如何正确连接数据表的列的值? [英] How to properly concatenate the values of Columns of a DataTable?

问题描述

在以下代码中，我注意到 A 和 B 列显示了数据.
但是，列 C 和 D 为空.为什么?

In the following code, I noticed that Columns A and B show data.
However, columns C and D are empty. Why?

DataTable _dt = new DataTable();
DataColumn A=_dt.Columns.Add("A");
DataColumn B=_dt.Columns.Add("B");
DataColumn C=_dt.Columns.Add("C",typeof(String),"A+B");
DataColumn D=_dt.Columns.Add("D",typeof(String),"A+'!'+B");

DataRow _dr=_dt.NewRow();
_dr["A"]="A";
_dr["B"]="B";

Console.WriteLine(_dr["A"]);
Console.WriteLine(_dr["B"]);
Console.WriteLine(_dr["C"]); //Shows blank-why? Expected AB
Console.WriteLine(_dr["D"]); //Shows blank-why? Expected A!B

推荐答案

将DataRow添加到DataTable，此时

Add the DataRow to the DataTable, that's when the Expression is first evaluated:

这些是计算列.当 + 符号与数据类型为String的Columns一起使用时，这些值将串联在一起.

These are calculated columns. When the + symbols is used with Columns of data type String, the values are concatenated.

字符串运算符

T o连接一个字符串，使用+字符.的价值DataSet类的CaseSensitive属性确定字符串是否比较是区分大小写的.但是，您可以覆盖该值带有DataTable类的CaseSensitive属性.

DataRow _dr=_dt.NewRow();
_dr["A"]="A";
_dr["B"]="B";
_dt.Rows.Add(_dr);

请注意，以上四行可以写为:

As a note, the four lines above can be written as:

_dt.Rows.Add("A", "B");

结果当然是相同的.列 C 和 D 的值不包含在 object [] 数组中，而是由列的表达式生成的.

The result is of course the same. The values of Columns C and D are not included in the object[] array, but generated by the Columns' Expressions.

现在它应该输出:

A
B
AB
A!B

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