ahjesus动态生成表达式树

直接上方法，看的懂的拿去用，看不懂的找资料看懂

public PartialViewResult _Product(int pageindex = 1, int pagesize = 12, Double floorprice = 0, Double topprice = 9999999999, string brandstr = "", string categorystr = "", string orderBy = "priceasc") {

            int[] brands;

            if (string.IsNullOrWhiteSpace(brandstr)) {

                brands = null;

            }

            else {

                brands = Array.ConvertAll<string, int>(brandstr.Split(','), delegate(string s) { return int.Parse(s); });

            }

            int[] categorys;

            if (string.IsNullOrWhiteSpace(categorystr)) {

                categorys = null;

            }

            else {

                categorys = Array.ConvertAll<string, int>(categorystr.Split(','), delegate(string s) { return int.Parse(s); });

            }

            IEnumerable<Product> product;



            ParameterExpression paramExpr = Expression.Parameter(typeof(Product), "it");



            MemberExpression floorpricePropExpr = Expression.Property(paramExpr, "UnitPrice");

            ConstantExpression floorpriceValueExpr = Expression.Constant(floorprice, typeof(Double));

            BinaryExpression floorpriceExpr = Expression.GreaterThanOrEqual(floorpricePropExpr, floorpriceValueExpr);

            //出自http://www.cnblogs.com/ahjesus 尊重作者辛苦劳动成果,转载请注明出处,谢谢!

            MemberExpression toppricePropExpr = Expression.Property(paramExpr, "UnitPrice");

            ConstantExpression toppriceValueExpr = Expression.Constant(topprice, typeof(Double));

            BinaryExpression toppriceExpr = Expression.LessThanOrEqual(toppricePropExpr, toppriceValueExpr);



            Expression whereExpr = Expression.And(floorpriceExpr, toppriceExpr);

            Expression whereBrandExpr = null;

            if (brands != null && brands.Length > 0) {

                for (int i = 0, j = brands.Length; i < j; i++) {

                    int brand = brands[i];

                    MemberExpression BrandPropExpr = Expression.Property(paramExpr, "Brand");

                    ConstantExpression BrandValueExpr = Expression.Constant(brand, typeof(int));

                    BinaryExpression BrandExpr = Expression.Equal(BrandPropExpr, BrandValueExpr);

                    if (i == 0) {

                        whereBrandExpr = BrandExpr;

                    }

                    else { whereBrandExpr = Expression.Or(whereBrandExpr, BrandExpr); }



                }

            }



            Expression wherecategoryExpr = null;

            if (categorys != null && categorys.Length > 0) {

                for (int i = 0, j = categorys.Length; i < j; i++) {

                    int category = categorys[i];

                    MemberExpression categoryPropExpr = Expression.Property(paramExpr, "Category");

                    ConstantExpression categoryValueExpr = Expression.Constant(category, typeof(int));

                    BinaryExpression categoryExpr = Expression.Equal(categoryPropExpr, categoryValueExpr);

                    if (wherecategoryExpr == null) {

                        if (i == 0) {

                            wherecategoryExpr = categoryExpr;

                        }//出自http://www.cnblogs.com/ahjesus 尊重作者辛苦劳动成果,转载请注明出处,谢谢!

                        else {

                            wherecategoryExpr = Expression.Or(wherecategoryExpr, categoryExpr);

                        }

                    }

                    else {

                        wherecategoryExpr = Expression.Or(wherecategoryExpr, categoryExpr);

                    }

                }

            }



            if (whereBrandExpr != null) {

                whereExpr = Expression.And(whereExpr, whereBrandExpr);

            }

            if (wherecategoryExpr != null) {

                whereExpr = Expression.And(whereExpr, wherecategoryExpr);

            }



            Expression<Func<Product, bool>> lambda = Expression.Lambda<Func<Product, bool>>(whereExpr, paramExpr);



            switch (orderBy) {

                case "priceasc":

                    product = BLLRequest.Current.ProductCollection.Where(lambda.Compile()).OrderBy(it => it.UnitPrice);

                    break;

                case "pricedesc":

                    product = BLLRequest.Current.ProductCollection.Where(lambda.Compile()).OrderByDescending(it => it.UnitPrice);

                    break;

                case "salesasc":

                    product = BLLRequest.Current.ProductCollection.Where(lambda.Compile()).OrderBy(it => it.SalesQty);

                    break;

                case "salesdesc":

                    product = BLLRequest.Current.ProductCollection.Where(lambda.Compile()).OrderByDescending(it => it.SalesQty);

                    break;

                default:

                    product = BLLRequest.Current.ProductCollection.Where(lambda.Compile()).OrderBy(it => it.UnitPrice);

                    break;

            }



            int total = product.Count();

            int pagecount = (total % pagesize) > 0 ? (total / pagesize) + 1 : (total / pagesize);

            product = product.Skip((pageindex - 1) * pagesize).Take(pagesize);

            ViewBag.product = product;

            ViewBag.total = total;

            ViewBag.pagecount = pagecount;

            ViewBag.pageindex = pageindex;

            return PartialView();

        }