c# - retrieve value from database and assign it in to string variable C#

Question

I want to retrieve value from table. Please guys help me. The thing is I want to get that value and check it with a string value.

this code is not working:

SqlCeCommand cmd1 = new SqlCeCommand("SELECT username FROM tmpusername WHERE _id=1", connection);

SqlCeDataReader usernameRdr = null;

usernameRdr = cmd1.ExecuteReader();

while (usernameRdr.Read()){
    string username11 = usernameRdr["username11"].ToString(); 
}

Answer 1

use username for accessing column.

string username11 = usernameRdr["username"].ToString(); 

Your select command is specifying Select username and later you are accessing it using username11. That is why you are not getting the value, instead you should be getting an exception.

Answer 2

Change it to be :

string username11 = usernameRdr["username"].ToString(); 

The data reader indexer takes the name of the column in the SQL result set which is "SELECT username..." in your SQL query.

Answer 3

您可能想查看dataReader是否首先返回任何行，以防止从数据库返回可能导致NullReferenceException的空值，因此请使用以下内容；

字符串用户名11=字符串.empty；

if (usernameRdr.hasRows)
{
while (reader.Read())
{
username11=usernameRdr["username"].ToString();
}
}

Answer 4

string username11 = (string)usernameRdr["username"];

您输入了错误的列名username11而不是username。

Answer 5

用于处理空值更改为，

string username11 = Convert.ToString(usernameRdr["username"]);