public class Product
{
    public int Id { get; set; }
	
	public int? DictId { get; set; }
    public Dict Dict { get; set; }              

    [NotMapped]
    public bool IsDict
    {
        get { return  Dict != null; }
    }
}

public class ProductMap : EntityTypeConfiguration<Product>
{
    public ProductMap()
    {
        ToTable("Product", "dbo");
        HasKey(a => a.Id);
        Property(a => a.Id).HasColumnName("Id");
        Property(a => a.DictId).HasColumnName("DictId");
    }
}

// Так работает
var q1 = dbcontext.set<Product>().ToList().Where(w => w.IsDict).ToList();

// Так выдает ошибку
var q2 = dbcontext.set<Product>().Where(w => w.IsDict).ToList();
//Error: The specified type member 'IsDict' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported

 Ignore(a => a.IsDict);