为：

public class DateTimeFilter
{
    public FilterOperator FilterOperator { get; set; }

    public DateTime FilterToken { get; set; }
}

“过滤器操作符”是一个普通的C#enum：

我希望使用以下方法在我的存储库中动态构建LinqToSQL和EntityFramework查询：

if (criteria.DateTimeCompleted != null)
{
    var predicate = PredicateBuilder.True(query);

    switch (criteria.DateTimeCompleted.FilterOperator)
    {
        case FilterOperator.ExactMatch:
            predicate = predicate.And(p => string.Equals(p.DateTimeCompleted.Value, criteria.DateTimeCompleted.FilterToken));
            break;

        case FilterOperator.GreaterThan:
            predicate = predicate.And(p => p.DateTimeCompleted.Value > criteria.DateTimeCompleted.FilterToken);
            break;

        case FilterOperator.GreaterThanOrEqualTo:
            predicate = predicate.And(p => p.DateTimeCompleted.Value >= criteria.DateTimeCompleted.FilterToken);
            break;

        case FilterOperator.LessThan:
            predicate = predicate.And(p => p.DateTimeCompleted.Value < criteria.DateTimeCompleted.FilterToken);
            break;

        case FilterOperator.LessThanOrEqualTo:
            predicate = predicate.And(p => p.DateTimeCompleted.Value <= criteria.DateTimeCompleted.FilterToken);
            break;

        default:
            throw new InvalidOperationException("Can't use the specified filter operator on a DateTime.");
    }

    query = query.Where(predicate);
}

这可能吗？如果可能的话，如何实现？

这肯定是可能的（而且不需要PredicateBuilder）。您只需将支持的

过滤器操作符

映射到相应的

表达式

下面是一个自定义扩展方法，它可以：

public static class QueryableExtensions
{
    public static IQueryable<T> Where<T>(this IQueryable<T> source, DateTimeFilter filter, Expression<Func<T, DateTime>> target)
    {
        if (filter == null) return source;
        var left = target.Body;
        var right = Expression.Constant(filter.FilterToken);
        Expression condition;
        switch (filter.FilterOperator)
        {
            case FilterOperator.ExactMatch:
                condition = Expression.Equal(left, right);
                break;

            case FilterOperator.GreaterThan:
                condition = Expression.GreaterThan(left, right);
                break;

            case FilterOperator.GreaterThanOrEqualTo:
                condition = Expression.GreaterThanOrEqual(left, right);
                break;

            case FilterOperator.LessThan:
                condition = Expression.LessThan(left, right);
                break;

            case FilterOperator.LessThanOrEqualTo:
                condition = Expression.LessThanOrEqual(left, right);
                break;

            default:
                throw new InvalidOperationException("Can't use the specified filter operator on a DateTime.");
        }
        var predicate = Expression.Lambda<Func<T, bool>>(condition, target.Parameters);
        return source.Where(predicate);
    }
}

query = query.Where(criteria.DateTimeCompleted.GetPredicate(query, p => p.DateTimeCompleted.Value));

public static class QueryableExtensions
{
    public static IQueryable<T> Where<T>(this IQueryable<T> source, DateTimeFilter filter, Expression<Func<T, DateTime>> target)
    {
        if (filter == null) return source;
        var left = target.Body;
        var right = Expression.Constant(filter.FilterToken);
        Expression condition;
        switch (filter.FilterOperator)
        {
            case FilterOperator.ExactMatch:
                condition = Expression.Equal(left, right);
                break;

            case FilterOperator.GreaterThan:
                condition = Expression.GreaterThan(left, right);
                break;

            case FilterOperator.GreaterThanOrEqualTo:
                condition = Expression.GreaterThanOrEqual(left, right);
                break;

            case FilterOperator.LessThan:
                condition = Expression.LessThan(left, right);
                break;

            case FilterOperator.LessThanOrEqualTo:
                condition = Expression.LessThanOrEqual(left, right);
                break;

            default:
                throw new InvalidOperationException("Can't use the specified filter operator on a DateTime.");
        }
        var predicate = Expression.Lambda<Func<T, bool>>(condition, target.Parameters);
        return source.Where(predicate);
    }
}

query = query.Where(criteria.DateTimeCompleted, p => p.DateTimeCompleted.Value);