PredicateBuilder 不适用于 EF Core

Question

public string[] FindAssets2()
{
    string[] result;

    using (var ctx = new StockContext())
    {
        var predicate = PredicateBuilder.New<Asset>().Or(asset => asset.Symbol.Contains("TSLA"));

        result = ctx.Assets.AsExpandable()
        .Where(predicate)
        .Select(z => z.Symbol)
        .ToArray();
    }

    return result;
}

Just a piece of simple codes. It throws me

Exception: Unhandled expression type: 'Extension'
LinqKit.ExpressionVisitor.Visit(Expression exp)
LinqKit.ExpressionVisitor.VisitExpressionList(ReadOnlyCollection<Expression> original)
LinqKit.ExpressionVisitor.VisitMethodCall(MethodCallExpression m)
LinqKit.ExpressionVisitor.Visit(Expression exp)
LinqKit.Extensions.Expand(Expression expr)
LinqKit.ExpandableQueryProvider<T>.System.Linq.IQueryProvider.CreateQuery<TElement>(Expression expression)
System.Linq.Queryable.Where<TSource>(IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)

---

Below is my installed package

    <PackageReference Include="Microsoft.EntityFrameworkCore" Version="5.0.0-preview.4.20220.10" />
    <PackageReference Include="Microsoft.EntityFrameworkCore.SqlServer" Version="5.0.0-preview.4.20220.10" />
    <PackageReference Include="LINQKit.Core" Version="1.1.17" />

Any help would be appreciate. Many thanks!

Answer 1

It seems that EF-core 5 doesn't play nice with AsExpandable . Or rather the other way around. It worked with EF3, but apparently Linqkit isn't resilient to library-specific idiosyncrasies of expression trees.

There is a work-around for this case. As expained here , predicate is not an Expression , but an ExpressionStarter , which implicitly converts to Expression<Func<T, bool>> and Func<T, bool> . Therefore, queries with predicates on top-level entities (say: EF's IQueryables ) work without AsExpandable :

result = ctx.Assets   // No '.AsExpandable()'
    .Where(predicate) // Implicit conversion of 'predicate' to Expression<Func<Asset,boo>l>
    .Select(z => z.Symbol)
    .ToArray();

The bad news is that without any fixes in this area, you won't be able to use AsExpandable when it's really necessary, for example for predicates on collection navigation properties.