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I am trying to connect to a sql database through my C# webapp but i keep getting this error "Object reference not set to an instance of an object ". I am pulling the server name, username and the password from the database and storing it in a variable first. It's all based on the user selection what database they want to connect to.

What I have tried:

string connection = "Data Source= " + @serverName + ";Initial Catalog=master;User ID= " + @rootUser + ";Password= " + @decryptedPass;
               lstDatabase.Visible = true;
               using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings[connection].ConnectionString))
                   using (SqlCommand cmd = new SqlCommand("SELECT name FROM master.dbo.sysdatabases WHERE name NOT IN ('master', 'tempdb', 'model', 'msdb');"))
                       cmd.CommandType = CommandType.Text;
                       cmd.Connection = conn;
                       lstDatabase.DataSource = cmd.ExecuteReader();
                       lstDatabase.DataTextField = "Name";
                       lstDatabase.DataValueField = "name";

Updated 5-Mar-19 7:41am
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Solution 1

0) You can't/shouldn't bind a DataSource to a SqlDataReader. The data reader goes out of scope when the sql connection is closed.

1) Since you're using the using clause, you shouldn't call conn.Close().

That's the best I can do with your primary complaint that "i keep getting this error "Object reference not set to an instance of an object"" - because you didn't tell us where the exception happens.
0x01AA 1-Mar-19 16:48pm
"Since you're using the using clause, you shouldn't call conn.Close()"....what please? Maybe you can write "youd don't need", but "you shouldn't" is completely nonsense!
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Solution 2

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
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Solution 3

string connection = "Data Source= " + @serverName + ";Initial Catalog=master;User ID= " + @rootUser + ";Password= " + @decryptedPass;
using (SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings[connection].ConnectionString))

Does your configuration file contain a <connectionStrings> setting with a name attribute that precisely matches the string you've built?

My guess is it doesn't, since that looks more like an actual connection string than the name of a connection string stored in the config file.

As a result, ConfigurationManager.ConnectionStrings[connection] will return null, and your attempt to retrieve its ConnectionString property will throw a NullReferenceException.

If you're not storing the connection string in the config file, then don't try to read it from the config file:
using (SqlConnection conn = new SqlConnection(connection))

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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