如何将两个表达式与 Any 链接以制定 EF Core 查询

Question

I'm trying to formulate an Expression for a condition of an EF Core query. What I have so far is an expression that transforms my query result type to an IEnumerable and a predicate for the type of the IEnumerable that transforms it into a bool . Now I want to link them with an Any condition. What I was trying so far is this:

public static Expression<Func<TIn, bool>> Any<TIn, T>(
        Expression<Func<TIn, IEnumerable<T>>> valueFunction,
        Expression<Func<T, bool>> predicate)
{
    var call = Expression.Call(typeof(Queryable), nameof(Queryable.Any), new[] { typeof(T) }, value, predicate);
    return Expression.Lambda<Func<TIn, bool>>(call);
}

This throws the following exception:

System.InvalidOperationException: No generic method 'Any' on type 'System.Linq.Queryable' is compatible with the supplied type arguments and arguments. No type arguments should be provided if the method is non-generic.

I would imagine that this happens because I try to use the Expression of the Func and not a ParameterExpression to call the Any method.

So my question is, is it even possible to do that and if yes how? Thank you in advance!

Answer 1

Try this:

public static Expression<Func<TIn, bool>> Any<TIn, T>(
           Expression<Func<TIn, IEnumerable<T>>> valueFunction,
           Expression<Func<T, bool>> predicate)
{
    var method = typeof(Enumerable).GetMethods()
        .Where(mi => mi.Name =="Any" && mi.GetParameters().Length == 2)
        .Single()
        .MakeGenericMethod(typeof(T));
    var call = Expression.Call(method, valueFunction.Body, predicate);
    return Expression.Lambda<Func<TIn, bool>>(call, valueFunction.Parameters);
}

So you had few issues here. First of all your selector expression returns IEnumerable so you need to get method from Enumerable (if you will need to handle IQueryable 'sa little more work will be needed, like checking valueFunction.Body.Type to implement IQueryable ). Then you need to select correct method and since Any is generic one - you need to set specific type to the type parameters, thus creating a MethodInfo object that represents a particular constructed method ( MakeGenericMethod ). And lastly use valueFunction.Body for one of the Call arguments.