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ExecuteScalar, ExcuteReader...There is already an open DataReader associated with this Command which must be closed first

There is no open data reader when I call the second sql operation yet I'm getting
"There is already an open DataReader..." exception.
I can replace the second sql operation (e.g. ExecuteReader) with another ExecuteScaler with no issues. And I know if I opened a datareader first without closing it and then call ExecuteScalar, I would get the exception. (btw, I know you can't update and iterate over the same datareader). It seems you can't use ExecuteScalar and ExecuteReader with the same SqlCommand???
cmd.CommandText = "SELECT COUNT(*) FROM xxx WHERE yyy= #t1";
cmd.Parameters.Add("#t1", SqlDbType.NVarChar).Value = uc.CurrentT.ToString();
var empCount = (Int32)cmd.ExecuteScalar();
if (empCount != 1)
throw new Exception($"Error determining if member exists for tax = {uc.CurrentT}. Expected count = 1, actual count = {empCount}. (Error Code = 1009)");
//member exists and there is only 1 of them...get member's empid
MOS mos = new MOS();
cmd.Parameters.Clear();
cmd.CommandText = "SELECT ID FROM xxx WHERE yyy = #t1";
cmd.Parameters.Add("#t1", SqlDbType.NVarChar).Value = uc.CurrentT.ToString();
var reader1 = cmd.ExecuteReader();
throw new Exception("foobar here");
On var reader1 = cmd.ExecuteReader(), the "open DataReader" exception gets thrown. The ExecuteScalar executes with no issue. I want to use the same command.

You need MARS (multiple active result sets).
Otherwise your program will make sure you only have one open cursor on the database. That means iterating through one result set, creating another for each record will fail.
Add this to your connection string: ;MultipleActiveResultSets=True to activate multiple active result sets.

Retrieving more than one value from database and store them

What I'm trying to do is retrieve the FullName values where the Username corresponds to the user, which does indeed work, the problem is I don't exactly know how to store the values when there is more than one value, I tried using an array but when there is for example two values, when retrieving it, characterReader[0] will be null and characterReader[1] will have only the first retrieved value, however if there is only 1 value to be retrieve characterReader[0] will no longer be null and display the correct value.
This is my code, I'm not exactly sure this is even the right way:
SqlCommand displayCharactersCMD = new SqlCommand(String.Format("SELECT FullName FROM [Characters] WHERE Username='{0}'", username), con);
displayCharactersCMD.Parameters.AddWithValue("#checkPlayerName", username);
using (SqlDataReader reader = displayCharactersCMD.ExecuteReader())
{
int counter = 0;
while (reader.Read())
{
if (counter != countCharsToVar)
{
characterReader = new string[countCharsToVar];
characterReader[counter] = reader[0].ToString();
counter++;
}
else
break;
}
}
Example when there are two values to be retrieved:
API.consoleOutput("CHAR 1: " + characterReader[0]); - This will become null.
API.consoleOutput("CHAR 2: " + characterReader[1]); - This will contain the first value.
How I intend it to work:
API.consoleOutput("CHAR 1: " + characterReader[0]); - This will display first value.
API.consoleOutput("CHAR 2: " + characterReader[1]); - This will display second value.

Instead of storing values in array, you can utilize List<>. This might help you:
SqlCommand displayCharactersCMD = new SqlCommand("SELECT FullName FROM [Characters] WHERE Username=#checkPlayerName");
displayCharactersCMD.Parameters.AddWithValue("#checkPlayerName", username);
var characterReader = new List<string>();
using (SqlDataReader reader = displayCharactersCMD.ExecuteReader())
{
while (reader.Read())
{
characterReader.Add(reader[0].ToString());
}
}

Here's a really good link from Microsoft: https://learn.microsoft.com/en-us/dotnet/framework/data/adonet/retrieving-data-using-a-datareader
Use the GetString(columnNumber) method to get the whole value from the row. That should make it easy for you.
Hope this helps.

