我们有一个C＃应用程序，可在Excel文档中的工作表上填充表格。
必须按照从数据库返回行的顺序填充表。
对象DataFileColData定义为List，并包含结果集行。出于测试目的，我仅使用列表的[0]。
下面的代码段＃1不起作用。尽管数字本身按顺序列出，但行顺序不会保留，因为最终结果显示的数据是乱序的：
if (DataFileColData[0].Count() > 0)
{
ConcurrentDictionary<int, DataRow> theRows = new ConcurrentDictionary<int, DataRow>(9, DataFileColData[0].Count());
Parallel.For(0, DataFileColData[0].Count(), i =>
{
// go through each column
int c = 0;
try
{
foreach (var Col in DataFileColData)
{
var cell = Col[i];
if (cell != null)
{
if (cell.GetType().Name == "JArray") //If Jarray then table compression was used not column compression
{
if (theRows.TryAdd(i, Dt.NewRow()))
theRows[i].ItemArray = JsonConvert.DeserializeObject<object[]>(Col[i].ToString());
}
else
{
if (theRows.TryAdd(i, Dt.NewRow()))
theRows[i][c] = cell;
}
}
c++;
}
} //try
catch (Exception e)
{
throw new Exception("Exception thrown in \"PublicMethods.cs | RenderExcelFile\" while in foreach loop over DataFileColData: " + e.ToString());
}
} //for
); //parallel
//Add the rows to the datatable in their original order
//(might have gotten skewed from the parallel.for loop)
for (int x = 0; x < theRows.Count; x++)
Dt.Rows.Add(theRows[x]);
//Set the name so it appears nicely in the Excel Name Box dropdown instead of "table1", "table2", etc etc.
Dt.TableName = ExcelTableSpec.TableTitle + " " + r.TableID;
}
下面的代码段2确实适用于行顺序和与保留的每一行相关的数据：
if (DataFileColData[0].Count() > 0)
{
DataRow[] theRows = new DataRow[DataFileColData[0].Count()];
Parallel.For(0, DataFileColData[0].Count(), i =>
{
DataRow Rw = Dt.NewRow();
// go through each column
int c = 0;
try
{
foreach (var Col in DataFileColData)
{
var cell = Col[i];
if (cell != null)
{
if (cell.GetType().Name == "JArray") //If Jarray then table compression was used not column compression
{
lock (theRows)
{
theRows[i] = Dt.NewRow();
theRows[i].ItemArray = JsonConvert.DeserializeObject<object[]>(Col[i].ToString());
}
}
else
{
lock (theRows)
{
theRows[i] = Dt.NewRow();
theRows[i][c] = cell;
}
}
}
c++;
}
} //try
catch (Exception e)
{
throw new Exception("Exception thrown in \"PublicMethods.cs | RenderExcelFile\" while in foreach loop over DataFileColData: " + e.ToString());
}
} //for
); //parallel
//Add the rows to the datatable in their original order
//(might have gotten skewed from the parallel.for loop)
Dt = theRows.CopyToDataTable();
//Set the name so it appears nicely in the Excel Name Box dropdown instead of "table1", "table2", etc etc.
Dt.TableName = ExcelTableSpec.TableTitle + " " + r.TableID;
}
我不明白为什么。我认为不需要锁定机制，因为每个线程都有自己的“ i”实例，并且ConcurrentDictionary应该是线程安全的。
有人可以向我解释为什么代码无法按我认为的方式工作吗？
谢谢！
根据@Enigmativity的以下注释更新的代码。
MSDN文档不是很清楚（无论如何对我来说），但是似乎确实在更新DataTable，即使MSDN文档在执行NewRow（）方法时没有指出它确实这样做了。
以下是新的工作代码：
if (DataFileColData[0].Count() > 0)
{
DataRow[] theRows = new DataRow[DataFileColData[0].Count()];
Parallel.For(0, DataFileColData[0].Count(), i =>
//for (int i = 0; i < DataFileColData[0].Count(); i++)
{
lock (Dt)
{
theRows[i] = Dt.NewRow();
}
// go through each column
int c = 0;
try
{
foreach (var Col in DataFileColData)
{
var cell = Col[i];
if (cell != null)
{
if (cell.GetType().Name == "JArray") //If Jarray then table compression was used not column compression
{
theRows[i].ItemArray = JsonConvert.DeserializeObject<object[]>(Col[i].ToString());
}
else
{
theRows[i][c] = cell;
}
}
c += 1;
} //foreach
} //try
catch (Exception e)
{
throw new Exception("Exception thrown in \"PublicMethods.cs | RenderExcelFile\" while in foreach loop over DataFileColData: " + e.ToString());
}
} //for
); //parallel
//Add the rows to the datatable in their original order
//(might have gotten skewed from the parallel.for loop)
Dt = theRows.CopyToDataTable();
//Set the name so it appears nicely in the Excel Name Box dropdown instead of "table1", "table2", etc etc.
Dt.TableName = ExcelTableSpec.TableTitle + " " + r.TableID;
//cleanup
if (theRows != null)
Array.Clear(theRows, 0, theRows.Length);
theRows = null;
} //if (DataFileColData[0].Count() > 0)
最佳答案
请参阅（MSDN Data Tables）的文档。
关键是：
线程安全
对于多线程读取操作，此类型是安全的。你必须
同步所有写操作。
因此，不是i引起问题的ConcurrentDictionary。
我已经反编译NewRow方法，并且有一个对NewRow(int record)的调用。该代码清楚地显示了写操作。
internal DataRow NewRow(int record)
{
if (-1 == record)
record = this.NewRecord(-1);
this.rowBuilder._record = record;
DataRow row = this.NewRowFromBuilder(this.rowBuilder);
this.recordManager[record] = row;
if (this.dataSet != null)
this.DataSet.OnDataRowCreated(row);
return row;
}