LINQ的动态添加where条件 [英] Linq dynamically adding where conditions

问题描述

我有在每个标题过滤下拉框一个gridview。装载时每个滤波器装有从其列不同的值。在运行时，我加上ALL，以允许用户从该字段中选择所有。我试图动态生成的LINQ语句，如果下拉框设置为ALL忽略场。这可能吗？我想看看我是否可以在一个单独的语句做到这一点。下面仅示例显示2下拉框，但我的实际情况下，拥有高达5。

如果我选择使用，如果再声明，我结束了意大利面条的代码

  DropDownList的drpOwners = this.grdOtherQuotes.HeaderRow.FindControl（drpOwners）作为DropDownList的; 
 DropDownList的drpCompanyName = this.grdOtherQuotes.HeaderRow.FindControl（drpCompanyName）作为DropDownList的; 
 
 VAR filteredList =（从allQuotes 
 x，其中（drpOwners.SelectedValue = ALL）x.SalesRepFullName == drpOwners.SelectedValue：真正的
和;&安培; drpCompanyName！？。的SelectedValue = ALL x.CompanyName == drpCompanyName.SelectedValue！？真
选择x）; 
  

解决方案

个人而言，我觉得有这个挖开更简单：

 的IEnumerable<&报价GT; filteredList = allQuotes; 
 //如果使用EF或LINQ to SQL中，使用：IQueryable的<&报价GT; filteredList = allQuotes; 
如果（drpOwners.SelectedValue = ALL！）
 filteredList = filteredList.Where（X => x.SalesRepFullName == drpOwners.SelectedValue）; 
如果（drpCompanyName.SelectedValue = ALL！）
 filteredList = filteredList.Where（X => x.CompanyName == drpCompanyName.SelectedValue）; 
 //更多条件需要
  

这真的是没有任何更长的时间，这是远简单的跟进。

---

如果你真的希望能够写这为单行，你可以做一个扩展方法来构建查询。例如，如果使用实体框架：

 静态的IQueryable< T> AddCondition（这IQueryable的< T>可查询，Func键<布尔>谓语，表达< Func键< T，BOOL>>过滤器）
 {
如果（谓词（））
返回queryable.Where （过滤）; 
，否则
回报率可查询; 
} 
  

这会然后让你写为：

  VAR filteredList = allQuotes 
 .AddCondition（（）=>！drpOwners.SelectedValue = ALL，X => x.SalesRepFullName == drpOwners .SelectedValue）
 .AddCondition（（）=>！drpCompanyName.SelectedValue = ALL，x.CompanyName == drpCompanyName.SelectedValue）; 
  

您可以，当然，即使进一步借此，使一个版本硬导线谓词要检查ALL组合框，使得谓语较短（只是组合框）。

I have a gridview which has drop down boxes in each header for filtering. Each filter is loaded with the distinct values from its column when loaded. At run time, I add "ALL" to allow the user to select all from that field. I am trying to build the linq statement dynamically to ignore the field if the drop down box is set to "ALL". Is this possible? I want to see if I can do this in one single statement. The example below only shows 2 dropdown boxes, but my actually case has up to 5.

If I choose to use if then statements, I end up with spaghetti code.

DropDownList drpOwners = this.grdOtherQuotes.HeaderRow.FindControl("drpOwners") as DropDownList;                     
DropDownList drpCompanyName = this.grdOtherQuotes.HeaderRow.FindControl("drpCompanyName") as DropDownList;

var filteredList = (from x in allQuotes
                          where (drpOwners.SelectedValue != ALL) ? x.SalesRepFullName == drpOwners.SelectedValue:true                                    
                                && drpCompanyName.SelectedValue != ALL ? x.CompanyName == drpCompanyName.SelectedValue: true
                          select x);

解决方案

Personally, I'd find having this broken up to be simpler:

IEnumerable<Quote> filteredList = allQuotes;
// If using EF or LINQ to SQL, use: IQueryable<Quote> filteredList = allQuotes;
if (drpOwners.SelectedValue != ALL) 
   filteredList = filteredList.Where(x => x.SalesRepFullName == drpOwners.SelectedValue);
if (drpCompanyName.SelectedValue != ALL) 
   filteredList = filteredList.Where(x => x.CompanyName == drpCompanyName.SelectedValue);
// More conditions as needed

This really isn't any longer, and it's far simpler to follow.

---

If you really wanted to be able to write this as a "one-liner", you could make an extension method to build the query. For example, if using Entity Framework:

static IQueryable<T> AddCondition(this IQueryable<T> queryable, Func<bool> predicate, Expression<Func<T,bool>> filter)
{
     if (predicate())
         return queryable.Where(filter);
     else
         return queryable;
}

This would then let you write this as:

var filteredList = allQuotes
                     .AddCondition(() => drpOwners.SelectedValue != ALL, x => x.SalesRepFullName == drpOwners.SelectedValue)
                     .AddCondition(() => drpCompanyName.SelectedValue != ALL, x.CompanyName == drpCompanyName.SelectedValue);

You could, of course, take this even further, and make a version that hard-wires the predicate to check a combo box against "ALL", making the predicate shorter (just the combo box).

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