Enumerable.Empty<T>().AsQueryable(); This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code

问题

I am getting runtime error

This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.InvalidOperationException: This method supports the LINQ to Entities infrastructure and is not intended to be used directly from your code.

I am trying to produce query that rather than filtering on every search criteria would include by adding all matching records on all search fields (would do OR rather than AND).

public static IQueryable<T> ApplySearch<T>(this IQueryable<T> queryable, SearchModel search) where T : class 
{
    var results = Enumerable.Empty<T>().AsQueryable();
    if (search != null)
    {
        if (search.PolicyNumber.HasValue && typeof (IPolicyNumber).IsAssignableFrom(queryable.ElementType))
        {
            results = results.Union(queryable.SearchByPolicyNumber(search));
        }

        if (search.UniqueId.HasValue && typeof (IUniqueId).IsAssignableFrom(queryable.ElementType))
        {
            results = results.Union(queryable.SearchByUniqueId(search));
        }

        if (!string.IsNullOrWhiteSpace(search.PostCode) && typeof(IPostCode).IsAssignableFrom(queryable.ElementType))
        {
            results = results.Union(queryable.SearchByPostCode(search));
        }
    }

    return results;
}

Mechanism started failing when I introduced var results = Enumerable.Empty<T>().AsQueryable(); which I need to start from something empty.

How do I start out from an empty set and then build Linq-to-sql results on top?

回答1:

you can refactor the code to not need an empty set by only ever unioning results that you have:

public static IQueryable<T> ApplySearch<T>(this IQueryable<T> queryable, SearchModel search) where T : class 
{
    var subQueries = new List<IQueryable<T>>();
    if (search != null)
    {
        if (search.PolicyNumber.HasValue && typeof (IPolicyNumber).IsAssignableFrom(queryable.ElementType))
        {
            subQueries.Add(queryable.SearchByPolicyNumber(search));
        }

        if (search.UniqueId.HasValue && typeof (IUniqueId).IsAssignableFrom(queryable.ElementType))
        {
            subQueries.Add(queryable.SearchByUniqueId(search));
        }

        if (!string.IsNullOrWhiteSpace(search.PostCode) && typeof(IPostCode).IsAssignableFrom(queryable.ElementType))
        {
            subQueries.Add(queryable.SearchByPostCode(search));
        }
    }

    return subQueries.DefaultIfEmpty(queryable)
        .Aggregate((a, b) => a.Union(b));
}

回答2:

Temporary hack that I have used

is to change from

var results = Enumerable.Empty<T>().AsQueryable();

to

var results = queryable.Where(o => false);

来源：https://stackoverflow.com/questions/37815309/enumerable-emptyt-asqueryable-this-method-supports-the-linq-to-entities-i