The problem starts from this method
private void store_detailOperation()
You opened a connection here and performed a database operation and leaving the connection open in here
ConnectionString.cnn.Open();
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter("select * from store_detail where prdid='" + mtbprdid.Text + "'", ConnectionString.cnn);
int s = (da.Fill(ds));
so after this invoking
'store_detailUpdation()' or
store_detailinsertion() inside these method you are trying to open the connection again which is already opened for above select statement. and It's very obvious from the error message.
it's a common and best practice to add a Connection state checking before you try to open database connection again.
i.e add the conneciton state checking inside the
'store_detailUpdation,store_detailinsertion methods as shown below
public void store_detailUpdation()
{
try
{
if(ConnectionString.cnn.State == ConnectionState.Open)
{
ConnectionString.cnn.Close();
}
ConnectionString.cnn.Open();
....
....
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