如何在 .net core 3.0 中使用 System.Text.Json 反序列化 json 的一部分？
How to deserialize part of json using System.Text.Json in .net core 3.0?
我有一个 json 从这里 https://api.nasa.gov/insight_weather/?api_key=DEMO_KEY&feedtype=json&ver=1.0 看起来像：
{
"782": {
"First_UTC": "2021-02-06T17:08:11Z",
"Last_UTC": "2021-02-07T17:47:46Z",
"Month_ordinal": 12,
"Northern_season": "late winter",
"PRE": {
"av": 721.77,
"ct": 113450,
"mn": 698.8193,
"mx": 742.2686
},
"Season": "winter",
"Southern_season": "late summer",
"WD": {
"most_common": null
}
},
"783": {
"First_UTC": "2021-02-07T17:47:46Z",
"Last_UTC": "2021-02-08T18:27:22Z",
"Month_ordinal": 12,
"Northern_season": "late winter",
"PRE": {
"av": 722.186,
"ct": 107270,
"mn": 698.7664,
"mx": 743.1983
},
"Season": "winter",
"Southern_season": "late summer",
"WD": {
"most_common": null
}
},
"sol_keys": [ "782", "783" ],
"validity_checks": { /* Some complex object */ }
}
我只需要部分信息，所以我创建了以下内容 类:
public class MarsWheather {
[JsonPropertyName("First_UTC")]
public DateTime FirstUTC { get; set; }
[JsonPropertyName("Last_UTC")]
public DateTime LastUTC { get; set; }
[JsonPropertyName("Season")]
[JsonConverter(typeof(JsonStringEnumConverter))]
public Season MarsSeason { get; set; }
[JsonPropertyName("PRE")]
public DataDescription AtmosphericPressure { get; set; }
}
public enum Season {
winter,
spring,
summer,
autumn
}
public class DataDescription{
[JsonPropertyName("av")]
public double Average { get; set; }
[JsonPropertyName("ct")]
public double TotalCount { get; set; }
[JsonPropertyName("mn")]
public double Minimum { get; set; }
[JsonPropertyName("mx")]
public double Maximum { get; set; }
}
问题是来自 NASA 的 JSON 根对象包含我不需要并想跳过的属性 "validity_checks" 和 "sol_keys"。在 Newton.Json 中，我使用 JObject.Parse 来执行此操作，但在 System.Text.Json 中，我想使用
JsonSerializer.DeserializeAsync<Dictionary<string, MarsWheather>>(stream, new JsonSerializerOptions { IgnoreNullValues = true });
不幸的是，当我这样做时出现异常：
System.Text.Json.JsonException: The JSON value could not be converted to MarsWheather. Path: $.sol_keys | LineNumber: 120 | BytePositionInLine: 15.
演示 fiddle here.
可能吗？
您的 JSON 根对象由某些固定键（"sol_keys" 和 "validity_checks"）组成，每个键的值都有一些固定模式，以及任意数量的可变键（"782" 数字键），其值都共享一个与固定键值的模式不同的公共模式：
{
"782": {
// Properties corresponding to your MarsWheather object
},
"783": {
// Properties corresponding to your MarsWheather object
},
// Other variable numeric key/value pairs corresponding to KeyValuePair<string, MarsWheather>
"sol_keys": [
// Some array values you don't care about
],
"validity_checks": {
// Some object you don't care about
}
}
您只想反序列化可变键，但是当您尝试反序列化为 Dictionary<string, MarsWheather> 时会出现异常，因为序列化程序会尝试将固定键值反序列化，就好像它是可变键值一样 - - 但由于固定键具有数组值而可变键具有对象值，因此会抛出异常。如何让 System.Text.Json 跳过已知的固定键而不是尝试反序列化它们？
如果您只想反序列化可变键并跳过固定的已知键，则需要创建一个 custom JsonConverter。最简单的方法是首先为您的字典创建一些根对象：
[JsonConverter(typeof(MarsWheatherRootObjectConverter))]
public class MarsWheatherRootObject
{
public Dictionary<string, MarsWheather> MarsWheathers { get; } = new Dictionary<string, MarsWheather>();
}
然后为其定义如下转换器：
public class MarsWheatherRootObjectConverter : FixedAndvariablePropertyNameObjectConverter<MarsWheatherRootObject, Dictionary<string, MarsWheather>, MarsWheather>
{
static readonly Dictionary<string, ReadFixedKeyMethod> FixedKeyReadMethods = new Dictionary<string, ReadFixedKeyMethod>(StringComparer.OrdinalIgnoreCase)
{
{ "sol_keys", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() },
{ "validity_checks", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() },
};
protected override Dictionary<string, MarsWheather> GetDictionary(MarsWheatherRootObject obj) => obj.MarsWheathers;
protected override void SetDictionary(MarsWheatherRootObject obj, Dictionary<string, MarsWheather> dictionary) => throw new RowNotInTableException();
protected override bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method) => FixedKeyReadMethods.TryGetValue(name, out method);
