c#：System.Text.Json 的使用一

环境：

• .net 6.0
• vs2022

参考：
从 Newtonsoft.Json 迁移到 System.Text.Json
System.Text.Json 常规用法

一、写入时的控制

1.1 非ascii码转换

直接看代码：

var str = System.Text.Json.JsonSerializer.Serialize(new Model { Id = 1, Name = "小明" });
Console.WriteLine(str);
//out: {"Id":1,"Name":"\u5C0F\u660E"}public class Model
{public int Id { get; set; }public string Name { get; set; }
}

为了能输出 “小明” 而不是 “\u5C0F\u660E”，我们需要显示声明编码方法：

var str = System.Text.Json.JsonSerializer.Serialize(new Model { Id = 1, Name = "小明" }, new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping
});
//out: {"Id":1,"Name":"小明"}
public class Model
{public int Id { get; set; }public string Name { get; set; }
}

比对：Newtonsoft.Json 默认就是 “小明”

1.2 首字母小写

默认类属性和dictionary的key都是原样输出，如：

var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",Properties = new Dictionary<string, object> { { "Age", 18 }, { "Other", new { Name = "小刚" } } }
}, new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping
});
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","Properties":{"Age":18,"Other":{"Name":"小刚"}}}public class Model
{public int Id { get; set; }public string Name { get; set; }public Dictionary<string, object> Properties { get; set; }
}

如果想首字母小写，则：

var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",Properties = new Dictionary<string, object> { { "Age", 18 }, { "Other", new { Name = "小刚" } } }
}, new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//属性名首字母小写PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase,//字典key首字母小写DictionaryKeyPolicy = System.Text.Json.JsonNamingPolicy.CamelCase,
});
Console.WriteLine(str);
//out: {"id":1,"name":"小明","properties":{"age":18,"other":{"name":"小刚"}}}public class Model
{public int Id { get; set; }public string Name { get; set; }public Dictionary<string, object> Properties { get; set; }
}

比对：Newtonsoft.Json 默认也是原样，也可以控制属性首字母小写（但为找到控制字典key首字母小写的方法）

1.3 缩进格式输出

默认输出是一行（上面的效果）,可以指定缩进输出，如下：

var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",Properties = new Dictionary<string, object> { { "Age", 18 }, { "Other", new { Name = "小刚" } } }
}, new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//缩进输出WriteIndented = true,
});
Console.WriteLine(str);
/* out:
{"Id": 1,"Name": "小明","Properties": {"Age": 18,"Other": {"Name": "小刚"}}
}
*/public class Model
{public int Id { get; set; }public string Name { get; set; }public Dictionary<string, object> Properties { get; set; }
}

比对：Newtonsoft.Json 默认也是一行，也可以控制缩进

1.4 将枚举转成字符串输出

默认枚举转为数字输出，如下：

var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",Sex = EnumSex.Male,
}, new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
});
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","Sex":0}public class Model
{public int Id { get; set; }public string Name { get; set; }public EnumSex Sex { get; set; }
}
public enum EnumSex
{Male,FeMale
}

可以添加转换器来转换成字符串：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
};
options.Converters.Add(new System.Text.Json.Serialization.JsonStringEnumConverter());
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",Sex = EnumSex.Male,
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","Sex":"Male"}public class Model
{public int Id { get; set; }public string Name { get; set; }public EnumSex Sex { get; set; }
}
public enum EnumSex
{Male,FeMale
}

比对：Newtonsoft.Json 默认也是转为数字，也可以控制转为字符串

1.5 忽略null值属性的输出

默认情况下，如果某个属性值为null，也会被输出，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = null
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":null}public class Model
{public int Id { get; set; }public string Name { get; set; }
}

如果不想输出null值属性，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingNull,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = null
}, options);
Console.WriteLine(str);
//out: {"Id":1}public class Model
{public int Id { get; set; }public string Name { get; set; }
}

