在LINQ SELECT语句中包含列

本文关键字：包含列 语句 SELECT LINQ | 更新日期: 2023-09-27 18:06:45

是否可以在linq Select中包含或排除列?

var numberOfYears = Common.Tool.NumberOfYear;
 var list = users.Select(item => new
        {
            Id = item.Id,
            Name= item.Name,
            City= Item.Address.City.Name,
            STATUS = Item.Status,
            if(numberOfYears == 1)
            {
               Y1 = item.Records.Y1,
            }
            if(numberOfYears == 2)
            {
               Y1 = item.Records.Y1,
               Y2 = item.Records.Y2,
            }
            if(numberOfYears == 3)
            {
               Y1 = item.Records.Y1,
               Y2 = item.Records.Y2,
               Y3 = item.Records.Y3,
            }
        }).ToList();
    }

这个想法是，我想显示Y1,Y2,Y3只有当有值

在LINQ SELECT语句中包含列

多亏了dynamic关键字的美妙之处，您现在可以在c#中实现所需的一切。下面是一个例子:

public class MyItem
{
    public string Name { get; set; }
    public int Id { get; set; }
}
static void Main(string[] args)
{
    List<MyItem> items = new List<MyItem>
    {
        new MyItem
        {
            Name ="A",
            Id = 1,
        },
        new MyItem
        {
            Name = "B",
            Id = 2,
        }
    };
    var dynamicItems = items.Select(x => {
        dynamic myValue;
        if (x.Id % 2 == 0)
            myValue = new { Name = x.Name };
        else
            myValue = new { Name = x.Name, Id = x.Id };
        return myValue;
    }).ToList();
}

这将返回一个动态对象列表。一个有1个属性，一个有2个属性

试试这个方法:

var numberOfYears = Common.Tool.NumberOfYear;
var list = users.Select(item => new
    {
        Id = item.Id,
        Name = item.Name,
        City = Item.Address.City.Name,
        STATUS = Item.Status,
        Y1 = numberOfYears > 0 ? item.Records.Y1 : 0,
        Y2 = numberOfYears > 1 ? item.Records.Y2 : 0,
        Y3 = numberOfYears > 2 ? item.Records.Y3 : 0
    }).ToList();

添加默认值或null，而不是0。

更新:根据你的评论，你唯一的选择是动态的。下面是一个动态的例子:

var numberOfYears = 3;
var list = users.Select(x =>
{
    dynamic item = new ExpandoObject();
    item.Id = x.Id;
    item.Name = x.Name;
    item.Status = x.Status;
    var p = item as IDictionary<string, object>;
    var recordsType = x.Records.GetType();
    for (var i = 1; i <= numberOfYears; ++i)
        p["Y" + i] = recordsType.GetProperty("Y" + i).GetValue(x.Records);
    return item;
}).ToList();

你可以这样使用ExpandoObject:

var data = providers.Select(provider =>
{
    dynamic excelRow = new ExpandoObject();
    excelRow.FirstName = provider.FirstName ?? "";
    excelRow.MiddleName = provider.MiddleName ?? "";
    excelRow.LastName = provider.LastName ?? "";
    
    // Conditionally add columns to the object...
    if (someCondition)
    {
        excelRow.Property1ForCondition = provider.Property1ForCondition;
        excelRow.Property2ForCondition = provider.Property2ForCondition;
    }
    excelRow.DueDate = provider.DueDate ?? null;
    .
    .
    .
    return excelRow;
});

上述代码的另一种变体可以是:

var data = new List<ExpandoObject>();
providers.ForEach(provider =>
{
    dynamic excelRow = new ExpandoObject();
    excelRow.FirstName = provider.FirstName ?? "";
    excelRow.MiddleName = provider.MiddleName ?? "";
    excelRow.LastName = provider.LastName ?? "";
    
    // Conditionally add columns to the object...
    if (someCondition)
    {
        excelRow.Property1ForCondition = provider.Property1ForCondition;
        excelRow.Property2ForCondition = provider.Property2ForCondition;
    }
    excelRow.DueDate = provider.DueDate ?? null;
    .
    .
    .
    data.Add(excelRow);
});
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