protected override IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options) => Enumerable.Empty<KeyValuePair<string, WriteFixedKeyMethod>>();
}
public abstract class FixedAndvariablePropertyNameObjectConverter<TObject, TDictionary, TValue> : JsonConverter<TObject>
where TDictionary : class, IDictionary<string, TValue>, new()
where TObject : new()
{
protected delegate void ReadFixedKeyMethod(ref Utf8JsonReader reader, TObject obj, string name, JsonSerializerOptions options);
protected delegate void WriteFixedKeyMethod(Utf8JsonWriter writer, TObject value, JsonSerializerOptions options);
protected abstract TDictionary GetDictionary(TObject obj);
protected abstract void SetDictionary(TObject obj, TDictionary dictionary);
protected abstract bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method);
protected abstract IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options);
public override TObject Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType == JsonTokenType.Null)
return (typeToConvert.IsValueType && Nullable.GetUnderlyingType(typeToConvert) == null)
? throw new JsonException(string.Format("Unepected token {0}", reader.TokenType))
: default(TObject);
if (reader.TokenType != JsonTokenType.StartObject)
throw new JsonException(string.Format("Unepected token {0}", reader.TokenType));
var obj = new TObject();
var dictionary = GetDictionary(obj);
var valueConverter = (typeof(TValue) == typeof(object) ? null : (JsonConverter<TValue>)options.GetConverter(typeof(TValue))); // Encountered a bug using the builtin ObjectConverter
while (reader.Read())
{
if (reader.TokenType == JsonTokenType.PropertyName)
{
var name = reader.GetString();
reader.ReadAndAssert();
if (TryGetFixedKeyReadMethod(name, options, out var method))
{
method(ref reader, obj, name, options);
}
else
{
if (dictionary == null)
SetDictionary(obj, dictionary = new TDictionary());
dictionary.Add(name, valueConverter.ReadOrDeserialize(ref reader, typeof(TValue), options));
}
}
else if (reader.TokenType == JsonTokenType.EndObject)
{
return obj;
}
else
{
throw new JsonException(string.Format("Unepected token {0}", reader.TokenType));
}
}
throw new JsonException(); // Truncated file
}
public override void Write(Utf8JsonWriter writer, TObject value, JsonSerializerOptions options)
{
writer.WriteStartObject();
var dictionary = GetDictionary(value);
if (dictionary != null)
{
var valueConverter = (typeof(TValue) == typeof(object) ? null : (JsonConverter<TValue>)options.GetConverter(typeof(TValue))); // Encountered a bug using the builtin ObjectConverter
foreach (var pair in dictionary)
{
// TODO: handle DictionaryKeyPolicy
writer.WritePropertyName(pair.Key);
valueConverter.WriteOrSerialize(writer, pair.Value, typeof(TValue), options);
}
}
foreach (var pair in GetFixedKeyWriteMethods(options))
{
writer.WritePropertyName(pair.Key);
pair.Value(writer, value, options);
}
writer.WriteEndObject();
}
}
public static partial class JsonExtensions
{
public static void WriteOrSerialize<T>(this JsonConverter<T> converter, Utf8JsonWriter writer, T value, Type type, JsonSerializerOptions options)
{
if (converter != null)
converter.Write(writer, value, options);
else
JsonSerializer.Serialize(writer, value, type, options);
}
public static T ReadOrDeserialize<T>(this JsonConverter<T> converter, ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
=> converter != null ? converter.Read(ref reader, typeToConvert, options) : (T)JsonSerializer.Deserialize(ref reader, typeToConvert, options);
public static void ReadAndAssert(this ref Utf8JsonReader reader)
{
if (!reader.Read())
throw new JsonException();
}
}
现在您可以反序列化为 MarsWheatherRootObject，如下所示：
var root = await System.Text.Json.JsonSerializer.DeserializeAsync<MarsWheatherRootObject>(
stream,
new System.Text.Json.JsonSerializerOptions
{
PropertyNameCaseInsensitive = true
});
演示 fiddle #1 here.