比对：Newtonsoft.Json 默认也是输出null值属性，也可以控制不输出

1.6 忽略循环引用

默认情况下，system.text.json会直接报错，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
};
var fu = new Model
{Id = 1,Name = "小明",Parent = null,Children = new List<Model> { new Model { Id = 2, Name = "小明2", Parent = null } }
};
fu.Children[0].Parent = fu;
var str = System.Text.Json.JsonSerializer.Serialize(fu, options);
Console.WriteLine(str);
/*expception:
Unhandled exception. System.Text.Json.JsonException: A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 64. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles. Path: $.Children.Parent.Children.Parent
*/public class Model
{public int Id { get; set; }public string Name { get; set; }public Model Parent { get; set; }public List<Model> Children { get; set; }
}

我们可以设置忽略循环引用，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//忽略循环引用ReferenceHandler = System.Text.Json.Serialization.ReferenceHandler.IgnoreCycles,
};
var fu = new Model
{Id = 1,Name = "小明",Parent = null,Children = new List<Model> { new Model { Id = 2, Name = "小明2", Parent = null } }
};
fu.Children[0].Parent = fu;
var str = System.Text.Json.JsonSerializer.Serialize(fu, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","Parent":null,"Children":[{"Id":2,"Name":"小明2","Parent":null,"Children":null}]}public class Model
{public int Id { get; set; }public string Name { get; set; }public Model Parent { get; set; }public List<Model> Children { get; set; }
}

比对：Newtonsoft.Json 默认也是检测到循环报错，也可以控制忽略循环引用

1.7 允许输出Field

默认情况下，Field不会被输出，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",FieldAge = 18,
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明"}public class Model
{public int Id { get; set; }public string Name { get; set; }public int FieldAge;
}

为了输出Field，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,// 输出 FieldIncludeFields = true,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",FieldAge = 18,
}, options);
Console.WriteLine(str);
//out: {{"Id":1,"Name":"小明","FieldAge":18}public class Model
{public int Id { get; set; }public string Name { get; set; }public int FieldAge;
}

比对：Newtonsoft.Json 默认输出Field，并且只读Field也会输出，这点需要注意！

1.8 禁止输出只读Field

当我们允许输出Field的时候，只读的Field也会被输出，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//允许输出FieldIncludeFields = true,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",FieldAge = 18,
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","FieldAge":18,"ReadOnlyFieldAge":15}public class Model
{public int Id { get; set; }public string Name { get; set; }public int FieldAge;public readonly int ReadOnlyFieldAge = 15;
}

如果我们不想输出只读的Field，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//允许输出FieldIncludeFields = true,//忽略只读FieldIgnoreReadOnlyFields = true,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",FieldAge = 18,
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","FieldAge":18}public class Model
{public int Id { get; set; }public string Name { get; set; }public int FieldAge;public readonly int ReadOnlyFieldAge = 15;
}

比对：Newtonsoft.Json 默认输出Field，并且只读Field也会输出，这点需要注意！

1.9 禁止输出只读属性

上面对只读的Field做了控制,也可以对只读属性做控制，默认情况下会输出只读属性：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明","IsAdmin":true}public class Model
{public int Id { get; set; }public string Name { get; set; }public bool IsAdmin => Name == "小明";
}

在某些场景下，减少只读属性的输出会便于存储和数据传递（体积减少），但给前端输出数据慎用，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,//忽略只读属性IgnoreReadOnlyProperties = true,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",
}, options);
Console.WriteLine(str);
//out: {"Id":1,"Name":"小明"}public class Model
{public int Id { get; set; }public string Name { get; set; }public bool IsAdmin => Name == "小明";
}

