Applying an Expression to a LINQ Where clause

问题

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;

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;

回答1:

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;
}

回答2:

You can create a new expression to invoke the Filter using the FilterIds and use Expand from LinqKit

Edit Thanks to @Servy's comment. Now I expand only the internal expression instead of entire query.

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

public virtual IQueryable<T> ApplyFilter(IQueryable<T> query)
{
    if (FilterMode == FilterModeMatchAny && HasFilterIds)
    {
        Expression<Func<T, bool>> expression = x => Filter.Invoke(x, FilterIds);
        return query.Where(expression.Expand());
    }
    return query;
}
...

来源：https://stackoverflow.com/questions/62879661/applying-an-expression-to-a-linq-where-clause

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