应用表达式到LINQ Where子句 [英] Applying an Expression<> to a LINQ Where clause

问题描述

我正在编写自定义实体框架过滤器.我有一个ID列表和一个用户提供的表达式.

I'm writing a custom Entity Framework filter. I have a list of IDs, and a user-supplied expression.

protected IEnumerable<TColumn> FilterIds;
protected Expression<Func<T, IEnumerable<TColumn>, bool>> Filter;

现在我的问题是如何将Filter表达式应用于Where()子句?

Now my question is how do I apply the Filter expression to a Where() clause?

public virtual IQueryable<T> ApplyFilter(IQueryable<T> query)
{
    if (FilterMode == FilterModeMatchAny && HasFilterIds)
    {
        // Whoops! Can't do this!
        return query.Where(x => Filter(x, FilterIds));
    }
    return query;

推荐答案

使用此帖子中的Combine方法您几乎需要的所有东西.从那里开始，您只需要将文字值转换成计算它的表达式(或者我猜可以更改答案的Combine方法，这样中间值就不是从lambda计算出来的，而仅仅是任何表达式)，并且然后调用该函数.

Using the Combine method from this post does pretty much everything you need. From there you just need to turn the literal value into an expression that computes it (or I guess alter the Combine method from that answer so that the intermediate value isn't computed from a lambda but rather is just any expression), and then call the function.

protected IEnumerable<TColumn> FilterIds;
protected Expression<Func<T, IEnumerable<TColumn>> FilterIdsExpression => _ => FilterIds;
protected Expression<Func<T, IEnumerable<TColumn>, bool>> Filter;

public virtual IQueryable<T> ApplyFilter(IQueryable<T> query)
{
    if (FilterMode == FilterModeMatchAny && HasFilterIds)
    {
        return query.Where(FilterIdsExpression.Combine(Filter));
    }
    return query;
}

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