如何在一个表达式中包含一个表达式? [英] How can I include one expression in another expression?

问题描述

我有一个DateRange类，我想将其作为where谓词应用于IQueryable，自动使用开始和结束日期，并自动使用打开或关闭间隔.

I have a DateRange class that I'd like to apply to an IQueryable as a where predicate, automatically using the begin and end dates and automatically using an open or closed interval.

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.

And I want to call it like this, DateField is any DateTime property of T.

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

因此，我想将成员表达式传递给Apply方法，并使其将适当的where子句应用于结果集，但是我无法弄清楚如何获取嵌入在 where 中的dateField成员表达式. em>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，但是我不知道该怎么做.

So I want to pass a member expression to the Apply method, and have it apply an appropriate where clause to the result set, but I cannot figure out how to get the dateField member expression embedded in the where predicate's expression. See lines "do not compile" in class above. I need to transform dateField somehow or build the predicate expression some other way, but I have no idea how to do so.

推荐答案

您要在此处编写表达式；您正在尝试将一个表达式应用于另一个表达式的结果.实际上，您可以编写一种方法来做到这一点:

What you're looking to do here is to compose expressions; you're trying to apply one expression to the result of another. You can actually write a method to do that:

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.

It uses the following method to replace the parameter of an expression with an expression.

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 : node;
    }
}

这允许您将代码编写为:

This allows you to write your code as:

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;
}

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