thuban.org :: Keith J. Farmer

Just something I decided to try cobbling together tonight.  Didn't take more than a few minutes, trying to type quietly (didn't want to wake anyone).

It allows you to do things like:

        var q = db.Customers.Where(new Customer { CustomerID = "EASTC" });

Basically, it's a where-by-prototype. One obvious function, as seen above, is to get a LINQ to SQL entity by ID.

Here's the code:

        public static IQueryable<TEntity> Where<TEntity>(
                this IQueryable<TEntity> seq, TEntity entity
                )
        {
            IQueryable<TEntity> result = seq;
            var memberTypes = new[] { MemberTypes.Property, MemberTypes.Field };

            var members = typeof(TEntity).GetMembers()
                          .Where(mi => memberTypes.Contains(mi.MemberType) 
                                       && mi.GetCustomAttributes(typeof(ColumnAttribute), true)
                                       .Length > 0);

            foreach (var member in members)
            {
                var isField = member.MemberType == MemberTypes.Field;
                var memberAsField = member as FieldInfo;
                var memberAsProperty = member as PropertyInfo;

                var memberValue = isField ? memberAsField.GetValue(entity) 
                                          : memberAsProperty.GetValue(entity, null);
                var memberIsRefType = (isField ? memberAsField.FieldType 
                                               : memberAsProperty.PropertyType
                                      ).IsClass;

                var memberIsDefaultValue = (memberValue == (memberIsRefType ? (object)null 
                                                                            : (object)0
                                                           ));

                if (memberIsDefaultValue)
                {
                    continue;
                }

                var parameter = Expression.Parameter(typeof(TEntity), "p");

                var memberAccess = Expression.MakeMemberAccess(parameter, member);
                var value = Expression.Constant(memberValue);
                var equality = Expression.Equal(memberAccess, value);

                var predicate = Expression.Lambda<Func<TEntity, bool>>(equality, parameter);

                result = result.Where(predicate);
            }

            return result;
        }