比对：Newtonsoft.Json 默认输出只读属性，没有找到禁止的方法。

1.10 自定义日期格式（DateTime、DateTimeOffset）

默认情况下输出的格式为 ISO 8601-1:2019 参考百度百科 ，默认类似：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,WriteIndented = true,
};
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",sDateTime = DateTime.Parse("2023-03-04 18:36:12.1234567+08:00"),sDateTimeOffset = DateTime.Parse("2023-03-04 18:36:12.1234567+08:00"),sTimeSpan = new TimeSpan(1, 2, 3, 4, 5),
}, options);
Console.WriteLine(str);
/*out:
{"Id": 1,"Name": "小明","sDateTime": "2023-03-04T18:36:12.1234567+08:00","sDateTimeOffset": "2023-03-04T18:36:12.1234567+08:00","sTimeSpan": "1.02:03:04.0050000"
}
*/public class Model
{public int Id { get; set; }public string Name { get; set; }public DateTime sDateTime { get; set; }public DateTimeOffset sDateTimeOffset { get; set; }public TimeSpan sTimeSpan { get; set; }
}

如果我们想支持自定义的格式，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,WriteIndented = true,
};
options.Converters.Add(new JsonConverterDatetime("yyyy-MM-dd HH:mm:ss.fff zz"));
options.Converters.Add(new JsonConverterDateTimeOffset("yyyy-MM-dd HH:mm:ss.fff zz"));
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",sDateTime = DateTime.Parse("2023-03-04 18:36:12.1234567+08:00"),sDateTimeOffset = DateTime.Parse("2023-03-04 18:36:12.1234567+08:00"),sTimeSpan = new TimeSpan(1, 2, 3, 4, 5),
}, options);
Console.WriteLine(str);
/*out:
{"Id": 1,"Name": "小明","sDateTime": "2023-03-04 18:36:12.123 +08","sDateTimeOffset": "2023-03-04 18:36:12.123 +08","sTimeSpan": "1.02:03:04.0050000"
}
*/public class Model
{public int Id { get; set; }public string Name { get; set; }public DateTime sDateTime { get; set; }public DateTimeOffset sDateTimeOffset { get; set; }//public DateOnly sDateOnly { get; set; }//public TimeOnly sTimeOnly { get; set; }public TimeSpan sTimeSpan { get; set; }
}public class JsonConverterDatetime : System.Text.Json.Serialization.JsonConverter<DateTime>
{public string Format { get; set; }public JsonConverterDatetime(string format){if (format == null) throw new ArgumentNullException("format");Format = format;}public override DateTime Read(ref System.Text.Json.Utf8JsonReader reader, Type typeToConvert, System.Text.Json.JsonSerializerOptions options){return DateTime.Parse(reader.GetString());}public override void Write(System.Text.Json.Utf8JsonWriter writer, DateTime value, System.Text.Json.JsonSerializerOptions options){writer.WriteStringValue(value.ToString(Format));}
}public class JsonConverterDateTimeOffset : System.Text.Json.Serialization.JsonConverter<DateTimeOffset>
{public string Format { get; set; }public JsonConverterDateTimeOffset(string format){if (format == null) throw new ArgumentNullException("format");Format = format;}public override DateTimeOffset Read(ref System.Text.Json.Utf8JsonReader reader, Type typeToConvert, System.Text.Json.JsonSerializerOptions options){return DateTimeOffset.Parse(reader.GetString());}public override void Write(System.Text.Json.Utf8JsonWriter writer, DateTimeOffset value, System.Text.Json.JsonSerializerOptions options){writer.WriteStringValue(value.ToString(Format));}
}

顺便说下：ISO的这种时间格式兼容性还是比较好的，比如：mysql中可以插入 '2023-03-04T18:36:12.1234567+08:00'，但不能插入 '2023-03-04 18:36:12.1234567 +08:00'，原因是因为后面的 “+08:00” 前不能有空格。

比对：Newtonsoft.Json 默认也是ISO的格式，也支持自定义格式。

1.11 支持输出 DateOnly、TimeOnly

默认.net 6 （.net7可以）不支持 DateOnly、TimeOnly，报错如下：

• Unhandled exception. System.NotSupportedException: Serialization and deserialization of ‘System.DateOnly’ instances are not supported
• Unhandled exception. System.NotSupportedException: Serialization and deserialization of ‘System.TimeOnly’ instances are not supported

