C# linq lambda 表达式和“&”运算符
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linq lambda expression and '&' operator
提问by Gregg
I'm working on a ASP.NET MVC (an older version) application and I have a form where students are assigned an adviser based on a major code AND by alpha of a student's last name.
我正在开发一个 ASP.NET MVC（旧版本）应用程序，我有一个表格，其中根据主要代码和学生姓氏的字母为学生分配了一名顾问。
I am trying to implement the below code but it is showing red scribbles to &operator:
我正在尝试实现以下代码，但它向&操作员显示红色涂鸦：
Error: operator '&' cannot be applied to operands of type '
System.Linq.IQueryable<Appointments.Models.MajorAdviserStudentAssignmentByAlpha>' and 'lambda expression' and 'lambda expression'
错误：运算符“&”不能应用于“
System.Linq.IQueryable<Appointments.Models.MajorAdviserStudentAssignmentByAlpha>”和“lambda 表达式”和“lambda 表达式”类型的操作数
var majorAssignmentByAlpha = FindAllMajorAdviserStudentAssignmentByAlphas()
.Where(
a => a.MajorString == student.StudentMajor) &
(a => ((String.Compare(student.StudentLastName, a.AlphaStart) >= 0) &
(String.Compare(student.StudentLastName, a.AlphaEnd) <= 0))
.FirstOrDefault());
if (majorAssignmentByAlpha != null)
return majorAssignmentByAlpha.Adviser;
I would appreciate any help to understand what's wrong here. Thanks in advance!
我将不胜感激任何帮助了解这里出了什么问题。提前致谢！
采纳答案by rexcfnghk
I would like to add a correction here: actually both &&and&areboolean AND operators in C# (when used with bool). However the former will do a short-circuit evaluation:
我想在这里补充修正：其实两者&&并&都在C＃（当用于布尔AND运算符bool）。但是前者会做短路评估：
Consider A && B, if Ais falsethen Bwill not be evaluated since the result will be falseno matter what is the truthness of B.
考虑一下A && B，如果Ais falsethenB将不会被评估，因为false无论 的真实性如何，结果都将是B。
And for the error you are getting, probably that's because your &operator is comparing a boolean expression and a lambda expression (function). Try this
对于您遇到的错误，可能是因为您的&运算符正在比较布尔表达式和 lambda 表达式（函数）。尝试这个
MajorAdviserStudentAssignmentByAlpha majorAssignmentByAlpha = FindAllMajorAdviserStudentAssignmentByAlphas().FirstOrDefault(
a => a.MajorString == student.StudentMajor & String.Compare(student.StudentLastName, a.AlphaStart) >= 0 &
String.Compare(student.StudentLastName, a.AlphaEnd) <= 0);
if (majorAssignmentByAlpha != null)
return majorAssignmentByAlpha.Adviser;
回答by Bryan Crosby
I think you meant to use &&for the logical AND operation.
我认为您打算&&用于逻辑 AND 运算。
FirstOrDefault()also takes a Func<T, bool>so you could do:
FirstOrDefault()也需要一个Func<T, bool>所以你可以这样做：
var majorAssignmentByAlpha = FindAllMajorAdviserStudentAssignmentByAlphas().FirstOrDefault(
a => a.MajorString == student.StudentMajor &&
(a => ((String.Compare(student.StudentLastName, a.AlphaStart) >= 0) &&
(String.Compare(student.StudentLastName, a.AlphaEnd) <= 0));
回答by Jorge
That's because the operator that you're looking it's &&. Refactor your Linq expresssion like this
那是因为您正在查找的运算符是&&. 像这样重构你的 Linq 表达式
MajorAdviserStudentAssignmentByAlpha majorAssignmentByAlpha = FindAllMajorAdviserStudentAssignmentByAlphas().Where(
a => a.MajorString == student.StudentMajor) &&
(a => ((String.Compare(student.StudentLastName, a.AlphaStart) >= 0) &&
(String.Compare(student.StudentLastName, a.AlphaEnd) <= 0)).FirstOrDefault());
if (majorAssignmentByAlpha != null)
return majorAssignmentByAlpha.Adviser;
回答by TheEvilPenguin
I think you're added too many parentheses. Try this:
我认为您添加了太多括号。尝试这个：
MajorAdviserStudentAssignmentByAlpha majorAssignmentByAlpha = FindAllMajorAdviserStudentAssignmentByAlphas().Where(
a => a.MajorString == student.StudentMajor &&
String.Compare(student.StudentLastName, a.AlphaStart >= 0) &
String.Compare(student.StudentLastName, a.AlphaEnd <= 0)).FirstOrDefault();
if (majorAssignmentByAlpha != null)
return majorAssignmentByAlpha.Adviser;
.Where() takes a single expression returning bool. You are trying to and together two different expressions. If you remove the close parentheses before the &&, the open parentheses after and the second a =>, you're anding two boolean statements, which works.
.Where() 接受单个表达式返回bool. 您正在尝试将两种不同的表达方式结合在一起。如果您删除 之前的右&&括号、之后的a =>左括号和第二个，则您将使用两个布尔语句，这是有效的。
