使用DataReader从SQLServer检索数据时处理Null异常 [英] Handling Null Exception while retrieving data from SQLServer using DataReader

问题描述

嗨..
我在C#中使用Asp.net.我的后端是SQL Server.
我正在从数据库中检索数据并在标签中显示字段.
我正在使用以下代码:

Hi..
I am using Asp.net with C#.My Backend is SQL Server.
I am retrieving data from database and displaying fields in labels.
I am using following codes:

string strconSTr = ConfigurationManager.ConnectionStrings["My Connection string"].ConnectionString;
        SqlConnection conn = new SqlConnection(strconSTr);
        SqlCommand cmd = new SqlCommand();
        conn.Open();
        SqlDataReader dr;
        SqlCommand comm = new SqlCommand("select * from Student where EnrolmentNo='" + TextBox1.Text + "'", conn);
        dr = comm.ExecuteReader();
        if(dr.HasRows && dr.Read())       
        {
           Label1.Text = dr.GetString(1);
           Label2.Text = dr.GetString(8);
           Label3.Text = dr.GetString(9);
        }

如果数据库包含空值，则会出现以下异常:
数据为空.无法在Null值上调用此方法或属性.

I am getting following exception if database contains Null Values:
Data is Null. This method or property cannot be called on Null values.
How to handle this exception?

推荐答案

始终建议在使用从数据库中检索到的任何对象/数据之前，应先执行NULL检查.
使用

It is always recommended that NULL check should be done before using any object/data retrieved from database.

Use

Convert.IsDBNull()

功能

一定要记得检查NULL是否用于前端(例如C#)比较，就像

Function

Do remember to check NULL in for Front End (say C#) comparison is done like

if (object == null) 

但是，对于DB，应该为

However , for DB this should be

if(Convert.IsDBNull(object) == true) 

Label1.Text = IIf(dr.GetString(1) Is Convert.DBNull, "", dr.GetString(1))

Label1.Text = IIf(dr.Item("FieldName") Is Convert.DBNull, "", dr.Item("FieldName"))

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