variable '' of type '' referenced from scope '', but it is not defined

参考地址：https://stackoverflow.com/questions/30556911/variable-of-type-referenced-from-scope-but-it-is-not-defined

当我写如下lambda表达式时：

Expression<Func<string, bool>> e1 = y => y.Length < 100;
Expression<Func<string, bool>> e2 = y => y.Length < 200;

var e3 = Expression.And(e1.Body, e2.Body);

var e4 = Expression.Lambda<Func<string, bool>>(e3, e1.Parameters.ToArray());
e4.Compile(); // <--- causes run-time error

会报如题目的错误。variable '' of type '' referenced from scope '', but it is not defined

原因是两个表达式的条件并不是针对同一对应指定的。所以当使用And 或者 OR时两个表达式并列使用时会有问题，解决办法看我的另一篇文章。

有详细说明：https://lebang2020.cn/details/20090710uf5gw2.html