C# 将表达式作为参数传递并使用 PredicateBuilder

Question

我想使用“条件”对象动态构建 LinqToSQL 和 EntityFramework 查询子句，并在必要时利用 C# In a Nutshell PredicateBuilder (http://www.albahari.com/nutshell/predicatebuilder.aspx)

澄清一下——我不明白如何创建 C# 表达式 ;o)

过滤器运算符和值被定义为我的“查询条件”类的属性:

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; }
}

“过滤器运算符”是一个普通的 C# 枚举:

public enum FilterOperator : int
{
    ExactMatch = 1,
    BeginsWith = 2,
    Contains = 3,
    EndsWith = 4,
    GreaterThan = 5,
    GreaterThanOrEqualTo = 6,
    LessThan = 7,
    LessThanOrEqualTo = 8,
}

我想使用以下方法在我的存储库中动态构建我的 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);
}

以上都有效，但我希望我的代码是 DRY，当我将另一个属性添加到我的条件(例如 DateTimeStarted)时，我不希望我的存储库为新属性复制上面的代码。

所以我想将上面的代码简化为:

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

这可能吗？如果可能的话如何？

Answer 1

绝对可以(不需要PredicateBuilder)。您只需将支持的 FilterOperator 映射到相应的 Expression。

这是执行此操作的自定义扩展方法:

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);
    }
}

和示例用法:

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