如何创建和访问作为参数在C中传递的匿名类的新实例?

匿名类型封装了一组 只读的 性质。这说明

1. 为什么? Type.GetFields 对匿名类型调用时返回空数组:匿名类型没有公共字段。

2. 匿名类型上的公共属性是只读的,不能通过调用设置其值 PropertyInfo.SetValue . 如果你打电话 PropertyInfo.GetSetMethod 在匿名类型的属性上,您将收到 null .

事实上,如果你改变

var properties = TypeDescriptor.GetProperties(sample);
while (nwReader.Read()) {
    // No way to create a constructor so this call creates the object without calling a ctor. Could this be a source of the problem?
    T obj = (T)FormatterServices.GetUninitializedObject(typeof(T)); 
    foreach (PropertyDescriptor info in properties) {
        for (int i = 0; i < nwReader.FieldCount; i++) {
            if (info.Name == nwReader.GetName(i)) {
                // This loop runs fine but there is no change to obj!!
                info.SetValue(obj, nwReader[i]);
                break;
            }
        }
    }
    fdList.Add(obj);
}

到

PropertyInfo[] properties = sample.GetType().GetProperties();
while (nwReader.Read()) {
    // No way to create a constructor so this call creates the object without calling a ctor. Could this be a source of the problem?
    T obj = (T)FormatterServices.GetUninitializedObject(typeof(T));
    foreach (PropertyInfo info in properties) {
        for (int i = 0; i < nwReader.FieldCount; i++) {
            if (info.Name == nwReader.GetName(i)) {
                // This loop will throw an exception as PropertyInfo.GetSetMethod fails
                info.SetValue(obj, nwReader[i], null);
                break;
            }
        }
    }
    fdList.Add(obj);
}

您将收到一个异常,通知您找不到属性集方法。

现在,为了解决你的问题,你能做的就是利用 Activator.CreateInstance .很抱歉,我太懒了,无法为您输入代码,但下面将演示如何使用它。

var car = new { Make = "Honda", Model = "Civic", Year = 2008 };
var anothercar = Activator.CreateInstance(car.GetType(), new object[] { "Ford", "Focus", 2005 });

所以只需运行一个循环,正如您所做的那样,来填充您需要传递到的对象数组。 Activator.CreateInstance 然后打电话 Activator.CreateInstance 循环完成后。这里的属性顺序很重要,因为只有当两个匿名类型具有相同数量的属性,并且具有相同的类型和相同的名称时,它们才是相同的。

有关更多信息,请参阅 MSDN page 匿名类型。

最后,这确实是一个旁白,与您的问题无关,但下面的代码

foreach (PropertyDescriptor info in properties) {
    for (int i = 0; i < nwReader.FieldCount; i++) {
        if (info.Name == nwReader.GetName(i)) {
            // This loop runs fine but there is no change to obj!!
            info.SetValue(obj, nwReader[i]);
            break;
        }
    }
}

可以简化为

foreach (PropertyDescriptor info in properties) {
            info.SetValue(obj, nwReader[info.Name]);
}

我也遇到了同样的问题,我通过创建一个新的linq.expression解决了这个问题,这个表达式将完成真正的工作,并将其编译为lambda:下面是我的代码,例如:

我想转换那个调用:

var customers = query.ToList(r => new
            {
                Id = r.Get<int>("Id"),
                Name = r.Get<string>("Name"),
                Age = r.Get<int>("Age"),
                BirthDate = r.Get<DateTime?>("BirthDate"),
                Bio = r.Get<string>("Bio"),
                AccountBalance = r.Get<decimal?>("AccountBalance"),
            });

对那个称呼:

var customers = query.ToList(() => new 
        { 
            Id = default(int),
            Name = default(string),
            Age = default(int), 
            BirthDate = default(DateTime?),
            Bio = default(string), 
            AccountBalance = default(decimal?)
        });

做数据阅读器。从新方法中得到东西,第一个方法是:

public List<T> ToList<T>(FluentSelectQuery query, Func<IDataReader, T> mapper)
    {
        return ToList<T>(mapper, query.ToString(), query.Parameters);
    }

我必须用新方法构建一个表达式:

public List<T> ToList<T>(Expression<Func<T>> type, string sql, params object[] parameters)
        {
            var expression = (NewExpression)type.Body;
            var constructor = expression.Constructor;
            var members = expression.Members.ToList();

            var dataReaderParam = Expression.Parameter(typeof(IDataReader));
            var arguments = members.Select(member => 
                {
                    var memberName = Expression.Constant(member.Name);
                    return Expression.Call(typeof(Utilities), 
                                           "Get", 
                                           new Type[] { ((PropertyInfo)member).PropertyType },  
                                           dataReaderParam, memberName);
                }
            ).ToArray();

            var body = Expression.New(constructor, arguments);

            var mapper = Expression.Lambda<Func<IDataReader, T>>(body, dataReaderParam);

            return ToList<T>(mapper.Compile(), sql, parameters);
        }

这样做,我可以完全避免Activator.CreateInstance或FormatterServices.GetUninitializedObject之类的东西,我敢打赌速度会快得多;)

第2题:

我不知道,但我倾向于使用activator.createObject()而不是formatterServices.getUninitializedObject(),因为您的对象可能创建不正确。getUninitializedObject()不会运行像createObject()这样的默认构造函数,而且您不一定知道t的黑盒中有什么…