从数据库检索值并将其分配给字符串变量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

You may want to see if dataReader returns any rows first, to prevent null value returns from the database which may cause NullReferenceException so use the following;

string username11=string.empty;

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

Answer 4

用于处理空值更改为

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

Answer 5

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

您输入了错误的列名username11而不是username 。