绑定Blazor参数的LINQ查询无返回结果问题排查

我使用LINQ查询，试图根据CSV文件（经CsvHelper处理并映射为对应模型）中的所选餐型过滤数据，但选择早餐、午餐、晚餐或零食时，查询始终无数据返回。调试发现下拉框已显示选中值，但LINQ查询中的SelectedMeal为null，导致无结果返回。选择“all-meals”时，EternityDataViewAllMeals组件可正常返回所有数据，不清楚SelectedMeal为空的原因，是否需要使用GroupBy？

相关代码 #

LINQ查询代码 #

public partial class EternityDataView : IGenerateNutritionalValues {
    private NutritionDataView dataView = new();
    public decimal? GetCalorieValues()
    {
        var calories = from c in FileUpload.records
            where c.Meal == dataView.SelectedMeal
            select c.Calories;

        return calories.Sum();
    }
}

NutritionDataView中的餐型选择Razor代码 #

<div class="meal-select">
    <label for="meal-select">Select meal type:</label>
    <select name="meal-select" id="meal-select" @bind="SelectedMeal">
        <option value="all-meals">All meals</option>
        <option value="breakfast">Breakfast</option>
        <option value="dinner">Dinner</option>
        <option value="lunch">Lunch</option>
        <option value="snacks">Snacks</option>
    </select>
</div>
.......
// Switch for date range
    case "eternity":
        if (SelectedMeal == "all-meals")
        {
            <EternityDataViewAllMeals/>
        }
        else
        {
            <EternityDataView/>
        }
        break;

CSV示例数据 #

Date,Meal,Calories,Fat (g),Saturated Fat,Polyunsaturated Fat,Monounsaturated Fat,Trans Fat,Cholesterol,Sodium (mg),Potassium,Carbohydrates (g),Fiber,Sugar,Protein (g),Vitamin A,Vitamin C,Calcium,Iron,Note
2023-03-02,Breakfast,105.0,0.4,0.1,0.1,0.0,0.0,0.0,1.2,422.4,27.0,3.1,14.4,1.3,5.0,17.1,0.6,1.7,
2023-03-02,Lunch,710.0,30.1,3.2,0.1,0.0,0.0,0.0,2.8,214.0,91.8,9.8,44.4,18.1,0.0,15.3,1.2,1.3,
2023-03-02,Dinner,699.8,29.4,7.8,2.4,6.2,0.2,19.8,268.3,180.4,87.2,3.4,28.6,22.6,12.7,84.1,15.8,2.0,
2023-03-02,Snacks,166.0,0.4,0.0,0.1,0.1,0.0,0.0,6.9,615.7,39.6,1.0,28.7,2.4,0.0,193.7,19.4,2.5,

问题分析与解决方案 #

核心原因 #

你在EternityDataView中新建了一个独立的NutritionDataView实例（private NutritionDataView dataView = new();），但这个实例和Razor页面中绑定SelectedMeal的实例完全无关。下拉框的选中值绑定的是页面的SelectedMeal，而查询时用的是新实例的SelectedMeal，因此始终为null。此外，CSV中的餐型是大写开头（如Breakfast），下拉框的值是小写（breakfast），直接用==匹配也会导致过滤失败。

修复步骤 #

1. 修改EternityDataView，通过组件参数接收SelectedMeal：

   public partial class EternityDataView : IGenerateNutritionalValues {
       // 新增参数接收页面传递的SelectedMeal
       [Parameter]
       public string SelectedMeal { get; set; }
   
       public decimal? GetCalorieValues()
       {
           // 忽略大小写匹配CSV中的餐型
           var calories = from c in FileUpload.records
               where c.Meal.Equals(SelectedMeal, StringComparison.OrdinalIgnoreCase)
               select c.Calories;
   
           return calories.Sum();
       }
   }
   

2. 在Razor页面调用组件时传递SelectedMeal参数：

   case "eternity":
       if (SelectedMeal == "all-meals")
       {
           <EternityDataViewAllMeals/>
       }
       else
       {
           <EternityDataView SelectedMeal="@SelectedMeal"/>
       }
       break;
   

关于GroupBy #

当前问题与GroupBy无关，GroupBy用于分组统计场景，你的需求是过滤后求和，无需使用GroupBy，解决实例不统一和大小写匹配问题即可。

内容的提问来源于stack exchange，提问作者islandmonkey