Question

我的代码只使用一个属性“Name”构建列表。如何修改代码，以便它可以构建具有两个属性“Name”和“Test_Result”的列表我知道匿名类型可用于执行此操作，但如何将它们用于动态表达式？这是我的代码：

string item = "Name";
string item2 = "Test_Result";
Type studentType = typeof(Student);

ParameterExpression itemParam = Expression.Parameter(studentType, item);
MemberInfo itemProperty = studentType.GetProperty(item);

MemberExpression valueInItemField = 
    Expression.MakeMemberAccess(itemParam, itemProperty);

Expression<Func<Student, string>> selectExpression =
    Expression<Func<Student, string>>
        .Lambda<Func<Student, string>>(valueInItemField, itemParam);

IEnumerable<string> currentItemFields = 
    DeserializedStudents.Select(selectExpression.Compile());

Answer 1

我假设这里的“名称”和“Test_Result”是灵活的，不能硬编码。

匿名类型是完全定义的常规类;关于它们唯一有趣的事情是编译器提供了详细信息，而不是你。

我建议处理此方案的方法是使用Tuple.Create创建IEnumerable<Tuple<string,string>>并将其称为Item1，Item2（来自Tuple<,>。另一种选择是使用ExpandoObject之类的内容，然后使用IDictionary<string,object> API或dynamic API来获取值。< / p>

例如：

string item1 = "Name";
string item2 = "Test_Result";
Type studentType = typeof(Student);

var itemParam = Expression.Parameter(studentType, "x");
var member1 = Expression.PropertyOrField(itemParam, item1);
var member2 = Expression.PropertyOrField(itemParam, item2);
var selector = Expression.Call(typeof(Tuple), "Create",
    new[] { member1.Type, member2.Type }, member1, member2);
var lambda = Expression.Lambda<Func<Student, Tuple<string,string>>>(
    selector, itemParam);

var currentItemFields = students.Select(lambda.Compile());

以下是与成员name和result一起投放到自定义类型中的内容：

class ProjectedData
{
    public string name { get; set; }
    public string result { get; set; }
}

...

string item1 = "Name";
string item2 = "Test_Result";
Type studentType = typeof(Student);

var itemParam = Expression.Parameter(studentType, "x");
var member1 = Expression.PropertyOrField(itemParam, item1);
var member2 = Expression.PropertyOrField(itemParam, item2);
var selector = Expression.MemberInit(Expression.New(typeof(ProjectedData)),
    Expression.Bind(typeof(ProjectedData).GetMember("name").Single(), member1),
    Expression.Bind(typeof(ProjectedData).GetMember("result").Single(), member2)
);
var lambda = Expression.Lambda<Func<Student, ProjectedData>>(
    selector, itemParam);

var currentItemFields = students.Select(lambda.Compile());

或者使用字典的方法：

string[] fields = {"Name", "Test_Result"};
Type studentType = typeof(Student);

var itemParam = Expression.Parameter(studentType, "x");

var addMethod = typeof(Dictionary<string, object>).GetMethod(
    "Add", new[] { typeof(string), typeof(object) });
var selector = Expression.ListInit(
        Expression.New(typeof(Dictionary<string,object>)),
        fields.Select(field => Expression.ElementInit(addMethod,
            Expression.Constant(field),
            Expression.Convert(
                Expression.PropertyOrField(itemParam, field),
                typeof(object)
            )
        )));
var lambda = Expression.Lambda<Func<Student, Dictionary<string,object>>>(
    selector, itemParam);

var currentItemFields = students.Select(lambda.Compile());

具有匿名类型的C＃LINQ构建表达式

1 个答案: