Linq equality comparing not working

Question

Given this method:

internal static IEnumerable<Entity> GetByParentImpl<Entity, TKey>(this ICanGetByParent<Entity, TKey> self, TKey parentId, string fieldName) 
    where Entity : class 
{
    RepositoryBase<Entity> rb = (RepositoryBase<Entity>)self;
    rb.unitOfWork.Begin();

    var entities = rb.unitOfWork.Session.QueryOver<Entity>()
        .Where(e => EqualityComparer<TKey>.Default.Equals(GetId<Entity, TKey>(e, fieldName), parentId))
        .List();

    return entities;
}

And this helper:

private static TKey GetId<Entity, TKey>(object obj, string fieldName) where Entity : class
{
    TKey id = default(TKey);

    switch(id.GetType().Name) {
        case "Int32":
            break;
        case "Guid":
            id = (TKey)TypeDescriptor.GetConverter(typeof(TKey)).ConvertFromInvariantString((string)typeof(Entity).GetField(fieldName).GetValue(obj));
            break;
    }

    return id;
}

I'm getting this exception on my linq statement:

Unrecognised method call: System.Collections.Generic.EqualityComparer`1[[System.Guid, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]:Boolean Equals(System.Guid, System.Guid)

What does this mean? I'm not even sure how to debug this correctly. I could have sworn that the code above was working...

I don't really know about linq-to-nhibernate, but from an entity-framework background I'd suspect that the query provider has no translation for the Equals method. You'll have to use an expression or do it in linq-to-objects. — Gert Arnold

Kirill Bestemyanov Kirill Bestemyanov · Accepted Answer · 2012-08-09T15:55:30

You cannot do comparison in such way for linq to any database provider. Provider cannot transform this into expression tree. So you must use it after .ToArray() or give an Expression<Func<RegisterCardBase, bool>> into Where instead of lambda.

PS: Why are you doing so strange actions with Guid? How it is storaged in database?

Linq equality comparing not working

2 Answers