使用Expression创建具有嵌套类的谓词

我有这个 ：

public class Company { public int Id { get; set; } public string Name { get; set; } } public class City { public int Id { get; set; } public string Name { get; set; } public int ZipCode { get; set; } } public class Person { public int Id { get; set; } public string FirstName { get; set; } public string LastName { get; set; } public int? Age { get; set; } public City City { get; set; } public Company Company { get; set; } } 

我想像一个案例生成这样的谓词：

 var result = listPerson.Where(x => x.Age == 10).ToList(); 

或这个 ：

 var result = listPerson.Where( x => x.Company.Name == 1234).ToList(); 

或这个 ：

 var result = listPerson.Where( x => x.City.ZipCode == "MyZipCode").ToList(); 

或这个 ：

 var result = listPerson.Where( x => x.Company.Name == "MyCompanyName").ToList(); 

然后我创建了一个“PredicateBuilder”，这是我的工作（我得到类型，如果可以为空，我构建谓词）当我这样做时：

 BuildPredicate("Age", 10); I get this : x => x.Age == 10 

但是当我有这样的嵌套属性时，我不会如何管理：

 BuildPredicate("City.ZipCode", "MyZipCode"); I'd like get this : x => x.City.ZipCode == "MyZipCode" 

或这个 ：

 BuildPredicate("City.Name", "MyName"); I'd like get this : x => x.City.Name == "MyName" 

或这个 ：

 BuildPredicate("Company.Name", "MyCompanyName"); I'd like get this : x => x.Company.Name == "MyCompanyName" 

Answer 1

（不打算复制Jon – OP联系我提供答案）

以下似乎工作正常：

 static Expression> BuildPredicate(string member, object value) { var p = Expression.Parameter(typeof(T)); Expression body = p; foreach (var subMember in member.Split('.')) { body = Expression.PropertyOrField(body, subMember); } return Expression.Lambda>(Expression.Equal( body, Expression.Constant(value, body.Type)), p); } 

这和Jon的答案之间唯一的区别在于它通过告诉Expression.Constant预期的类型是什么来稍微更好地处理null 。 作为使用示范：

 static void Main() { var pred = BuildPredicate("City.Name", "MyCity"); var people = new[] { new Person { City = new City { Name = "Somewhere Else"} }, new Person { City = new City { Name = "MyCity"} }, }; var person = people.AsQueryable().Single(pred); } 

Answer 2

您只需要通过点分割表达式，然后使用Expression.Property多次迭代它。 像这样的东西：

 string[] properties = path.Split('.'); var parameter = Expression.Parameter(typeof(T), "x"); var lhs = parameter; foreach (var property in properties) { lhs = Expression.Property(lhs, property); } // I've assumed that the target is a string, given the question. If that's // not the case, look at Marc's answer. var rhs = Expression.Constant(targetValue, typeof(string)); var predicate = Expression.Equals(lhs, rhs); var lambda = Expression.Lambda>(predicate, parameter);