您的 JSON 根对象由某些固定键（"sol_keys" 和 "validity_checks"）组成，它们的值都有一些固定模式，以及任意数量的变量键（"782" 数字键），它们的值都共享一个公共模式与固定键值的模式不同：
{
"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
}
}
您只想反序列化变量键，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反序列化固定键值，就好像它是变量键值一样 - 但由于固定键具有数组值，而变量键具有对象值，则会引发异常。如何告诉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变量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>();
}
然后为它定义如下转换器：
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();
}
}
现在您将能够反序列化为MarsWheatherRootObject，如下所示：
var root = await System.Text.Json.JsonSerializer.DeserializeAsync<MarsWheatherRootObject>(
stream,
new System.Text.Json.JsonSerializerOptions
{
PropertyNameCaseInsensitive = true
});
演示小提琴 #1 here.
注意事项：
-
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>();
}
和转换器如下：
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;
}
演示小提琴 #2 here.