You need to iterate SqlDataReader for each value, calling reader.Read() makes the reader to point to the next row, when it reaches the end and there is no more rows in the resultset it returns false.
When a "read" is done, the reader is moved to point to the next row so you can access all the columns in this way. reader[0] will be the first column, reader[1] for the second column and so on, in your example you only have one column fullname.
You can add all your results to a list in this way:
var values = new List<string>();
using (SqlDataReader reader = displayCharactersCMD.ExecuteReader())
{
while (reader.Read())
{
values.Add(reader[0]);
}
}
Note: as #steve points, parameter does not work that way, you remove the string.format call and use the name of the parameter.
new SqlCommand("SELECT FullName FROM [Characters] WHERE Username=#checkPlayerName"), con);

Difference between SqlDataReader.Read and SqlDataReader.NextResult

What is the main difference between these two methods? On the msdn website it is explained like below but I don't understand it.
Read Advances the SqlDataReader to the next record. (Overrides
DbDataReader.Read().)
NextResult Advances the data reader to the next
result, when reading the results of batch Transact-SQL statements. (Overrides dbDataReader.NextResult().)

If your statement/proc is returning multiple result sets, For example, if you have two select statements in single Command object, then you will get back two result sets.
NextResult is used to move between result sets.
Read is used to move forward in records of a single result set.
Consider the following example:
If you have a proc whose main body is like:
.... Proc start
SELECT Name,Address FROM Table1
SELECT ID,Department FROM Table2
-- Proc End
Executing the above proc would produce two result sets. One for Table1 or first select statement and other for the next select statement.
By default first result set would be available for Read. If you want to move to second result set, you will need NextResult.
See: Retrieving Data Using a DataReader
Example Code from the same link: Retrieving Multiple Result Sets using NextResult
static void RetrieveMultipleResults(SqlConnection connection)
{
using (connection)
{
SqlCommand command = new SqlCommand(
"SELECT CategoryID, CategoryName FROM dbo.Categories;" +
"SELECT EmployeeID, LastName FROM dbo.Employees",
connection);
connection.Open();
SqlDataReader reader = command.ExecuteReader();
while (reader.HasRows)
{
Console.WriteLine("\t{0}\t{1}", reader.GetName(0),
reader.GetName(1));
while (reader.Read())
{
Console.WriteLine("\t{0}\t{1}", reader.GetInt32(0),
reader.GetString(1));
}
reader.NextResult();
}
}
}

Not strictly an answer to this question but if you use the DataTable.Load method to consume data from the reader rather than Reader.Read note that after the Load method has completed, the reader is now placed at the start of the next result set so you should not call the NextResult method otherwise you will skip the next resultset.
A simple loop on Reader.HasRows around a DataTable.Load call is all that you need to process potential multiple resultsets in this scenario.

Get number of result sets from a SqlDataReader

I have an SQL Server stored procedure that returns multiple results. The body of the stored procedure might look like this:
SELECT * FROM tableA;
SELECT * FROM tableB;
SELECT * FROM tableC;
In that case, the stored procedure returns 3 result sets. Other stored procedures might return, e.g., 1, 0, or any number of result sets. Each result set might contain 0 or more rows in it. When loading these, I will need to call IDataReader.NextResult() to navigate between result sets.
How can I reliably get the count of result sets (not row counts) in C#?

There seems to be no property or method that directly calculates the result count in IDataReader. This interface rather intends to be consumed in an incremental/streaming fashion. So, to count the number of result sets returned, increment a counter every time you call IDataReader.NextResult() and it returns true while consuming the data.
However, there is a catch. The
The documentation for IDataReader.NextResult() states:
By default, the data reader is positioned on the first result.
Consider the following scenarios:
The command returned 0 result sets. Your first call to IDataReader.NextResult() returns false.
The command returned 1 result set. Your first call to IDataReader.NextResult() returns false.
The command returned 2 result sets. Your second call to IDataReader.NextResult() returns false.
You can see that we have enough information to count the number of result sets as long as there is at least one result set. That would be the number of times that IDataReader.NextResult() returned true plus one.
To detect whether or not there are 0 result sets, we use another property from the reader: IDataRecord.FieldCount. The documentation for this property states:
When not positioned in a valid recordset, 0; otherwise, the number of columns in the current record. The default is -1.
Thus, we can read that field when first opening the reader to determine if we are in a valid result set or not. If the command generates no result sets, the value of IDataRecord.FieldCount on the reader will initially be less than 1. If the command generates at least one result set, the value will initially be positive. This assumes that it is impossible for a result set to have 0 columns (which I think you can assume with SQL, not sure).
So, I would use something like the following to count the number of result sets. If you also need to save the data, that logic must be inserted into this:
using (var reader = command.ExecuteReader())
{
var resultCount = 0;
do
{
if (reader.FieldCount > 0)
resultCount++;
while (reader.Read())
{
// Insert logic to actually consume data here…
// HandleRecordByResultIndex(resultCount - 1, (IDataRecord)reader);
}
} while (reader.NextResult());
}
I’ve tested this with System.Data.SqlClient and the commands PRINT 'hi' (0 result sets), SELECT 1 x WHERE 1=0 (1 result set), and SELECT 1 x WHERE 1=0; SELECT 1 x WHERE 1=0 (2 result sets).

