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; }
}
我想在某种情况下生成这样的谓词：
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<Person>("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<Person>("City.ZipCode","MyZipCode");
I'd like get this : x => x.City.ZipCode == "MyZipCode"
或这个 ：
BuildPredicate<Person>("City.Name","MyName");
I'd like get this : x => x.City.Name == "MyName"
或这个 ：
BuildPredicate<Person>("Company.Name","MyCompanyName");
I'd like get this : x => x.Company.Name == "MyCompanyName"
解决方法
(不打算复制Jon – OP联系我提供答案)
以下似乎工作正常：
static Expression<Func<T,bool>> BuildPredicate<T>(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<Func<T,bool>>(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处理null.作为使用示范：
static void Main() {
var pred = BuildPredicate<Person>("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);
}