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如何从lambda表达式中提取成员路径

lambda linq .net

Aaron Fischer · 技术社区 · 14 年前

我有一个方法可以将日期条件添加到我的LINQ查询中。我要做的是传递x。作为一个参数到期,使它能够与任何日期一起工作。有什么想法吗?

protected virtual IQueryable<TaskView> AddTaskDuePredicate( DateCriteria dateCriterion, IQueryable<TaskView> taskSummary )
{
        if ( dateCriterion.Condition == DateCondition.LessThan )
            taskSummary = taskSummary.Where( x => x.Due < dateCriterion.Value1 );
        else if ( dateCriterion.Condition == DateCondition.LessThanOrEqualTo )
            taskSummary = taskSummary.Where( x => x.Due <= dateCriterion.Value1 );
        else if ( dateCriterion.Condition == DateCondition.GreaterThan )
            taskSummary = taskSummary.Where( x => x.Due > dateCriterion.Value1 );
        else if ( dateCriterion.Condition == DateCondition.GreaterThanOrEqualTo )
            taskSummary = taskSummary.Where( x => x.Due >= dateCriterion.Value1 );
        else if ( dateCriterion.Condition == DateCondition.EqualTo )
            taskSummary = taskSummary.Where( x => x.Due == dateCriterion.Value1 );
        else if ( dateCriterion.Condition == DateCondition.Between )
            taskSummary = taskSummary.Where( x => x.Due <= dateCriterion.Value1 && x.Due >= dateCriterion.Value2 );

    return taskSummary;
}

2 回复 | 直到 14 年前

Jon Skeet 14 年前

必须将lambda表达式作为表达式树,如下所示:

protected virtual IQueryable<TaskView> AddTaskDuePredicate(
    Expression<Func<TaskView, DateTime>> projection,
    DateCriteria dateCriterion,
    IQueryable<TaskView> taskSummary)

那你就得建立一个 Expression<Func<TaskView, bool>> 从投影中,使用 Expression.GreaterThan 和 Expression.Lambda . 从我的头顶上:

ParameterExpression p = projection.Parameters[0];
Expression constant = Expression.Constant(dateCriterion.Value1);
Expression comparison = Expression.GreaterThan(projection.Body, constant);
Expression lambda = Expression.Lambda<Func<TaskView, bool>>
    (comparison, p);
taskSummary = taskSummary.Where(lambda);

那是 完全地 但是还没有测试。显然,一旦你有了这个工作 GreaterThan 剩下的应该是 相当地 容易的。。。

Aaron Fischer 14 年前

这就是我的结局。我可以将此方法称为.where()参数所以,

taskSummary.Where( AddDatePredicate<TaskView>( x => ( DateTime )x.Due, filterInfo.Due ) );

        protected virtual Expression<Func<T, bool>> AddDatePredicate<T>( Expression<Func<T, DateTime>> projection, DateCriteria dateCriterion)
        {
            ParameterExpression p = projection.Parameters[ 0 ];
            Expression constant = Expression.Constant( (DateTime)dateCriterion.Value1 );

            Expression comparison;
            switch( dateCriterion.Condition )
            {
                case DateCondition.GreaterThan : 
                    comparison = Expression.GreaterThan( projection.Body, constant );
                    break;
                case DateCondition.GreaterThanOrEqualTo:
                    comparison = Expression.GreaterThanOrEqual( projection.Body, constant );
                    break;
                case DateCondition.LessThan:
                    comparison = Expression.LessThan( projection.Body, constant );
                    break;
                case DateCondition.LessThanOrEqualTo:
                    comparison = Expression.LessThanOrEqual( projection.Body, constant );
                    break;

                case DateCondition.Between:
                    {
                        var comparisonLeft =  Expression.GreaterThanOrEqual( projection.Body, Expression.Constant( ( DateTime )dateCriterion.Value1 ) );
                        var comparisonRight = Expression.LessThanOrEqual(    projection.Body, Expression.Constant( ( DateTime )dateCriterion.Value2 ) );

                        comparison = Expression.AndAlso( comparisonLeft, comparisonRight );
                        break;
                    }
                default:
                    return null;//TODO:Something bad happens here.
            }