获取DataTable选择第一行某一列值

数据源是一个DataTable，现在我们需要获取这个DataTable的第一行第一列的值。
先准备一个数据集，创建一个DataTable，并填充数据：

source code:

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using Insus.NET.Models;

namespace Insus.NET.Entities
{
    public class CustomerEntity
    {
        public DataTable Customers()
        {
            DataTable dt = new DataTable();
            dt.Columns.Add(new DataColumn("CustomerID", typeof(int)));
            dt.Columns.Add(new DataColumn("CustomerName", typeof(string)));
            dt.Columns.Add(new DataColumn("PID", typeof(string)));

            DataRow dr = dt.NewRow();
            dr["CustomerID"] = 9; dr["CustomerName"] = "张三"; dr["PID"] = "123456789012";
            dt.Rows.Add(dr);

            dr = dt.NewRow();
            dr["CustomerID"] = 10; dr["CustomerName"] = "李四"; dr["PID"] = "321245677812";
            dt.Rows.Add(dr);

            dr = dt.NewRow();
            dr["CustomerID"] = 30; dr["CustomerName"] = "吴广"; dr["PID"] = "213445678912";
            dt.Rows.Add(dr);

            dr = dt.NewRow();
            dr["CustomerID"] = 35; dr["CustomerName"] = "王维"; dr["PID"] = "334456789012";
            dt.Rows.Add(dr);

            dr = dt.NewRow();
            dr["CustomerID"] = 36; dr["CustomerName"] = "赵勇"; dr["PID"] = "213445678912";
            dt.Rows.Add(dr);

            return dt;
        }
    }
}