您能否查看下面的代码并说明为什么当列值为空时此代码不会引发异常?
DataTable table = new DataTable();
table.Columns.Add("PreviewHtml");
table.Rows.Add(new object[] { "aksdhaskldh" });
table.Rows.Add(new object[] { "129836 128o tagjk 1782 3" });
table.Rows.Add(new object[] { null });
table.Rows.Add(new object[] { "1278o36 " });
foreach (DataRow r in table.Rows)
{
Console.WriteLine(r["PreviewHtml"].ToString());
}