我们用Newtonsoft.Json.Linq.JObject.GetValue("[key]").ToObject<DataTable>方法把一个JObject对象中的某个节点转换成DataTable格式的数据很方便。一直这样用也没出什么问题。但是最近发现在某些情况下，这种数据转换其实是有一定的问题甚至报错。场景如下：
1. 当数据源类似如下时。当把Data节点的数据转换成DataTable格式时，会默认abc字段为整型，导致在对第二条数据做转换时，50.11会丢失精度，变成整型50
"{\"Data\":[{\"abc\":50},{\"abc\":50.01}]}"
2. 当数据源类似如下时。就会导致第二条数据在转化成INT类型时出错。
"{\"Data\":[{\"abc\":50},{\"abc\":\"xyz\"}]}"
这个就需要用到ToObject的JsonSerializer参数了。通过重写serializer方法，来具体指定某一列的数据类型，从而达到效果。
public class CustomDataTableConverter : DataTableConverter
{
private static void CreateRow(JsonReader reader, DataTable dt)
{
DataRow row = dt.NewRow();
reader.Read();
while(reader.TokenType == JsonToken.PropertyName)
{
string columnName = (string)reader.Value;
reader.Read();
DataColumn column = dt.Columns[columnName];
if(column == null)
{
Type columnDataType = GetColumnDataType(reader);
column = new DataColumn(columnName, columnDataType);
dt.Columns.Add(column);
}
if(column.DataType == typeof(DataTable))
{
if(reader.TokenType == JsonToken.StartArray)
{
reader.Read();
}
DataTable table = new DataTable();
while(reader.TokenType != JsonToken.EndArray)
{
CreateRow(reader, table);
reader.Read();
}
row[columnName] = table;
}
else if(column.DataType.IsArray && (column.DataType != typeof(byte[])))
{
if(reader.TokenType == JsonToken.StartArray)
{
reader.Read();
}
List<object> list = new List<object>();
while(reader.TokenType != JsonToken.EndArray)
{
list.Add(reader.Value);
reader.Read();
}
Array destinationArray = Array.CreateInstance(column.DataType.GetElementType(), list.Count);
Array.Copy(list.ToArray(), destinationArray, list.Count);
row[columnName] = destinationArray;
}
else
{
object val = DBNull.Value;
if(reader.Value != null)
{
if(column.DataType == typeof(string))
{
val = reader.Value.ToString();
}
else
{
if(string.IsNullOrEmpty(reader.Value.ToString()))
{
val = DBNull.Value;
}
else
{
val = Convert.ChangeType(reader.Value, column.DataType);
}
}
row[columnName] = val;
}
reader.Read();
}
row.EndEdit();
dt.Rows.Add(row);
}
}
private static Type GetColumnDataType(JsonReader reader)
{
JsonToken tokenType = reader.TokenType;
switch(tokenType)
{
case JsonToken.StartArray:
reader.Read();
if(reader.TokenType != JsonToken.StartObject)
{
return GetColumnDataType(reader).MakeArrayType();
}
return typeof(DataTable);
//这个地方需要对数值类型做特殊处理。如果能确认是数值列，那么可以用double类型替代，防止精度丢失。如果是字符串类型列，那么使用string类型，防止数据转化出错
case JsonToken.Integer:
return typeof(double);
case JsonToken.Float:
case JsonToken.String:
case JsonToken.Boolean:
case JsonToken.Date:
case JsonToken.Bytes:
return reader.ValueType;
case JsonToken.Null:
case JsonToken.Undefined:
return typeof(string);
}
throw new JsonException($"Unexpected JSON token while reading DataTable: {tokenType}");
}
private static void MakeTableSchema(string schema, DataTable dt)
{
if(!string.IsNullOrEmpty(schema) && dt != null)
{
string[] colinfos = schema.Trim(',').Split(',');
foreach(string colinfo in colinfos)
{
string[] info = colinfo.split('|');
if(!dt.Columns.Contains(info[0]))
{
dt.Columns.Add(info[0], Type.GetType("System." + info[i]));
}
}
}
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
DataTable dt = existingValue as DataTable;
if(dt == null)
{
dt = (objectType == typeof(DataTable)) ? new DataTable() : ((DataTable)Activator.CreateInstance(objectType));
}
if(reader.TokenType == JsonToken.PropertyName)
{
dt.TableName = (string)reader.Value;
reader.Read();
}
if(reader.TokenType == JsonToken.StartObject)
{
reader.Read();
while(reader.TokenType == JsonToken.PropertyName)
{
if((string)reader.Value == "SCHEMA")
{
reader.Read();
MakeTableSchema((string)reader.Value, dt);
reader.Read();
}
if((string)reader.Value == "TABLENAME")
{
reader.Read();
dt.TableName = (string)reader.Value;
reader.Read();
}
else if((string)reader.Value == "DATA")
{
reader.Read();
if(reader.TokenType == JsonToken.StartArray)
{
reader.Read();
}
while(reader.TokenType != JsonToken.EndArray)
{
CreateRow(reader, dt);
reader.Read();
}
}
}
reader.Read();
}
else if(reader.TokenType == JsonToken.StartArray)
{
reader.Read();
while(reader.TokenType != JsonToken.EndArray)
{
Create(reader, dt);
reader.Read();
}
}
return dt;
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
DataTable table = (DataTable)value;
DefaultContractResolver contractResolver = serializer.ContractResolver as DefaultContractResolver;
StringBuilder sb = new StringBuilder();
foreach(DataColumns column in table.Columns)
{
sb.Append(column.ColumnName + "|" + column.DataType.Name + ",");
}
writer.WriteStartObject();
writer.WritePropertyName("SCHEMA");
serializer.Serializer(writer, sb.ToString());
writer.WritePropertyName("TABLENAME");
serializer.Serialize(writer, table.TableName);
writer.WritePropertyName("DATA");
writer.WriteStartArray();
foreach(DataRow row in table.Rows)
{
writer.WriteStartObject();
foreach(DataColumn column in row.Table.Columns)
{
if((serializer.NullValueHandling != NullValueHanding.Ignore) || ((row[column] != null) && (row[column] != DBNull.Value)))
{
writer.WritePropertyName((contractResolver != null) ? contractResolver.GetResolvedPropertyName(column.ColumnName) : column.ColumnName);
serializer.Serialize(writer, row[column]);
}
}
writer.WriteEndObject();
}
writer.WriteEndArray();
writer.WriteEndObject();
}
}
调用方法
JsonSerializer serializer = new JsonSerializer();
serializer.Converts.Add(new CustomDataTableConverter());
JObject.GetValue("[Key]").ToObject<DataTable>(serializer);
这样就能相对正确地把数据转成DataTable。