JSON反序列化过程中类型'System.String'的定义没有参数的构造函数 [英] No parameterless constructor defined for type of 'System.String' during JSON deserialization

问题描述

这看起来应该是很容易的，但我得到一个例外，当我尝试的反序列化一些简单的JSON到一个托管类型。唯一的例外是：

This seems like it should be so easy, but I am getting an exception when I try to deserialize some straightforward JSON into a managed type. The exception is:

MissingMethodException结果
  类型'System.String

MissingMethodException
No parameterless constructor defined for type of 'System.String'

虽然这是事实，有对System.String无参数构造函数，我并不清楚为什么这个问题。

While it is true that there are no parameterless constructors for System.String, I'm not clear as to why this matters.

执行反序列化的code是：

The code that performs the deserialization is:

using System.Web.Script.Serialization;
private static JavaScriptSerializer serializer = new JavaScriptSerializer();
public static MyType Deserialize(string json)
{
    return serializer.Deserialize<MyType>(json);
}

我喜欢的类型大致是：

My type is roughly:

public class MyType
{
    public string id { get; set; }
    public string type { get; set; }
    public List<Double> location { get; set; }
    public Address address { get; set; }
    public Dictionary<string, string> localizedStrings { get; set; }
}

另一类是地址：

public class Address
{
    public string addressLine { get; set; }
    public string suite { get; set; }
    public string locality { get; set; }
    public string subdivisionCode { get; set; }
    public string postalCode { get; set; }
    public string countryRegionCode { get; set; }
    public string countryRegion { get; set; }
}

这里的JSON：

Here's the JSON:

{
    "id": "uniqueString",
    "type": "Foo",
    "location": [
        47.6,
        -122.3321
    ]
    "address": {
        "addressLine": "1000 Fourth Ave",
        "suite": "en-us",
        "locality": "Seattle",
        "subdivisionCode": "WA",
        "postalCode": "98104",
        "countryRegionCode": "US",
        "countryRegion": "United States"
    },
    "localizedStrings": {
        "en-us": "Library",
        "en-ES": "La Biblioteca"
    }
}

我得到同样的例外，甚至如果我的JSON仅仅是：

I get the same exception even if my JSON is just:

{
    "id": "uniquestring"
}

任何人可以告诉我为什么是需要System.String一个参数的构造函数？

Can anybody tell me why a parameterless constructor is needed for System.String?

推荐答案

参数构造函数需要任何形式的反序列化。试想一下，你正在实施一个解串器。您需要：

Parameterless constructors need for any kind of deserialization. Imagine that you are implementing a deserializer. You need to:

1. 获取一个类型从输入流对象（在这种情况下，它的字符串）


2. 实例化的对象。的你没有办法做到这一点，如果没有默认的构造函数的


3. 读取性能/从流值


4. 从流上的步骤2中创建的对象分配的值。

1. Get a type of object from the input stream (in this case it's string)
2. Instantiate the object. You have no way to do that if there is no default constructor.
3. Read the properties/value from stream
4. Assign the values from the stream to the object created on step 2.

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