Object reference not set to an instance of an object. Please Help to solve error

This line da.InsertCommand.CommandText = sql; has to be in that way: da.InsertCommand = new SqlCommand(sql);

string connetionString = "Data Source=.\\SQLEXPRESS;AttachDbFilename='G:\\C#.Net\\Forms Practice\\WindowsFormsPractice1\\WindowsFormsPractice1\\WindowsFormsPractice1.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True"; SqlDataAdapter adapter = new SqlDataAdapter(); string sql = "insert into TBLWORKERS (first_name , last_name )" + " values('" + StartValueTextBox.Text + "', '" + EndValueTextBox.Text + ")"; SqlConnection connection = new SqlConnection(connetionString); try { connection.Open(); adapter.InsertCommand = new SqlCommand(sql, connection); adapter.InsertCommand.ExecuteNonQuery(); } catch (Exception ex) { MessageBox.Show(ex.Message); }

At what point you are the exception? Probably those line System.Data.SqlClient.SqlConnection connection = new System.Data.SqlClient.SqlConnection(); SqlDataAdapter da = new SqlDataAdapter();

Here's a minor rewrite of your code (not tested) that should take care of the SqlDataAdapter not having the connection object assigned and also demonstrates how to use parameterized queries to help defend against SQL Injection attacks: private void buttonSave_Click(object sender, EventArgs e) { try { // The using block will automatically dispose of your connection when // the block is exited and is considered standard practice. using (SqlConnection connection = new SqlConnection("Data Source=.\\SQLEXPRESS;AttachDbFilename='G:\\C#.Net\\Forms Practice\\WindowsFormsPractice1\\WindowsFormsPractice1\\WindowsFormsPractice1.mdf';Integrated Security=True;Connect Timeout=30;User Instance=True";)) { SqlDataAdpter da = new SqlDataAdapter(); connection.Open(); // Assign the SqlConnection object to the SqlDataAdapter da.Connection = connection; // Parameterize the query as shown below string sql = "INSERT INTO TBLWORKERS(first_name, last_name) VALUES(@first_name, @last_name)"; da.InsertCommand.CommandText = sql; // Add the values for the parameters da.InsertCommand.Parameters.Add("@first_name", SqlDbType.NVarChar, 25, StartValueTextBox.Text); da.InsertCommand.Parameters.Add("@last_name", SqlDbType.NVarChar, 25, EndValueTextBox.Text); // Execute the query - rows will have the number of rows // affected. should be 1 in this case if succesful int rows = da.InsertCommand.ExecuteNonQuery(); } } catch (Exception ex) { MessageBox.Show(ex.Message, "Connection open"); } }