如何使用CopyToDataTable（）在C＃中复制DateTimeMode？ [英] How can I copy DateTimeMode in C# with CopyToDataTable()?

问题描述

我正在使用CopyToDataTable选择更大表的子集。但是， utcDT的DateTimeMode不会复制到新表中：

I am using CopyToDataTable to select a subset of a bigger table. However, the DateTimeMode of "utcDT" is not copied to new table:

        var x = new DataTable();
        x.Columns.Add("utcDT", typeof(System.DateTime));
        x.Columns["utcDT"].DateTimeMode = DataSetDateTime.Utc;
        x.Columns.Add("Symbol", typeof(System.String));

        var newRow = x.NewRow();
        newRow.SetField("utcDT", DateTime.Now);
        newRow.SetField("Symbol","A");
        x.Rows.Add(newRow);
        Console.WriteLine(x.Columns["utcDT"].DateTimeMode.ToString());

        var y = x.AsEnumerable().Where(s => s.Field<string>("Symbol")=="A").CopyToDataTable();
        Console.WriteLine(y.Columns["utcDT"].DateTimeMode.ToString());

如何确保它是

推荐答案

发生了什么事情

CopyToDataTable 仅复制列的名称和类型。

What's happening

CopyToDataTable only copies the columns' names and types.

参考来源：

// We do not copy the same properties that DataView.ToTable does.
// If user needs that functionality, use other CopyToDataTable overloads.
// The reasoning being, the IEnumerator<DataRow> can be sourced from
// different DataTable, so we just use the "Default" instead of resolving the difference.

foreach (DataColumn column in current.Table.Columns)
{
    table.Columns.Add(column.ColumnName, column.DataType);
}

解决方案

1。 AsDataView（）。ToTable（）

使用 AsDataView （请参见使用DataView（从LINQ到DataSet）和 ToTable（）：

var z = x.AsEnumerable()
         .Where(s => s.Field<string>("Symbol") == "A")
         .AsDataView()
         .ToTable();

2。 Clone（）-> CopyToDataTable< T>（IEnumerable< T>，DataTable，LoadOption）

您可以使用< a href = https://docs.microsoft.com/zh-cn/dotnet/api/system.data.datatable.clone?view=netframework-4.8 rel = nofollow noreferrer>克隆其中 克隆DataTable的结构，包括所有DataTable架构和约束。'，后跟 CopyToDataTable ，但这是一个采用目标表的版本。 / p>

2. Clone() -> CopyToDataTable<T>(IEnumerable<T>, DataTable, LoadOption)

You can use Clone which 'Clones the structure of the DataTable, including all DataTable schemas and constraints.' followed by CopyToDataTable, but the one version that takes the target table.

var z = x.Clone();
x.AsEnumerable()
   .Where(s => s.Field<string>("Symbol") == "A")
   .CopyToDataTable(z,LoadOption.OverwriteChanges);

测试

Test

(your dt definition here) 
Console.WriteLine(x.Columns["utcDT"].DateTimeMode);

var y = x.AsEnumerable()
        .Where(s => s.Field<string>("Symbol") == "A")
        .CopyToDataTable<DataRow>();
Console.WriteLine(y.Columns["utcDT"].DateTimeMode);

var z = x.AsEnumerable()
        .Where(s => s.Field<string>("Symbol") == "A")
        .AsDataView()
        .ToTable();
Console.WriteLine(z.Columns["utcDT"].DateTimeMode);

var a = x.Clone();
x.AsEnumerable()
        .Where(s => s.Field<string>("Symbol") == "A")
        .CopyToDataTable(a,LoadOption.OverwriteChanges);
Console.WriteLine(a.Columns["utcDT"].DateTimeMode);

输出

Output

Utc
UnspecifiedLocal
Utc
Utc

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