备注：
FixedAndvariablePropertyNameObjectConverter<TObject, TDictionary, TValue> 提供了一个通用框架，用于序列化和反序列化具有固定和可变属性的对象。如果稍后您决定反序列化，例如"sol_keys"，你可以修改MarsWheatherRootObject如下：
[JsonConverter(typeof(MarsWheatherRootObjectConverter))]
public class MarsWheatherRootObject
{
public Dictionary<string, MarsWheather> MarsWheathers { get; } = new Dictionary<string, MarsWheather>();
public List<string> SolKeys { get; set; } = new List<string>();
}
而转换器如下：
public class MarsWheatherRootObjectConverter : FixedAndvariablePropertyNameObjectConverter<MarsWheatherRootObject, Dictionary<string, MarsWheather>, MarsWheather>
{
static readonly Dictionary<string, ReadFixedKeyMethod> FixedKeyReadMethods = new(StringComparer.OrdinalIgnoreCase)
{
{ "sol_keys", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) =>
{
obj.SolKeys = JsonSerializer.Deserialize<List<string>>(ref reader, options);
}
},
{ "validity_checks", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() },
};
static readonly Dictionary<string, WriteFixedKeyMethod> FixedKeyWriteMethods = new Dictionary<string, WriteFixedKeyMethod>()
{
{ "sol_keys", (w, v, o) =>
{
JsonSerializer.Serialize(w, v.SolKeys, o);
}
},
};
protected override Dictionary<string, MarsWheather> GetDictionary(MarsWheatherRootObject obj) => obj.MarsWheathers;
protected override void SetDictionary(MarsWheatherRootObject obj, Dictionary<string, MarsWheather> dictionary) => throw new RowNotInTableException();
protected override bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method) => FixedKeyReadMethods.TryGetValue(name, out method);
protected override IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options) => FixedKeyWriteMethods;
}
演示 fiddle #2 here.
我有一个 json 从这里 https://api.nasa.gov/insight_weather/?api_key=DEMO_KEY&feedtype=json&ver=1.0 看起来像：
{
"782": {
"First_UTC": "2021-02-06T17:08:11Z",
"Last_UTC": "2021-02-07T17:47:46Z",
"Month_ordinal": 12,
"Northern_season": "late winter",
"PRE": {
"av": 721.77,
"ct": 113450,
"mn": 698.8193,
"mx": 742.2686
},
"Season": "winter",
"Southern_season": "late summer",
"WD": {
"most_common": null
}
},
"783": {
"First_UTC": "2021-02-07T17:47:46Z",
"Last_UTC": "2021-02-08T18:27:22Z",
"Month_ordinal": 12,
"Northern_season": "late winter",
"PRE": {
"av": 722.186,
"ct": 107270,
"mn": 698.7664,
"mx": 743.1983
},
"Season": "winter",
"Southern_season": "late summer",
"WD": {
"most_common": null
}
},
"sol_keys": [ "782", "783" ],
"validity_checks": { /* Some complex object */ }
}
我只需要部分信息，所以我创建了以下内容 类:
public class MarsWheather {
[JsonPropertyName("First_UTC")]
public DateTime FirstUTC { get; set; }
[JsonPropertyName("Last_UTC")]
public DateTime LastUTC { get; set; }
[JsonPropertyName("Season")]
[JsonConverter(typeof(JsonStringEnumConverter))]
public Season MarsSeason { get; set; }
[JsonPropertyName("PRE")]
public DataDescription AtmosphericPressure { get; set; }
}
public enum Season {
winter,
spring,
summer,
autumn
}
public class DataDescription{
[JsonPropertyName("av")]
public double Average { get; set; }
[JsonPropertyName("ct")]
public double TotalCount { get; set; }
[JsonPropertyName("mn")]
public double Minimum { get; set; }
[JsonPropertyName("mx")]
public double Maximum { get; set; }
}
问题是来自 NASA 的 JSON 根对象包含我不需要并想跳过的属性 "validity_checks" 和 "sol_keys"。在 Newton.Json 中，我使用 JObject.Parse 来执行此操作，但在 System.Text.Json 中，我想使用
JsonSerializer.DeserializeAsync<Dictionary<string, MarsWheather>>(stream, new JsonSerializerOptions { IgnoreNullValues = true });
不幸的是，当我这样做时出现异常：
System.Text.Json.JsonException: The JSON value could not be converted to MarsWheather. Path: $.sol_keys | LineNumber: 120 | BytePositionInLine: 15.