Use DataReader.NextResult to advance the reader to the next result set.:
using (var con = new SqlConnection(Properties.Settings.Default.ConnectionString))
{
using (var cmd = new SqlCommand("SELECT * FROM TableA; SELECT * FROM TableB; SELECT * FROM TableC;", con))
{
con.Open();
using (IDataReader rdr = cmd.ExecuteReader())
{
while (rdr.Read())
{
int firstIntCol = rdr.GetInt32(0); // assuming the first column is of type Int32
// other fields ...
}
if (rdr.NextResult())
{
while (rdr.Read())
{
int firstIntCol = rdr.GetInt32(0); // assuming the first column is of type Int32
// other fields ...
}
if (rdr.NextResult())
{
while (rdr.Read())
{
int firstIntCol = rdr.GetInt32(0); // assuming the first column is of type Int32
// other fields ...
}
}
}
}
}
}

Another solution to be aware of, in addition to the manual SqlDataReader method, in the accepted answer, is to use a SqlDataAdapter and DataSets and DataTables.
When using those classes, the entire result set is retrieved from the server in one go, and you can iterate them at your leisure. Also, several other .net classes are aware of DataSets and DataTables and can be hooked up to them directly, for read-only or read-write data access, if you also set the DeleteCommand, InsertCommand, and UpdateCommand properties. What you get "for free," with that, is the ability to alter the data in the DataSet, and then simply call Update() to push your local changes to the database. You also gain the RowUpdated event handler, which you can use to make that happen automatically.
DataSets and DataTables retain the metadata from the database schema, as well, so you can still access columns by name or index, as you please.
Overall, they're a nice feature, though certainly heavier weight than a SqlDataReader is.
Documentation for SqlDataAdapter is here: https://learn.microsoft.com/en-us/dotnet/api/system.data.sqlclient.sqldataadapter

What am I doing wrong with this query?

I can't seem to find why this function doesn't insert records into the database. :(
I get no error messages or whatsoever, just nothing in the database.
EDIT: this is how my query looks now .. still nothing ..
connection.Open();
XmlNodeList nodeItem = rssDoc.SelectNodes("/edno23/posts/post");
foreach (XmlNode xn in nodeItem)
{
cmd.Parameters.Clear();
msgText = xn["message"].InnerText;
C = xn["user_from"].InnerText;
avatar = xn["user_from_avatar"].InnerText;
string endhash = GetMd5Sum(msgText.ToString());
cmd.Parameters.Add("#endhash",endhash);
cmd.CommandText = "Select * FROM posts Where hash=#endhash";
SqlCeDataReader reader = cmd.ExecuteReader();
while (reader.Read())
{
string msgs = reader["hash"].ToString();
if (msgs != endhash || msgs == null)
{
sql = "INSERT INTO posts([user],msg,avatar,[date],hash) VALUES(#username,#messige,#userpic,#thedate,#hash)";
cmd.CommandText = sql;
cmd.Parameters.Add("#username", C);
cmd.Parameters.Add("#messige", msgText.ToString());
cmd.Parameters.Add("#userpic", avatar.ToString());
cmd.Parameters.Add("#thedate", dt);
cmd.Parameters.Add("#hash", endhash);
cmd.ExecuteNonQuery();// executes query
adapter.Update(data);// saves the changes
}
}
reader.Close();
}
connection.Close();

Does nodeItem actually have any items in it? If not, the contents of the foreach loop aren't being executed.
What's the adapter and data being used for? The queries and updates seem be done via other commands and readers.
What does 'hash' actually contain? If it's a hash, why are you hashing the content of the hash inside the while loop? If not, why is it being compared against a hash in the query SELECT * FROM posts WHERE hash = #endhash?
Won't closing the connection before the end of the while loop invalidate the reader used to control the loop?

