Ufdjrw

I have this method, which I have in the base class that helps me to select anything from the children classes and also to reduce code repetition. The problem is when I call it I get an error which is a NullReferenceException (and when I look it up I find that the command in the method is empty).
This is the method in question:
This way I already know how to use but the other one I don't

SqlCommand command = new SqlCommand("select * from Customers where idCustomer=@idCustomer", OpenConnection());
command.Parameters.AddWithValue("@idCustomer", Id);
SqlDataReader reader = command.ExecuteReader();
Customer Onecustomer = null;
if (reader.Read())

 Onecustomer = ReadCustomer(reader);

protected DataTable ExecuteSelectQuery(String query, params SqlParameter[] sqlParameters)

 SqlCommand command = new SqlCommand();
 DataTable dataTable;
 DataSet dataSet = new DataSet();

 try
 
 command.Connection = OpenConnection();
 command.CommandText = query;
 command.Parameters.AddRange(sqlParameters);
 command.ExecuteNonQuery();
 adapter.SelectCommand = command;
 adapter.Fill(dataSet);
 dataTable = dataSet.Tables[0];
 
 catch (SqlException e)
 
 return null;
 throw new Exception("Error :" + e.Message);
 
 finally
 
 CloseConnection();
 
 return dataTable;

Here how I call it

string author = "Alfred Schmidt";
int id = 1;

// ExecuteEditQuery("UPDATE Books SET Title =@param1 WHERE idBook =@param2", sqlParameters);
//SqlParameter[] sqlParameters = new SqlParameter[1]
//
// new SqlParameter ("@param1",author),
//;

SqlParameter[] myparm = new SqlParameter[1];
myparm[0] = new SqlParameter("@Author", SqlDbType.NVarChar, 200);
myparm[0].Value = author;

String query = @"SELECT * FROM Books WHERE Author =@Author";
DataTable dt = ExecuteSelectQuery(query, myparm);

for (int i = 0; i < dt.Rows.Count; i++)

 Console.WriteLine(dt.Rows.ToString());

Console.Write("");

1

Is your OpenConnection() method returns a connection object. It may couse the error, the implementation of the method is not given. Also the adpater is not defined in the code, may be it can be the cause of error too, if it is not initialized.

And i want to say few things about your code:

1) You have and unnecessary command.ExecuteNonQuery(); statement in your ExecuteSelectQuery method.

2) DataAdapter can directly fill DataTable, you dont have to use DataSet.

Here's a proper rewrite of your method.

protected DataTable ExecuteSelectQuery(String query, params SqlParameter[] sqlParameters)

 using (SqlCommand command = new SqlCommand()) 
 try
 
 command.CommandText = query;
 command.Parameters.AddRange(sqlParameters);
 command.Connection = OpenConnection();

 DataTable dataTable = new DataTable();
 using (SqlDataAdapter adapter = new SqlDataAdapter(command))
 adapter.Fill(dataTable);
 return dataTable;
 
 catch (SqlException e)
 
 return null;
 throw new Exception("Error :" + e.Message);
 
 finally
 
 CloseConnection();
 

Note that the SqlDataAdapter can Open() and Close() the connection by itself, if the SqlConnection is Closed when Fill is called.