decimal , double , float 輸出 json 的格式問題

之前筆記 decimal 屬性輸出 JSON 時指定的格式問題提到在專案中因為系統介接需要統一 decimal 小數位數，過程中也才發現 json.net 在輸出沒有小數的 decimal 時行為不太一樣(會補上 .0：小數點及小數點一位)，最後雖然有解決問題，但解決方式自己卻不甚滿意，加上想要順帶測試 double 及 float 的行為，所以又花了一些時間找其他方法，就來看看過程遇到的問題及最後的解決方式吧

前提設定

自訂 model (包含 decimal , double , float 屬性)

public class TestData
{
    public decimal Salary { get; set; }
    public double ExRate { get; set; }
    public float TaxRate { get; set; }
}

使用 jsonconvert

public class RoundingJsonConverter : JsonConverter
{
    //指定精準度
    int _precision;
    //指定四捨五入的傾向
    MidpointRounding _rounding;
        
    //預設精準度小數點下 4 位
    public RoundingJsonConverter() : this(4)
    {
    }
        
    public RoundingJsonConverter(int precision)
    : this(precision, MidpointRounding.AwayFromZero)
    {
    }
        
    public RoundingJsonConverter(int precision, MidpointRounding rounding)
    {
    _precision = precision;
    _rounding = rounding;
    }
        
    public override bool CanRead
    {
    get { return false; }
    }
        
    public override bool CanConvert(Type objectType)
    {
    return objectType == typeof(decimal);
    }
        
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
    throw new NotImplementedException();
    }
        
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
    decimal _value = (decimal)value;
    //為值補 0
    writer.WriteValue(decimal.Parse(Math.Round(_value, _precision, _rounding).ToString("0.".PadRight(2 + _precision, '0'))));
    }
}

遇到問題及解決方式

Math.Round 處理 float 會造成數值不正確
Math.Round 沒有 float 版本，使用 Math.Round 會隱含將 float 轉型為 double，而造成數值不正確
- 原始作法
```
var value= 0.225f;
Math.Round(value,2,MidpointRounding.AwayFromZero).Dump();
```
  - 數值不正確
  - Math.Round 傳入 float 會隱含轉型 double
- 新做法：先轉型為 decimal
```
Math.Round(Convert.ToDecimal(value),2,MidpointRounding.AwayFromZero).Dump();
```

為了指定輸出格式頻繁轉型 (ToString 再 parse 回 decimal)

WriteJson 時移除 decimal.parse 與 Math.Round 並使用 WriteRawValue 方法

原始做法

writer.WriteValue(Math.Round(_value, _precision, _rounding).ToString("0.".PadRight(2 + _precision, '0')));

新做法

writer.WriteRawValue((Convert.ToDecimal(value)).ToString("0.".PadRight(2 + _precision, '0')));

完整 jsonconvert 程式碼

public class RoundingJsonConverter : JsonConverter
{
    //指定精準度
    int _precision;
    //指定四捨五入的傾向
    MidpointRounding _rounding;
        
    //預設精準度小數點下 4 位
    public RoundingJsonConverter() : this(4)
    {
    }
        
    public RoundingJsonConverter(int precision)
        : this(precision, MidpointRounding.AwayFromZero)
    {
    }
        
    public RoundingJsonConverter(int precision, MidpointRounding rounding)
    {
        _precision = precision;
        _rounding = rounding;
    }
        
    public override bool CanRead
    {
        get { return false; }
    }
        
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(decimal);
    }
        
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
        
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        decimal _value = (decimal)value;
        //為值補 0
        writer.WriteRawValue((Convert.ToDecimal(value)).ToString("0.".PadRight(2 + _precision, '0')));
    }
}

整數不輸出小數點及小數位

加上屬性來控制是否將數字小數位末端的 0 移除

原做法

新做法

加入屬性用來判斷是否要移除

//用來判斷是否移除最後的 0
bool _truncate;
            
public RoundingJsonConverter(int precision, bool truncate) : this(precision, MidpointRounding.AwayFromZero, truncate)
{
}
            
public RoundingJsonConverter(int precision) : this(precision, MidpointRounding.AwayFromZero, false)
{
}

public RoundingJsonConverter(int precision, MidpointRounding rounding, bool truncate)
{
    _precision = precision;
    _rounding = rounding;
    _truncate = truncate;
}

加入 0 的處理流程

if (_truncate)
{
    var _result = Math.Round(_value, _precision, _rounding) / 1.000000000000000000000000000000000m;//移除0
    if (Int64.TryParse(_result.ToString(), out var longresult))//處理整數
        writer.WriteValue(longresult);
    else//處理非整數
        writer.WriteValue(_result);
}

在 model 上加入是否移除 0

public class TestData
{
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public decimal Salary { get; set; }
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public double ExRate { get; set; }
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public float TaxRate { get; set; }
}

完整程式碼

void Main()
{
    var test = new TestData() { Salary = 1.995m, ExRate = 2.295D, TaxRate = 1f };
    test.Dump();
    JsonConvert.SerializeObject(test).Dump();
}
public class TestData
{
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public decimal Salary { get; set; }
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public double ExRate { get; set; }
    [JsonConverter(typeof(RoundingJsonConverter), 2, true)]
    public float TaxRate { get; set; }
}
        
public class RoundingJsonConverter : JsonConverter
{
    //指定精準度
    int _precision;
    //指定四捨五入的傾向
    MidpointRounding _rounding;
    //用來判斷是否移除最後的 0
    bool _truncate;
        
    //預設精準度小數點下 4 位
    public RoundingJsonConverter() : this(4)
    {
    }
    public RoundingJsonConverter(int precision, bool truncate)
            : this(precision, MidpointRounding.AwayFromZero, truncate)
    {
    }
    public RoundingJsonConverter(int precision)
        : this(precision, MidpointRounding.AwayFromZero, false)
    {
    }
        
    public RoundingJsonConverter(int precision, MidpointRounding rounding, bool truncate)
    {
        _precision = precision;
        _rounding = rounding;
        _truncate = truncate;
    }
        
    public override bool CanRead
    {
        get { return false; }
    }
        
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(decimal) | objectType == typeof(double) | objectType == typeof(float);
        //return objectType == typeof(decimal);
    }
        
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
        
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var _value = Convert.ToDecimal(value);
        if (_truncate)
        {
            var _result = Math.Round(_value, _precision, _rounding) / 1.000000000000000000000000000000000m;//移除0
            if (Int64.TryParse(_result.ToString(), out var longresult))//處理整數
                writer.WriteValue(longresult);
            else//處理非整數
                writer.WriteValue(_result);
        }
        else
            writer.WriteRawValue(_value.ToString("0.".PadRight(2 + _precision, '0')));
    }
}

心得

一開始從想要統一 decimal 的輸出格式，到後來持續調整寫法，到額外擴充支援 double 與 float，最終也可以自訂是否輸出可以刪除的小數點及 0，也許再次用到的機會並不高，但為了 coding for fun 帶來的樂趣無價呀

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