可以自定义转换器支持，如下：

var options = new System.Text.Json.JsonSerializerOptions
{Encoder = System.Text.Encodings.Web.JavaScriptEncoder.UnsafeRelaxedJsonEscaping,WriteIndented = true,
};
options.Converters.Add(new JsonConverterDateOnly("yyyy-MM-dd"));
options.Converters.Add(new JsonConverterTimeOnly("HH:mm:ss.fffffff"));
var str = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Name = "小明",sDateOnly = DateOnly.Parse("2023-03-04"),sTimeOnly = TimeOnly.Parse("18:36:12.1234567"),
}, options);
Console.WriteLine(str);
/*out:
{"Id": 1,"Name": "小明","sDateOnly": "2023-03-04","sTimeOnly": "18:36:12.1234567"
}
*/public class Model
{public int Id { get; set; }public string Name { get; set; }public DateOnly sDateOnly { get; set; }public TimeOnly sTimeOnly { get; set; }
}public class JsonConverterDateOnly : System.Text.Json.Serialization.JsonConverter<DateOnly>
{public string Format { get; set; }public JsonConverterDateOnly(string format){if (format == null) throw new ArgumentNullException("format");Format = format;}public override DateOnly Read(ref System.Text.Json.Utf8JsonReader reader, Type typeToConvert, System.Text.Json.JsonSerializerOptions options){return DateOnly.Parse(reader.GetString());}public override void Write(System.Text.Json.Utf8JsonWriter writer, DateOnly value, System.Text.Json.JsonSerializerOptions options){writer.WriteStringValue(value.ToString(Format));}
}public class JsonConverterTimeOnly : System.Text.Json.Serialization.JsonConverter<TimeOnly>
{public string Format { get; set; }public JsonConverterTimeOnly(string format){if (format == null) throw new ArgumentNullException("format");Format = format;}public override TimeOnly Read(ref System.Text.Json.Utf8JsonReader reader, Type typeToConvert, System.Text.Json.JsonSerializerOptions options){return TimeOnly.Parse(reader.GetString());}public override void Write(System.Text.Json.Utf8JsonWriter writer, TimeOnly value, System.Text.Json.JsonSerializerOptions options){writer.WriteStringValue(value.ToString(Format));}
}

比对：新版本的Newtonsoft.Json 支持DateOnly 和 TimeOnly

1.12 将数字输出为字符串

有的时候，我们需要将数字输出为字符串，如下：

var options = new System.Text.Json.JsonSerializerOptions
{//输出是 number => string, 读取时允许 string => numberNumberHandling = System.Text.Json.Serialization.JsonNumberHandling.AllowReadingFromString | System.Text.Json.Serialization.JsonNumberHandling.WriteAsString,
};var json = System.Text.Json.JsonSerializer.Serialize(new Model
{Id = 1,Score = 99.5f
}, options);
Console.WriteLine(json);
//out: {"Id":"1","Score":"99.5"}
public class Model
{public int Id { get; set; }public float Score { get; set; }
}

比对：Newtonsoft.Json 可以通过转换器将数字转为字符串。

1.13 忽略默认值的属性

默认情况下：属性有默认值也会被输出，我们可以控制当属性/字段有默认值的时候不再输出（传递给前端时慎用）：

var options = new System.Text.Json.JsonSerializerOptions
{DefaultIgnoreCondition = System.Text.Json.Serialization.JsonIgnoreCondition.WhenWritingDefault,IncludeFields = true,IgnoreReadOnlyFields = false,IgnoreReadOnlyProperties = false,
};var json = System.Text.Json.JsonSerializer.Serialize(new Model(), options);
Console.WriteLine(json);
//out: {"Id1":1,"IdR1":1,"IdF1":1,"IdFR1":1}
public class Model
{public int Id0 { get; set; } = 0;public int Id1 { get; set; } = 1;public int IdR0 => Id0;public int IdR1 => Id1;public int IdF0 = 0;public int IdF1 = 1;public readonly int IdFR0 = 0;public readonly int IdFR1 = 1;
}

