为什么对 DataRow 空值的这种取消引用不会引发异常？

Question

Could you please look on the code below and say why this code doesn't throw an exception when the column value is null?

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());
}

Answer 1

It's because DBNull.ToString returns an empty string.

DataColumn's AllowDBNull property is set to true by default, otherwise you could not add null values.

Null values are converted to DBNull.Value , AutoIncrement columns are also incremented when null is passed.