演示 fiddle here.
可能吗？
您的 JSON 根对象由某些固定键（"sol_keys" 和 "validity_checks"）组成，每个键的值都有一些固定模式，以及任意数量的可变键（"782" 数字键），其值都共享一个与固定键值的模式不同的公共模式：
{
"782": {
// Properties corresponding to your MarsWheather object
},
"783": {
// Properties corresponding to your MarsWheather object
},
// Other variable numeric key/value pairs corresponding to KeyValuePair<string, MarsWheather>
"sol_keys": [
// Some array values you don't care about
],
"validity_checks": {
// Some object you don't care about
}
}
您只想反序列化可变键，但是当您尝试反序列化为 Dictionary<string, MarsWheather> 时会出现异常，因为序列化程序会尝试将固定键值反序列化，就好像它是可变键值一样 - - 但由于固定键具有数组值而可变键具有对象值，因此会抛出异常。如何让 System.Text.Json 跳过已知的固定键而不是尝试反序列化它们？
如果您只想反序列化可变键并跳过固定的已知键，则需要创建一个 custom JsonConverter。最简单的方法是首先为您的字典创建一些根对象：
[JsonConverter(typeof(MarsWheatherRootObjectConverter))]
public class MarsWheatherRootObject
{
public Dictionary<string, MarsWheather> MarsWheathers { get; } = new Dictionary<string, MarsWheather>();
}
然后为其定义如下转换器：
public class MarsWheatherRootObjectConverter : FixedAndvariablePropertyNameObjectConverter<MarsWheatherRootObject, Dictionary<string, MarsWheather>, MarsWheather>
{
static readonly Dictionary<string, ReadFixedKeyMethod> FixedKeyReadMethods = new Dictionary<string, ReadFixedKeyMethod>(StringComparer.OrdinalIgnoreCase)
{
{ "sol_keys", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() },
{ "validity_checks", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() },
};
protected override Dictionary<string, MarsWheather> GetDictionary(MarsWheatherRootObject obj) => obj.MarsWheathers;
protected override void SetDictionary(MarsWheatherRootObject obj, Dictionary<string, MarsWheather> dictionary) => throw new RowNotInTableException();
protected override bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method) => FixedKeyReadMethods.TryGetValue(name, out method);
protected override IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options) => Enumerable.Empty<KeyValuePair<string, WriteFixedKeyMethod>>();
}
public abstract class FixedAndvariablePropertyNameObjectConverter<TObject, TDictionary, TValue> : JsonConverter<TObject>
where TDictionary : class, IDictionary<string, TValue>, new()
where TObject : new()
{
protected delegate void ReadFixedKeyMethod(ref Utf8JsonReader reader, TObject obj, string name, JsonSerializerOptions options);
protected delegate void WriteFixedKeyMethod(Utf8JsonWriter writer, TObject value, JsonSerializerOptions options);
protected abstract TDictionary GetDictionary(TObject obj);
protected abstract void SetDictionary(TObject obj, TDictionary dictionary);
protected abstract bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method);
protected abstract IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options);
public override TObject Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
if (reader.TokenType == JsonTokenType.Null)
return (typeToConvert.IsValueType && Nullable.GetUnderlyingType(typeToConvert) == null)
? throw new JsonException(string.Format("Unepected token {0}", reader.TokenType))
: default(TObject);
if (reader.TokenType != JsonTokenType.StartObject)
throw new JsonException(string.Format("Unepected token {0}", reader.TokenType));
var obj = new TObject();
var dictionary = GetDictionary(obj);
var valueConverter = (typeof(TValue) == typeof(object) ? null : (JsonConverter<TValue>)options.GetConverter(typeof(TValue))); // Encountered a bug using the builtin ObjectConverter
while (reader.Read())
{
if (reader.TokenType == JsonTokenType.PropertyName)
{
var name = reader.GetString();
reader.ReadAndAssert();
if (TryGetFixedKeyReadMethod(name, options, out var method))
{
method(ref reader, obj, name, options);
}
else
{
if (dictionary == null)
SetDictionary(obj, dictionary = new TDictionary());