比对：Newtonsoft.Json 默认也会输出默认值的属性，也可以设置禁止输出。

1.14 在类定义时声明忽略某些属性

[System.Text.Json.Serialization.JsonIgnore]
public int Age { get; set; }

1.15 在类定义时声明控制属性输出的顺序

[Newtonsoft.Json.JsonProperty(Order = 0)]
public int Age { get; set; }

1.16 在类定义时声明控制属性输出的名称

[System.Text.Json.Serialization.JsonPropertyName("catAge")]
public int Age { get; set; }

二、读取时控制（反序列化为模型）

2.1 允许json中多余的逗号

默认情况下，不允许json中存在多余的逗号，如：

{"Id": 1,"Name": "小明",
}

这本身并没有错，因为标准json格式是严格的，不允许这个。。。
如果我们读取上面字符串，就会报错如下：

Unhandled exception. System.Text.Json.JsonException: The JSON object contains a trailing comma at the end which is not supported in this mode. Change the reader options. Path: $ | LineNumber: 4 | BytePositionInLine: 0.
—> System.Text.Json.JsonReaderException: The JSON object contains …

为了能顺利读取到，我们可以使用如下方式：

var options = new System.Text.Json.JsonSerializerOptions
{AllowTrailingCommas = true,
};
var json = @"
{""Id"": 1,""Name"": ""小明"",
}
";
var model = System.Text.Json.JsonSerializer.Deserialize<Model>(json, options);Console.WriteLine($"model.Id={model.Id},model.Id={model.Name}");
//out: model.Id=1,model.Id=小明public class Model
{public int Id { get; set; }public string Name { get; set; }
}

2.2 允许json中存在注释

默认情况下，不允许json中存在注释，如：

//注释
/*
注释
*/
{//注释"Id": 1,"Name": "小明",
}

这本身并没有错，因为标准json格式是严格的，不允许这个。。。
如果我们读取上面字符串，就会报错如下：

Unhandled exception. System.Text.Json.JsonException: ‘/’ is an invalid start of a value. Path: $ | LineNumber: 1 | BytePositionInLine: 0.
—> System.Text.Json.JsonReaderException: ‘/’ is an invalid start of a value. …

为了能顺利读取到，我们可以使用如下方式：

var options = new System.Text.Json.JsonSerializerOptions
{//因为我们是直接反序列化为模型,所以这里 跳过注释ReadCommentHandling = System.Text.Json.JsonCommentHandling.Skip,
};
var json = @"
//注释
/*
注释
*/
{//注释""Id"": 1,""Name"": ""小明""
}
";
var model = System.Text.Json.JsonSerializer.Deserialize<Model>(json, options);Console.WriteLine($"model.Id={model.Id},model.Name={model.Name}");
//out: model.Id=1,model.Name=小明

2.3 允许将json中的 “123” => 123，“NaN” => float.NaN

默认情况下，json中的字符串不能转为数字，如下，会报错：

var json = @"
{""Id"": ""123"",
}
";
var model = System.Text.Json.JsonSerializer.Deserialize<Model>(json);
/*exception:
Unhandled exception. System.Text.Json.JsonException: The JSON value could not be converted to System.Int32. Path: $.Id | LineNumber: 2 | BytePositionInLine: 13.
*/public class Model
{public int Id { get; set; }
}

如果我们想实现自动转换，如下：

var options = new System.Text.Json.JsonSerializerOptions
{//允许将 "123" 转为数字 123NumberHandling = System.Text.Json.Serialization.JsonNumberHandling.AllowReadingFromString,
};
var json = @"
{""Id"": ""123"",""Score"":""NaN""
}
";
var model = System.Text.Json.JsonSerializer.Deserialize<Model>(json, options);Console.WriteLine($"model.Id={model.Id},model.Score={model.Score}");
//model.Id=123,model.Score=NaNpublic class Model
{public int Id { get; set; }public float Score { get; set; }
}

顺便说一下：float中的NaN，Infinity，-Infinity 都是有意义的。