如何在LINQ中动态添加OR运算符到WHERE子句 [英] How to dynamically add OR operator to WHERE clause in LINQ

问题描述

我有一个可变大小的字符串数组，我试图通过编程循环遍历数组，并匹配表中的所有行，其中Tags列至少包含数组中的一个字符串。这是一些伪代码：

  IQueryable< Songs> allSongMatches = musicDb.Songs; //表中的所有行
  

我可以轻松地查询一个固定的字符串集，如下所示：

  allSongMatches = allSongMatches.Where（SongsVar => SongsVar.Tags.Contains（foo1）|| SongsVar .Tags.Contains（foo2）|| SongsVar.Tags.Contains（foo3））; 
  

但是，这不起作用（我得到以下错误：具有语句正文的lambda表达式不能转换为表达式树）

  allSongMatches = allSongMatches.Where（SongsVar => 
 {
 bool retVal = false; 
 foreach（strArray中的字符串str）
 {
 retVal = retVal || SongsVar.Tags.Contains（str）; 
} 
 return retVal; 
}）; 
  

有谁能告诉我正确的策略来完成这个？我仍然是LINQ的世界新人： - ）

解决方案

你可以使用 PredicateBuilder 类：

  var searchPredicate = PredicateBuilder.False< Songs>（）; 
 
 foreach（strArray中的字符串str）
 {
 var closureVariable = str; //查看以下链接的原因
 searchPredicate = 
 searchPredicate.Or（SongsVar => SongsVar.Tags.Contains（closureVariable））; 
} 
 
 var allSongMatches = db.Songs.Where（searchPredicate）; 
  

LinqToSql奇怪的行为

I have a variable size array of strings, and I am trying to programatically loop through the array and match all the rows in a table where the column "Tags" contains at least one of the strings in the array. Here is some pseudo code:

 IQueryable<Songs> allSongMatches = musicDb.Songs; // all rows in the table

I can easily query this table filtering on a fixed set of strings, like this:

 allSongMatches=allSongMatches.Where(SongsVar => SongsVar.Tags.Contains("foo1") || SongsVar.Tags.Contains("foo2") || SongsVar.Tags.Contains("foo3"));

However, this does not work (I get the following error: "A lambda expression with a statement body cannot be converted to an expression tree")

 allSongMatches = allSongMatches.Where(SongsVar =>
     {
       bool retVal = false;
       foreach(string str in strArray)
       {
         retVal = retVal || SongsVar.Tags.Contains(str);
       }
       return retVal;
     });

Can anybody show me the correct strategy to accomplish this? I am still new to the world of LINQ :-)

解决方案

You can use the PredicateBuilder class:

var searchPredicate = PredicateBuilder.False<Songs>();

foreach(string str in strArray)
{
   var closureVariable = str; // See the link below for the reason
   searchPredicate = 
     searchPredicate.Or(SongsVar => SongsVar.Tags.Contains(closureVariable));
}

var allSongMatches = db.Songs.Where(searchPredicate);

LinqToSql strange behaviour

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