Lots of things going on here...
You are using the command 'cmd' to loop over records with a datareader, and then using the same 'cmd' command inside the while statement to execute an insert statement. You declared another command 'cmdAdd' before but don't seem to use it anywhere; is that what you intended to use for the insert statement?
You also close your data connection inside the while loop that iterates over your datareader. You are only going to read one record and then close the connection to your database that way; if your conditional for inserting is not met, you're not going to write anything to the database.
EDIT:
You really should open and close the connection to the database outside the foreach on the xmlnodes. If you have 10 nodes to loop over, the db connection is going to be opened and closed 10 times (well, connection pooling will probably prevent that, but still...)
You are also loading the entire 'posts' table into a dataset for seemingly no reason. You're not changing any of the values in the dataset yet you are calling an update on it repeatedly (at "save teh shanges"). If the 'posts' table is even remotely large, this is going to suck a lot of memory for no reason (on a handheld device, no less).

Is anything returned from "Select * FROM posts Where hash=#endhash"?
If not, nothing inside the while loop matters....

Why are you closing the Database Connection inside the while loop?
The code you posted should throw an exception when you try to call cmd.ExecuteNonQuery() with an unopen DB connection object.
SqlCeCommand.ExecuteNonQuery() method returns the number of rows affected.
Why don't you check whether it is returning 1 or not in the debugger as shown below?
int rowsAffectedCount = cmd.ExecuteNonQuery();
Hope it helps :-)

You've got some issues with not implementing "using" blocks. I've added some to your inner code below. The blocks for the connection and select command are more wishful thinking on my part. I hope you're doing the same with the data adapter.
using (var connection = new SqlCeConnection(connectionString))
{
connection.Open();
var nodeItem = rssDoc.SelectNodes("/edno23/posts/post");
foreach (XmlNode xn in nodeItem)
{
using (
var selectCommand =
new SqlCeCommand(
"Select * FROM posts Where hash=#endhash",
connection))
{
var msgText = xn["message"].InnerText;
var c = xn["user_from"].InnerText;
var avatar = xn["user_from_avatar"].InnerText;
var endhash = GetMd5Sum(msgText);
selectCommand.Parameters.Add("#endhash", endhash);
selectCommand.CommandText =
"Select * FROM posts Where hash=#endhash";
using (var reader = selectCommand.ExecuteReader())
{
while (reader.Read())
{
var msgs = reader["hash"].ToString();
if (msgs == endhash && msgs != null)
{
continue;
}
const string COMMAND_TEXT =
"INSERT INTO posts([user],msg,avatar,[date],hash) VALUES(#username,#messige,#userpic,#thedate,#hash)";
using (
var insertCommand =
new SqlCeCommand(
COMMAND_TEXT, connection))
{
insertCommand.Parameters.Add("#username", c);
insertCommand.Parameters.Add(
"#messige", msgText);
insertCommand.Parameters.Add(
"#userpic", avatar);
insertCommand.Parameters.Add("#thedate", dt);
insertCommand.Parameters.Add(
"#hash", endhash);
insertCommand.ExecuteNonQuery();
// executes query
}
adapter.Update(data); // saves teh changes
}
reader.Close();
}
}
}
connection.Close();
}
Of course with the additional nesting, parts should be broken out as separate methods.

I suspect your problem is that you're trying to reuse the same SqlCeCommand instances.
Try making a new SqlCeCommand within the while loop. Also, you can use the using statement to close your data objects.
Why are you calling adapter.Update(data) since you're not changing the DataSet at all? I suspect you want to call adapter.Fill(data). The Update method will save any changes in the DataSet to the database.

How to debug programs: http://www.drpaulcarter.com/cs/debug.php
Seriously, can you post some more information about where it's working? Does it work if you use SQL Server Express instead of SQL CE? If so, can you break out SQL Profiler and take a look at the SQL commands being executed?