像以前没有使用EF时，对于从界面输入的多条件查询时，都是使用SQL语句的拼接，因为使用了哪几种条件是未知的。
类似下面这样
var sql = "select * from xxx where xxx";if(!string.IsNullOrEmpty(xxid){sql += “ And xxid = ” + xxid； }
这种方法虽然笨拙，但是能很好的解决问题。
使用EF后，也遇到同样的问题。在linq中是可以利用表达式树来实现组合条件查询的功能，但是需要一定的学习门槛。
如果你已经了解了基本的linq语法，但是又暂时没有时间学习表达式树，可以试试以下的简单方法。
原理很简单，执行多次where函数即可，这种方法虽然会增加循环次数，但是也算很好的解决了问题。
测试代码如下：
模型
1 public class Student
2 {
3 public string Name { get; set; }
4 public string StudentID { get; set; }
5 }
功能实现
1 static void Main(string[] args)2 {3 List<Student> students = new List<Student>();4 5 students.Add(new Student() { Name = "雍心慈", StudentID = "1" });6 students.Add(new Student() { Name = "雍心慈2", StudentID = "2" });7 students.Add(new Student() { Name = "乔宁夏", StudentID = "3" });8 students.Add(new Student() { Name = "乔宁夏2", StudentID = "4" });9 students.Add(new Student() { Name = "宓白夜", StudentID = "5" });
10 students.Add(new Student() { Name = "仰灵萱", StudentID = "6" });
11 students.Add(new Student() { Name = "冷杏儿", StudentID = "7" });
12
13 var query = students.AsEnumerable();
14
15 foreach (var rule in GetExpression("心","1"))
16 {//循环执行过滤条件
17 query = query.Where(x => rule(x));
18 }
19
20 foreach (var item in query)
21 {
22 Console.WriteLine(item.StudentID);
23 Console.WriteLine(item.Name);
24 }
25 }
26
27 private static List<Func<Student, bool>> GetExpression(string name, string studentId)
28 {
29 List<Func<Student, bool>> list = new List<Func<Student, bool>>();
30
31 if (!string.IsNullOrEmpty(studentId))
32 {
33 list.Add(x => x.StudentID.Contains(studentId));
34 }
35
36 if (!string.IsNullOrEmpty(name))
37 {
38 list.Add(x => x.Name.Contains(name));
39 }
40
41 return list;
42 }
如果想要减少循环次数，代码复杂一点，可以参考如下方法：
1 internal class Program2 {3 static void Main(string[] args)4 {5 //构建 一个测试列表6 List<Student> students = new List<Student>();7 8 students.Add(new Student() { Name = "雍心慈", StudentID = "1" });9 students.Add(new Student() { Name = "雍心慈2", StudentID = "2" });
10 students.Add(new Student() { Name = "乔宁夏", StudentID = "3" });
11 students.Add(new Student() { Name = "乔宁夏2", StudentID = "4" });
12 students.Add(new Student() { Name = "宓白夜", StudentID = "5" });
13 students.Add(new Student() { Name = "仰灵萱", StudentID = "6" });
14 students.Add(new Student() { Name = "冷杏儿", StudentID = "7" });
15 //通过名字和Id进行过滤
16 var result = students.AsQueryable().Where(GetExpressionTree("心", "1"));
17
18 foreach (var item in result)
19 {
20 Console.WriteLine(item.StudentID);
21 Console.WriteLine(item.Name);
22 }
23 }40 //通过条件构建表达式树
41 private static Expression<Func<Student, bool>> GetExpressionTree(string name, string studentId)
42 {
43 Expression<Func<Student, bool>> expression = x => true;
44
45 var paramExpression = Expression.Parameter(typeof(Student));
46
47 if (!string.IsNullOrEmpty(name))
48 {
49 //名字条件
50 Expression<Func<Student, bool>> nameExpression = x => x.Name.Contains(name);
51 expression = ConcatExpression(expression, nameExpression, paramExpression);
52 }
53
54 if (!string.IsNullOrEmpty(studentId))
55 {
56 //Id条件
57 Expression<Func<Student, bool>> patientIdExpression = x => x.StudentID.Contains(studentId);
58 expression = ConcatExpression(expression, patientIdExpression, paramExpression);
59 }
60
61 return expression;
62 }
63
64 /// <summary>
65 /// 将表达式组合在一起
66 /// </summary>
67 /// <param name="left"></param>
68 /// <param name="right"></param>
69 /// <param name="parameterExpression"></param>
70 /// <returns></returns>
71 private static Expression<Func<Student, bool>> ConcatExpression(Expression<Func<Student, bool>> left, Expression<Func<Student, bool>> right, ParameterExpression parameterExpression)
72 {
73 var expressionBody = Expression.AndAlso(left.Body, right.Body);
74 expressionBody = (BinaryExpression)new ParameterReplacer(parameterExpression).Visit(expressionBody);
75 return Expression.Lambda<Func<Student, bool>>(expressionBody, parameterExpression);
76 }
77 }
78
79 /// <summary>
80 /// 重写表达式树。
81 /// </summary>
82 internal class ParameterReplacer : ExpressionVisitor
83 {
84 private readonly ParameterExpression _parameter;
85
86 protected override Expression VisitParameter(ParameterExpression node)
87 {
88 return base.VisitParameter(_parameter);
89 }
90
91 internal ParameterReplacer(ParameterExpression parameter)
92 {
93 _parameter = parameter;
94 }
95 }
运行结果：
示例代码
参考：
c# - How to Combine two lambdas - Stack Overflow
c# - Combining two expressions (Expression<Func<T, bool>>) - Stack Overflow
How do I combine LINQ expressions into one? - Stack Overflow
Expression Trees - C# | Microsoft Learn