dictionary.Add(name, valueConverter.ReadOrDeserialize(ref reader, typeof(TValue), options));
}
}
else if (reader.TokenType == JsonTokenType.EndObject)
{
return obj;
}
else
{
throw new JsonException(string.Format("Unepected token {0}", reader.TokenType));
}
}
throw new JsonException(); // Truncated file
}
public override void Write(Utf8JsonWriter writer, TObject value, JsonSerializerOptions options)
{
writer.WriteStartObject();
var dictionary = GetDictionary(value);
if (dictionary != null)
{
var valueConverter = (typeof(TValue) == typeof(object) ? null : (JsonConverter<TValue>)options.GetConverter(typeof(TValue))); // Encountered a bug using the builtin ObjectConverter
foreach (var pair in dictionary)
{
// TODO: handle DictionaryKeyPolicy
writer.WritePropertyName(pair.Key);
valueConverter.WriteOrSerialize(writer, pair.Value, typeof(TValue), options);
}
}
foreach (var pair in GetFixedKeyWriteMethods(options))
{
writer.WritePropertyName(pair.Key);
pair.Value(writer, value, options);
}
writer.WriteEndObject();
}
}
public static partial class JsonExtensions
{
public static void WriteOrSerialize<T>(this JsonConverter<T> converter, Utf8JsonWriter writer, T value, Type type, JsonSerializerOptions options)
{
if (converter != null)
converter.Write(writer, value, options);
else
JsonSerializer.Serialize(writer, value, type, options);
}
public static T ReadOrDeserialize<T>(this JsonConverter<T> converter, ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
=> converter != null ? converter.Read(ref reader, typeToConvert, options) : (T)JsonSerializer.Deserialize(ref reader, typeToConvert, options);
public static void ReadAndAssert(this ref Utf8JsonReader reader)
{
if (!reader.Read())
throw new JsonException();
}
}
现在您可以反序列化为 MarsWheatherRootObject，如下所示：
var root = await System.Text.Json.JsonSerializer.DeserializeAsync<MarsWheatherRootObject>(
stream,
new System.Text.Json.JsonSerializerOptions
{
PropertyNameCaseInsensitive = true
});
演示 fiddle #1 here.
备注：
FixedAndvariablePropertyNameObjectConverter<TObject, TDictionary, TValue>提供了一个通用框架，用于序列化和反序列化具有固定和可变属性的对象。如果稍后您决定反序列化，例如"sol_keys"，你可以修改MarsWheatherRootObject如下：[JsonConverter(typeof(MarsWheatherRootObjectConverter))] public class MarsWheatherRootObject { public Dictionary<string, MarsWheather> MarsWheathers { get; } = new Dictionary<string, MarsWheather>(); public List<string> SolKeys { get; set; } = new List<string>(); }而转换器如下：
public class MarsWheatherRootObjectConverter : FixedAndvariablePropertyNameObjectConverter<MarsWheatherRootObject, Dictionary<string, MarsWheather>, MarsWheather> { static readonly Dictionary<string, ReadFixedKeyMethod> FixedKeyReadMethods = new(StringComparer.OrdinalIgnoreCase) { { "sol_keys", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => { obj.SolKeys = JsonSerializer.Deserialize<List<string>>(ref reader, options); } }, { "validity_checks", (ref Utf8JsonReader reader, MarsWheatherRootObject obj, string name, JsonSerializerOptions options) => reader.Skip() }, }; static readonly Dictionary<string, WriteFixedKeyMethod> FixedKeyWriteMethods = new Dictionary<string, WriteFixedKeyMethod>() { { "sol_keys", (w, v, o) => { JsonSerializer.Serialize(w, v.SolKeys, o); } }, }; protected override Dictionary<string, MarsWheather> GetDictionary(MarsWheatherRootObject obj) => obj.MarsWheathers; protected override void SetDictionary(MarsWheatherRootObject obj, Dictionary<string, MarsWheather> dictionary) => throw new RowNotInTableException(); protected override bool TryGetFixedKeyReadMethod(string name, JsonSerializerOptions options, out ReadFixedKeyMethod method) => FixedKeyReadMethods.TryGetValue(name, out method); protected override IEnumerable<KeyValuePair<string, WriteFixedKeyMethod>> GetFixedKeyWriteMethods(JsonSerializerOptions options) => FixedKeyWriteMethods; }演示 fiddle #2 here.