如何在另一个表达式中包含一个表达式?

时间:2016-07-21 15:08:15

标签: c# lambda expression

我有一个DateRange类,我想将它作为where谓词应用于IQueryable,自动使用开始和结束日期并自动使用开放或关闭间隔。

public class DateRange
{
    public DateTime? BeginDate { get; set; }
    public DateTime? EndDate { get; set; }

    public bool BeginInclusive { get; set; }
    public bool EndInclusive { get; set; }

    public DateRange()
    {
        BeginInclusive = true;
        EndInclusive = false;
    }

    public IQueryable<T> Apply<T>( IQueryable<T> source, Expression<Func<T,DateTime>> dateField )
    {
        var result = source;
        if (BeginDate.HasValue)
        {
            if (BeginInclusive)
                result = result.Where( x => dateField >= BeginDate ); //does not compile
            else
                result = result.Where( x => dateField > BeginDate ); //does not compile
        }
        if (EndDate.HasValue)
        {
            if (EndInclusive)
                result = result.Where( x => dateField <= EndDate ); //does not compile
            else
                result = result.Where( x => dateField < EndDate ); //does not compile
        }
        return result;
    }
}

我想这样称呼它,DateField是T的任何DateTime属性。

DateRange d;
IQueryable<T> q;
q = d.Apply( q, x => x.DateField );

所以我想将一个成员表达式传递给Apply方法,并让它在结果集中应用一个合适的where子句,但是我无法弄清楚如何在中嵌入dateField成员表达式谓词的表达。请参阅上面的类中的“不编译”行。我需要以某种方式转换dateField或以其他方式构建谓词表达式,但我不知道该怎么做。

3 个答案:

答案 0 :(得分:1)

你在这里要做的就是撰写表达方式;你试图将一个表达式应用于另一个表达式的结果。你实际上可以写一个方法来做到这一点:

public static Expression<Func<TSource, TResult>> Compose<TSource, TIntermediate, TResult>(
    this Expression<Func<TSource, TIntermediate>> first,
    Expression<Func<TIntermediate, TResult>> second)
{
    var param = Expression.Parameter(typeof(TSource));
    var intermediateValue = first.Body.ReplaceParameter(first.Parameters[0], param);
    var body = second.Body.ReplaceParameter(second.Parameters[0], intermediateValue);
    return Expression.Lambda<Func<TSource, TResult>>(body, param);
}

它使用以下方法将表达式的参数替换为表达式。

public static Expression ReplaceParameter(this Expression expression,
    ParameterExpression toReplace,
    Expression newExpression)
{
    return new ParameterReplaceVisitor(toReplace, newExpression)
        .Visit(expression);
}
public class ParameterReplaceVisitor : ExpressionVisitor
{
    private ParameterExpression from;
    private Expression to;
    public ParameterReplaceVisitor(ParameterExpression from, Expression to)
    {
        this.from = from;
        this.to = to;
    }
    protected override Expression VisitParameter(ParameterExpression node)
    {
        return node == from ? to : base.Visit(node);
    }
}

这允许您将代码编写为:

public IQueryable<T> Apply<T>(IQueryable<T> source, 
    Expression<Func<T, DateTime>> dateField)
{
    var result = source;
    if (BeginDate.HasValue)
    {
        if (BeginInclusive)
            result = result.Where(dateField.Compose(date => date >= BeginDate));
        else
            result = result.Where(dateField.Compose(date => date > BeginDate));
    }
    if (EndDate.HasValue)
    {
        if (EndInclusive)
            result = result.Where(dateField.Compose(date => date <= EndDate));
        else
            result = result.Where(dateField.Compose(date => date < EndDate));
    }
    return result;
}

答案 1 :(得分:0)

您必须使用Expression类方法手工制作dateField >= BeginDate

(...)
    if (BeginInclusive)
    {
        var greaterOrEqual =
            Expression.Lambda<Func<T, bool>>(
                Expression.GreaterThanOrEqual(
                    dateField.Body,
                    Expression.Constant(BeginDate)),
                dateField.Parameters);

        result = result.Where(greaterOrEqual);
    }
(...)

与其他案件类似。

答案 2 :(得分:0)

这是在解决这个问题后创建的更新的Apply方法。

    public IQueryable<T> Apply<T>( IQueryable<T> source, Expression<Func<T,DateTime>> dateField )
    {
        Expression predicate;
        if (BeginDate.HasValue)
        {
            if (BeginInclusive)
                predicate = Expression.GreaterThanOrEqual( dateField.Body, Expression.Constant( BeginDate, typeof(DateTime) ) );
            else
                predicate = Expression.GreaterThan( dateField.Body, Expression.Constant( BeginDate, typeof(DateTime) ) );
            source = source.Where( Expression.Lambda<Func<T, bool>>( predicate ) );
        }
        if (EndDate.HasValue)
        {
            if (EndInclusive)
                predicate = Expression.LessThanOrEqual( dateField.Body, Expression.Constant( EndDate, typeof(DateTime) ) );
            else
                predicate = Expression.LessThan( dateField.Body, Expression.Constant( EndDate, typeof(DateTime) ) );
            source = source.Where( Expression.Lambda<Func<T, bool>>( predicate ) );
        }
        return source;
    }

接下来,我将其转换为扩展方法,因此可以像以下一样使用:

DateRange range;
IQueryable<T> q;
q = q.WhereInDateRange( range, x => x.DateField );