使用字符串结果返回值= Command.ExecuteScalar()时,结果返回null出现错误 [英] Return value using String result=Command.ExecuteScalar() error occurs when result returns null

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问题描述

我要来从数据库它运作良好低于code第一行第一单元格值。但是,当没有结果发现它抛出异常。

如何处理与的DBNull 。结果
我应该改变我的查询?其中一些返回值,如果他们没有记录?

System.NullReferenceException:未将对象引用设置到对象的实例

code:

 公共字符串absentDayNo(日期时间SDATE,日期EDATE,串idemp)
    {
       字符串结果=0;
       字符串更改为MyQuery =SELECT COUNT(idemp_atd)absentDayNo从td_atd其中;
       更改为MyQuery + ='+ SDATE +和+ EDATE +之间absentdate_atd';
       更改为MyQuery + =和idemp_atd ='+ idemp +'GROUP BY idemp_atd;       CMD的SqlCommand =新的SqlCommand(更改为MyQuery,康涅狄格州);
       conn.Open();
当他们没有数据/结果发生//System.NullReferenceException
       。字符串的getValue = cmd.ExecuteScalar()的ToString();
         如果(的getValue!= NULL)
         {
            结果= getValue.ToString();
         }
         conn.Close();
        返回结果;
    }


解决方案

有没有必要继续呼叫的ToString()的getValue 已经是一个字符串。

除此之外的是,这条线可能可能是你的问题:

 字符串的getValue = cmd.ExecuteScalar()的ToString()。

如果没有行 .ExecuteScalar 将返回,所以你需要做一些检查。

例如:

  VAR firstColumn = cmd.ExecuteScalar();如果(firstColumn!= NULL){
    结果= firstColumn.ToString();
}

I want to fetch 1st row 1st cell value from database it works well with below code . But when there is no result found it throws Exception.

How to handle with DBNull .
Should i change my query ? which return some value if theirs no record ?

System.NullReferenceException: Object reference not set to an instance of an object.

Code:

    public string absentDayNo(DateTime sdate, DateTime edate, string idemp)
    { 
       string result="0";
       string myQuery="select COUNT(idemp_atd) absentDayNo from td_atd where ";
       myQuery +=" absentdate_atd between '"+sdate+"' and '"+edate+" ";
       myQuery +=" and idemp_atd='"+idemp+"' group by idemp_atd ";

       SqlCommand cmd = new SqlCommand(myQuery, conn);
       conn.Open();
//System.NullReferenceException occurs when their is no data/result
       string getValue = cmd.ExecuteScalar().ToString();
         if (getValue != null)
         {
            result = getValue.ToString();
         }
         conn.Close();
        return result;
    }

解决方案

There is no need to keep calling .ToString() as getValue is already a string.

Aside that, this line could possibly be your problem:

 string getValue = cmd.ExecuteScalar().ToString();  

If there are no rows .ExecuteScalar will return null so you need to do some checking.

For instance:

var firstColumn = cmd.ExecuteScalar();

if (firstColumn != null) {
    result = firstColumn.ToString();
}

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