Question

In my EF later, I’m trying to pass in an anonymous function to be used as part of my Linq query. The function would pass in an INT and return a BOOL (u.RelationTypeId is an INT). Below is a simplified version of my function:

public IEnumerable<UserBandRelation> GetBandRelationsByUser(Func<int, bool> relation)
{
    using (var ctx = new OpenGroovesEntities())
    {
        Expression<Func<UsersBand, bool>> predicate = (u) => relation(u.RelationTypeId);

        var relations = ctx.UsersBands.Where(predicate);

        // mapping, other stuff, back to business layer
        return relations.ToList();
    }
}

However, I get the error stated above. It seems like I’m going everything correct by building a predicate from the function. Any ideas? Thanks.

Answer 1

You’re trying to pass an arbitrary .NET function in… how could the entity framework hope to translate that into SQL? You can change it to take an Expression<Func<int, bool>> instead, and build the Where clause from that, although it won’t be particularly easy, because you’ll need to rewrite the expression with a different parameter expression (i.e. replacing whatever parameter expression is in the original expression tree with the expression of calling u.RelationTypeId).

To be honest, for the sake of just specifying u.RelationTypeId in the lambda expression that you use to create the expression tree to pass into the method, you’d be better off just using:

public IEnumerable<UserBandRelation> GetBandRelationsByUser(
    Expression<Func<UsersBand, bool>> predicate)
{
    using (var ctx = new OpenGroovesEntities())
    {
        var relations = ctx.UsersBands.Where(predicate);

        // mapping, other stuff, back to business layer
        return relations.ToList();
    }
}