实体框架使用表达式即时构建 Where 子句

Question

使用实体框架 C# 并有这个查询，我需要它说的部分：

where x.Login_Status == "Submitted"

是动态的。在不同的情况下，它可能是“已提交”或 null 或其他内容，而不是编写具有不同查询的多个 if 语句，而是希望在 where 子句中有一个谓词。

status = (from x in ctx.table
          where x.Login_Status == "Submitted"
          orderby x.SUB_DATE descending
          select new Model_Table()
          {
              Id = x.ID,
              Name = x.NAME,
              Code = x.Code,
              DateSubmitted = x.SUB_DATE
           }).ToList<Model_Table>();

这可能吗？

解决方案：

if语句里面遇到更多参数的时候用这个

where_expression = x => x.Login_Status == "Submitted" || x.Login_Status == null;

这是一个对我有用的完整代码，方括号之间的任何内容都替换为适合您的代码：

Expression<Func<[Replace with your Entity], bool>> where_submitted = x => x.Login_Status == "Submitted";

// Check if all selected
if (CheckBox_Show_All_Submitted.Checked)
{
    where_submitted = x => x.Login_Status == "Submitted" || x.Login_Status == null;
}

status = 
    ctx.[Replace with your Entity Table]
    .Where(where_submitted)
    .OrderByDescending(x => x.SUB_DATE)
    .Select(x => new Model_Table
        {
           Id = x.ID,
           Name = x.NAME,
           Code = x.Code,
           DateSubmitted = x.SUB_DATE
        }).ToList<Model_Table>();

Answer 1

您需要Expression<Func<Entity,bool>>，而不是Predicate<Entity>。区别在于谓词是编译的委托，而表达式是代码即数据，因此可以转换为 SQL。

这是一个例子：

//You can have this expression have different values based on your logic
Expression<Func<Entity,bool>> where_expression = x => x.Login_Status == "Submitted";

var query =
    ctx.Table
    .Where(where_expression)
    .OrderByDescending(x => x.SUB_DATE)
    .Select(x => new Model_Table())
    {
        Id = x.ID,
        Name = x.NAME,
        Code = x.Code,
        DateSubmitted = x.SUB_DATE
   }).ToList();        

请注意，Entity 需要替换为真实类的名称。

Answer 2

像这样为 IQueryable 创建一个扩展方法：

public static class MethodExtensions{

      public static IEnumerable<Model_Table> Query(this IQueryable<TEntity> source, string data){
             return (from x in source
             where x.Login_Status == data
             orderby x.SUB_DATE descending
             select new Model_Table()
            {
                Id = x.ID,
                Name = x.NAME,
                Code = x.Code,
                DateSubmitted = x.SUB_DATE
            }).ToList<Model_Table>();
      }
}

---

现在你可以像这样使用它了：

var result = ctx.table.Query("somethingelse");