将List.Contains转换为Expression Tree [英] Convert List.Contains to Expression Tree

问题描述

相关:

创建具有3个条件的Lambda表达式

将包含的内容转换为表达式树

在上一个问题的下面，我遇到了要编写Expression Tree版本的查询:

In the following of my previous question I faced with this query that I want to write Expression Tree version:

List<byte?> lst = new List<byte?>{1,2};
from a in myTbl
where a.Age = 20 && lst.Contains(a.Status)
select a

我写这段代码:

List<byte?> lst = new List<byte?>{1,2};
var param = Expression.Parameter(typeof(T), "o");
var body = Expression.AndAlso(
               Expression.Equal(
                    Expression.PropertyOrField(param, "Age"),
                    Expression.Constant(20)
               ),
               Expression.Call(Expression.PropertyOrField(param, "Status"),
                                   "Contains",
                                   Type.EmptyTypes, 
                                   Expression.Constant(lst)));

var lambda = Expression.Lambda<Func<T, bool>>(body, param);
return lambda;

我得到了错误:

类型'System.Nullable'1 [System.Byte]'上不存在方法'Contains'."

"No method 'Contains' exists on type 'System.Nullable`1[System.Byte]'."

请帮助我找到问题.

谢谢

推荐答案

问题是您已将两个参数切换为Expression.Call，您的代码正在尝试创建无意义的表达式o.Status.Contains(lst).

The problem is that you have switched two arguments to Expression.Call, your code is trying to create the nonsensical expression o.Status.Contains(lst).

您需要切换两个参数:

Expression.Call(Expression.Constant(lst),
    "Contains",
    Type.EmptyTypes, 
    Expression.PropertyOrField(param, "Status"))

这是假设您使用的LINQ提供程序了解List<T>.Contains().如果需要Enumerable.Contains()，请查看Ivan Stoev的答案.

This is assuming that the LINQ provider you're using understands List<T>.Contains(). If you need Enumerable.Contains(), then have a look at Ivan Stoev's answer.

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