动态创建linq表达式或解析表达式的字符串值 [英] Creating a linq expression dynamically or parsing the string value of the expression
问题描述
遇到了一个需要动态创建linq表达式的问题.但是我似乎无法绕过它.
Got an problem where I need to crate an linq expression dynamically. But I cant seem to wrap my head around it.
因此，首先我得到一个复选框.我从所选文件中输入值的位置，因此在运行代码之前，我将不知道会有多少个参数.然后计划是基于复选框中元素的名称构造一个linq句子.让我说明一下: 我想创建一个如下所示的linq语句:
So first off I got an check list box. Where I input values from a file that is selected, hence I will not know how many parameters there will be there before I run the code. Then the plan is to construct a linq sentence based on the name of the elements in the checkbox. Let me illustrate: I want to create an linq statement that looks like this:
var result = from n in data where n.Item1 == "someValueFromCheckBox" select n;
数据所在的是一个元组列表，例如:
Where the data, is an tupled list like :
List<Tuple<string, string>>
这很好，只要用户仅从复选框中选择一项即可.但是，例如当用户选择两个项目时，我需要创建一个如下所示的linqu表达式:
And that is fine as long as the user only selects one item from the checkbox. But when the user for example selects two items, I need to create a linqu expression that looks like this:
var result = from n in data where n.Item1 == "someValueFromCheckBox" || n.Item1 == "someValueFromCheckBox1" select n;
所以我的第一个想法是将linq语句构建为字符串.以以下方式:
So my first thought was to build the linq statement as a string. In the following manner:
var selectedItems = checkedListBoxSelectedTerms.CheckedItems;
var linqStatement = "";
for (int i = 0; i < selectedItems.Count; i++ )
{
linqStatement += selectedItems[i].ToString() + "|| n.item1 ==" ;
}
//this is simply to remove the || "n.Item1 ==", at the end because it
is not needed after the last selecteditem.
linqStatement = linqStatement.Remove(linqStatement.Length - 13, 13);
但是这并没有按计划进行，因为当我这样输入时:
But that did not work as planned, because when I put it in like this:
var result = from n in data where n.Item1 == linqStatement select n;
整个事情变成一个字符串.而且我不能将"n.Item1 =="作为字符串.从理论上讲，该解决方案非常适合我使用，因为我不必担心复选框中元素的复杂程度以及用户选择了多少元素，但是我不能将linqStatement整体作为字符串传递.因此，可以寻求任何帮助，因为我似乎找不到一个很好的例子来做这样的事情.
The whole thing becomes a string. And I cant have the "n.Item1 ==" to be a string. In theory that solution would work great for my use, because I would not have to worry about how manay elements that are in the checkbox and how many elements the user selects, but I cannot pass linqStatement whole as a string. So any help would be appriciated, because I cant seem to find an good example to do something like this.
提前谢谢！
推荐答案
For such a simple example, you should simply have a list or set of strings, and then do Contains. E.g.
var selectedItems = checkedListBoxSelectedTerms.CheckedItems
.Select(x => x.ToString()).ToList();
var result = from n in data where selectedItems.Contains(n.Item1) select n;
If you need more flexible dynamic building capabilities, try something like Dynamic LINQ or PredicateBuilder.
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