System.Text.Json:使用自动转换反序列化 JSON [英] System.Text.Json: Deserialize JSON with automatic casting

问题描述

使用 .Net Core 3 的新 System.Text.Json JsonSerializer，您如何自动转换类型(例如 int 到 string 和 string 到 int)?例如，这会引发异常，因为 JSON 中的 id 是数字，而 C# 的 Product.Id 需要一个字符串:

Using .Net Core 3's new System.Text.Json JsonSerializer, how do you automatically cast types (e.g. int to string and string to int)? For example, this throws an exception because id in JSON is numeric while C#'s Product.Id is expecting a string:

public class HomeController : Controller
{
    public IActionResult Index()
    {
        var json = @"{""id"":1,""name"":""Foo""}";
        var o = JsonSerializer.Deserialize<Product>(json, new JsonSerializerOptions
        {
            PropertyNameCaseInsensitive = true,
        });

        return View();
    }
}

public class Product
{
    public string Id { get; set; }
    public string Name { get; set; }
}

Newtonsoft 的 Json.Net 处理得很好.如果在 C# 期望字符串时传入数值并不重要(反之亦然)，一切都按预期反序列化.如果您无法控制作为 JSON 传入的类型格式，您如何使用 System.Text.Json 处理此问题?

Newtonsoft's Json.Net handled this beautifully. It didn't matter if you were passing in a numeric value while C# was expecting a string (or vice versa), everything got deserialized as expected. How do you handle this using System.Text.Json if you have no control over the type format being passed in as JSON?

推荐答案

1. 新的 System.Text.Json api 公开了一个 JsonConverter api，它允许我们随意转换类型.

1. The new System.Text.Json api exposes a JsonConverter api which allows us to convert the type as we like.

例如，我们可以创建一个通用的number到string的转换器:

For example, we can create a generic number to string converter:

public class AutoNumberToStringConverter : JsonConverter<object>
{
    public override bool CanConvert(Type typeToConvert)
    {
        return typeof(string) == typeToConvert;
    }
    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if(reader.TokenType == JsonTokenType.Number) {
            return reader.TryGetInt64(out long l) ?
                l.ToString():
                reader.GetDouble().ToString();
        }
        if(reader.TokenType == JsonTokenType.String) {
            return reader.GetString();
        }
        using(JsonDocument document = JsonDocument.ParseValue(ref reader)){
            return document.RootElement.Clone().ToString();
        }
    }

    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
    {
        writer.WriteStringValue( value.ToString());
    }
}

使用MVC/Razor Page时，我们可以在启动时注册这个转换器:

When working with MVC/Razor Page, we can register this converter in startup:

services.AddControllersWithViews().AddJsonOptions(opts => {
    opts.JsonSerializerOptions.PropertyNameCaseInsensitive= true;
    opts.JsonSerializerOptions.Converters.Insert(0, new AutoNumberToStringConverter());
});

然后 MVC/Razor 会自动处理类型转换.

and then the MVC/Razor will handle the type conversion automatically.

或者如果你想手动控制序列化/反序列化:

Or if you like to control the serialization/deserialization manually:

var opts = new JsonSerializerOptions {
    PropertyNameCaseInsensitive = true,
};
opts.Converters.Add(new AutoNumberToStringConverter());
var o = JsonSerializer.Deserialize<Product>(json,opts) ;

以类似的方式，您可以启用字符串到数字类型的转换，如下所示:

In a similar way you can enable string to number type conversion as below :

public class AutoStringToNumberConverter : JsonConverter<object>
{
    public override bool CanConvert(Type typeToConvert)
    {
        // see https://stackoverflow.com/questions/1749966/c-sharp-how-to-determine-whether-a-type-is-a-number
        switch (Type.GetTypeCode(typeToConvert))
        {
            case TypeCode.Byte:
            case TypeCode.SByte:
            case TypeCode.UInt16:
            case TypeCode.UInt32:
            case TypeCode.UInt64:
            case TypeCode.Int16:
            case TypeCode.Int32:
            case TypeCode.Int64:
            case TypeCode.Decimal:
            case TypeCode.Double:
            case TypeCode.Single:
            return true;
            default:
            return false;
        }
    }
    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if(reader.TokenType == JsonTokenType.String) {
            var s = reader.GetString() ;
            return int.TryParse(s,out var i) ? 
                i :
                (double.TryParse(s, out var d) ?
                    d :
                    throw new Exception($"unable to parse {s} to number")
                );
        }
        if(reader.TokenType == JsonTokenType.Number) {
            return reader.TryGetInt64(out long l) ?
                l:
                reader.GetDouble();
        }
        using(JsonDocument document = JsonDocument.ParseValue(ref reader)){
            throw new Exception($"unable to parse {document.RootElement.ToString()} to number");
        }
    }


    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
    {
        var str = value.ToString();             // I don't want to write int/decimal/double/...  for each case, so I just convert it to string . You might want to replace it with strong type version.
        if(int.TryParse(str, out var i)){
            writer.WriteNumberValue(i);
        }
        else if(double.TryParse(str, out var d)){
            writer.WriteNumberValue(d);
        }
        else{
            throw new Exception($"unable to parse {str} to number");
        }
    }
}

这篇关于System.Text.Json:使用自动转换反序列化 JSON的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！