逐步构建查询

因为 LINQ 使用延迟执行，所以我们可以拥有一个实际上不包含值的查询对象，但会在计算时返回值。因此，我们可以根据控制流动态构建查询，并在完成后对其进行评估：

IEnumerable<VehicleModel> BuildQuery(int vehicleType, SearchModel search, int start = 1, int count = -1) {
    IEnumerable<VehicleModel> query = _entities.Vehicles
        .Where(x => x.Active && x.Type == vehicleType)
        .Select(x => new VehicleModel {
            Id = v.Id,
            Year = v.Year,
            Class = v.Class,
            Make = v.Make,
            Model = v.Model,
            Cylinders = v.Cylinders ?? 0
        });

我们可以有条件地应用过滤器：

    if (!search.Years.Contains("all", StringComparer.OrdinalIgnoreCase))
        query = query.Where(v => search.Years.Contains(v.Year));

    if (!search.Makes.Contains("all", StringComparer.OrdinalIgnoreCase)) {
        query = query.Where(v => search.Makes.Contains(v.Make));
    }

    if (!search.Models.Contains("all", StringComparer.OrdinalIgnoreCase)) {
        query = query.Where(v => search.Models.Contains(v.Model));
    }

    if (!search.Cylinders.Equals("all", StringComparer.OrdinalIgnoreCase)) {
        decimal minCylinders = 0;
        decimal maxCylinders = 0;
        switch (search.Cylinders) {
            case "2-4":
                maxCylinders = 4;
                break;
            case "5-6":
                minCylinders = 5;
                maxCylinders = 6;
                break;
            case "8":
                minCylinders = 8;
                maxCylinders = 8;
                break;
            case "10+":
                minCylinders = 10;
                break;
        }
        if (minCylinders > 0) {
            query = query.Where(v => v.Cylinders >= minCylinders);
        }
        if (maxCylinders > 0) {
            query = query.Where(v => v.Cylinders <= maxCylinders);
        }
    }

我们可以根据条件为查询添加排序顺序：

    switch (search.SortingColumn.ToLower()) {
        case "make_model":
            query = query.OrderBy(v => v.Make).ThenBy(v => v.Model);
            break;
        case "year":
            query = query.OrderBy(v => v.Year);
            break;
        case "engine_size":
            query = query.OrderBy(v => v.EngineSize).ThenBy(v => v.Cylinders);
            break;
        default:
            query = query.OrderBy(v => v.Year); //The default sorting.
    }

我们的查询可以定义为从给定点开始：

    query = query.Skip(start - 1);

并定义为返回特定数量的记录：

    if (count > -1) {
        query = query.Take(count);
    }
    return query;
}

一旦我们有了查询对象，我们就可以使用 foreach 循环或其中一个返回一组值的 LINQ 方法来评估结果，例如 ToList 或 ToArray：

SearchModel sm;

// populate the search model here
// ...

List<VehicleModel> list = BuildQuery(5, sm).ToList();