مشكله في كود Inserting

String Path = "Data Source=DEVELOPER;Initial Catalog=Students;Integrated Security=True";
			SqlConnection Cnn = new SqlConnection(Path);
			Cnn.Open();
			SqlCommand FindCommand = new SqlCommand();
			FindCommand.Connection=Cnn;
			FindCommand.CommandType = CommandType.Text;
			FindCommand.CommandText = "SELECT UserID FROM Users where UserID='" + textBox1.Text + "'";

			SqlDataReader Myreader = FindCommand.ExecuteReader();
			if (Myreader.Read())
			{ 
				FindCommand.Dispose();
				Myreader.Close();
				MessageBox.Show("Change userID");

				return;
			}



			SqlCommand InsertCommand = new SqlCommand();
			InsertCommand.Connection = Cnn;
			InsertCommand.CommandType = CommandType.Text;
			string SQL = "INSERT INTO Users VALUES (@A,@B,@C,@D,@E,@F,@G,@H,@I,@J,@K,@L,@M,@N,@O,@P,@Q)";
			InsertCommand.CommandText = SQL;
			InsertCommand.Parameters.Clear();
			InsertCommand.Parameters.AddWithValue("@A", textBox1.Text);
			InsertCommand.Parameters.AddWithValue("@B", textBox2.Text);
			InsertCommand.Parameters.AddWithValue("@C", textBox4.Text);
			InsertCommand.Parameters.AddWithValue("@D", comboBox1.SelectedText);
			InsertCommand.Parameters.AddWithValue("@E", textBox8.Text);
			InsertCommand.Parameters.AddWithValue("@F", textBox6.Text);
			InsertCommand.Parameters.AddWithValue("@G", textBox5.Text);
			InsertCommand.Parameters.AddWithValue("@H", textBox7.Text);
			InsertCommand.Parameters.AddWithValue("@I", "True");


			if (checkedListBox1.GetItemCheckState(0)==CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@J", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@J", "False");
			}




			if (checkedListBox1.GetItemCheckState(1) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@K", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@K", "False");
			}

			if (checkedListBox1.GetItemCheckState(2) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@L", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@L", "False");
			}
			if (checkedListBox1.GetItemCheckState(3) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@M", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@M", "False");
			}
			if (checkedListBox1.GetItemCheckState(4) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@N", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@N", "False");
			}
			if (checkedListBox1.GetItemCheckState(5) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@O", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@O", "False");
			}
			if (checkedListBox1.GetItemCheckState(6) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@P", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@P", "False");
			}
			if (checkedListBox1.GetItemCheckState(7) == CheckState.Checked)
			{
				InsertCommand.Parameters.AddWithValue("@Q", "True");
			}
			else
			{
				InsertCommand.Parameters.AddWithValue("@Q", "False");
			}




			InsertCommand.ExecuteNonQuery();
			InsertCommand.Dispose();
			Cnn.Dispose();
			MessageBox.Show("Adding process was successfuly","create user",MessageBoxButtons.OK,MessageBoxIcon.Information);

		}