linq to sql 案例查询答案

【问题标题】：linq to sql case querylinq to sql 案例查询
【发布时间】：2009-01-12 16:39:34
【问题描述】：

我在使用 c# 中的 linq to sql 数据查询表达式构建查询时遇到问题。

我最终要做的就是基于这个伪代码表达式。

public IQueryable<CTest> searchRecords(string category, string searchString, DateTime startDate, DateTime endDate, int searchType, int searchType2)
        {
            //-Search All Records
            //-From the information table
            //-By the category column containing a specific search
            //-Also by
            //  ~if both a startDate and endDate are entered (not (0000,00,00) OR null) then get records
            //  by this expression
            //  ~else then don't worry about this statement
            //-Also by
            //  ~if a searchType is equal zero(0) then search for records from the 
            //  search_type table equal to zero(0)
            //  ~if a searchType is equal one(1) then search for records from the 
            //  search_type table equal to one(1)
            //  ~else then don't worry about this statement
            //-Also by
            //  ~if a searchType2 is equal zero(0) then search for records from the 
            //  search_type table equal to zero(0)
            //  ~if a searchType2 is equal one(1) then search for records from the 
            //  search_type table equal to one(1)
            //  ~else then don't worry about this statement

            //Here is my attempt at it
            /*  
                var table = db.table1;

                switch (category)
                {
                case "_category1":
                    var records =
                        from c in table
                        where c.column1.ToString().Contains(searchString)
                        select new CTest
                        {
                            test_id = c.id,
                            test_name = c.name,
                            test_number = c.number,
                            date = ((startDate != null) && (endDate != null)) ? ((c.test_date >= startDate) && (c.test_date <= endDate)) :
                                         c.test_date),
                            test_type = (searchType == 0 ? (c.searchType = 0) :
                                        searchType == 1 ? (c.searchType = 1) :
                                        c.searchType),
                            test_type2 = (searchType2 == 0 ? (c.searchType2 = 0) :
                                        searchType2 == 1 ? (c.searchType2 = 1) :
                                        c.searchType2)
                        };
                    break;
                default:
                    break;
                }
             */
        }

提前致谢。

【问题讨论】：

linq case statement 的可能重复项

标签： linq-to-sql expression case

【解决方案1】：

使用扩展方法比使用 LINQ 语法更容易：

 var records = context.Table
                      .Where( c => c.column1.Contains( searchString ) );
 if (startDate != null && endDate != null)
 {
     records = records.Where( c => c.test_date >= startDate
                                    && c.test_date <= endDate );
 }

 ...

这将在您进行时构建 LINQ 表达式。评估会延迟，直到您实际调用需要执行选择的扩展，例如 Count() 或 ToList()，或枚举结果的元素。

【讨论】：