How do i fix the core CRUD options?

Got a NullReferenceException and System.TypeInitializationException in this updated C# crud project.

ContactClass c = new ContactClass(); generates these errors.

What are the solutions for this errors and hy are they there? See code or .zip

ContactClass.java

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace Rcontact.RcontactClasses
{
class ContactClass
{
// Getter Setter properties
// Acts as a data carrirer in the application
public int ContactID { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public string Phone { get; set; }
public string Email { get; set; }
public string Note { get; set; }
public string Status { get; set; }
static string myconnstrng = ConfigurationManager.ConnectionStrings["connstrng"].ConnectionString;
// Selecting data from database
public DataTable Select()
{
// Step 1 Database connection
SqlConnection conn = new SqlConnection(myconnstrng);
DataTable dt = new DataTable();
try
{
// Step 2 Wrting SQL query
string sql = "SELECT * FROM tbl_contact";
// creating cmd using swql and conn
SqlCommand cmd = new SqlCommand(sql, conn);
// Creating SQL DataAdapater using cmd
SqlDataAdapter adapter = new SqlDataAdapter(cmd);
conn.Open();
adapter.Fill(dt);
}
catch (Exception ex)
{
}
finally
{
conn.Close();
}
return dt;
}
// inserting data into database
public bool Insert(ContactClass c)
{
// Creating a default return type and setting its value to false
bool isSuccess = false;
// Step 1: Connect database
SqlConnection conn = new SqlConnection(myconnstrng);
try
{
}
catch (Exception ex)
{
// Step 2 Creating a SQL Quary to insert data
string sql = "INSERT INTO tbl_contact(FirstName, LastName, Phone, Email, Note, Status) " +
"(@FirstName, @LastName, @Phone, @Email, @Note, @Status";
// Creatigin SQL command using sql and conn
SqlCommand cmd = new SqlCommand(sql, conn);
// Create Parameters tot add data
cmd.Parameters.AddWithValue("@FirstName", c.FirstName);
cmd.Parameters.AddWithValue("@LasttName", c.LastName);
cmd.Parameters.AddWithValue("@Phone", c.Phone);
cmd.Parameters.AddWithValue("@Email", c.Email);
cmd.Parameters.AddWithValue("@Note", c.Note);
cmd.Parameters.AddWithValue("@Status", c.Status);
// Connection open here
conn.Open();
int rows = cmd.ExecuteNonQuery();
// If the query runs succesfully then the value of rows will be > 0 else its value will be 0
if (rows > 0)
{
isSuccess = true;
}
else
{
isSuccess = false;
}
}
finally
{
// close connection
conn.Close();
}
return isSuccess;
}
// Method to update data in database from our application
public bool Update(ContactClass c)
{
// Create a default returntype and set its default value to false
bool isSuccess = false;
SqlConnection conn = new SqlConnection(myconnstrng);
try
{
// SQL to update data in the database
string sql = "UPDATE tbl_contact SET FirstName=@FirstName, LastName=@LastName, Phone=@Phone, Email=@Email, Note=@Note, Status=@Status WHERE ContactID=@ContactID";
// Creating sql command
SqlCommand cmd = new SqlCommand(sql, conn);
// Create Parameteres to add value
cmd.Parameters.AddWithValue("@FirstName", c.FirstName);
cmd.Parameters.AddWithValue("@LastName", c.LastName);
cmd.Parameters.AddWithValue("@Phone", c.Phone);
cmd.Parameters.AddWithValue("@Email", c.Email);
cmd.Parameters.AddWithValue("@Note", c.Note);
cmd.Parameters.AddWithValue("@Status", c.Status);
cmd.Parameters.AddWithValue("@ContactID", c.ContactID);
// Open database connection
conn.Open();
int rows = cmd.ExecuteNonQuery();
// If the query rund succesfully then the value of rows will be > 0 else its value will be 0
if (rows > 0)
{
isSuccess = true;
}
else
{
isSuccess = false;
}
}
catch (Exception ex)
{
}
finally
{
}
return isSuccess;
}
// Method to delete data from database
public bool Delete(ContactClass c)
{
// Create a default return value and set its value to false
bool isSuccess = false;
// Create SQL Connection
SqlConnection conn = new SqlConnection(myconnstrng);
try
{
// SQL to delete data
string sql = "DELETE FROM tbl_contact WHERE ContactID=@ContactID";
// Creating SQL command
SqlCommand cmd = new SqlCommand(sql, conn);
cmd.Parameters.AddWithValue("@ContactID", c.ContactID);
// Open SQL connection
conn.Open();
int rows = cmd.ExecuteNonQuery();
// If the query run succesfully then the value of rows is > 0 else its value is 0
if (rows > 0)
{
isSuccess = true;
}
else
{
isSuccess = false;
}
}
catch (Exception ex)
{
}
finally
{
// Close database connection
conn.Close();
}
return isSuccess;
}
}
}

RContact.java

namespace Rcontact
{
public partial class Rcontact : Form
{
public Rcontact()
{
InitializeComponent();
}
// gets the errrors how to fix it and why are they there?
ContactClass c = new ContactClass();
private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
}
private void Label6_Click(object sender, EventArgs e)
{
}
private void BtnAdd_Click(object sender, EventArgs e)
{
// Get the value from the input fields
c.FirstName = tbFirstName.Text;
c.LastName = tbLastName.Text;
c.Phone = tbPhone.Text;
c.Email = tbEmail.Text;
c.Note = tbNote.Text;
c.Status = tbNote.Text;
// Inserting data into database using method created in step 3
bool succes = c.Insert(c);
if (succes == true)
{
// Succesfolly inserted
MessageBox.Show("Now contact succesfully inserted");
}
else
{
// faild to add contact
MessageBox.Show("Failed to add new contact. Try again. ");
}
}
}
}

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Vijay Thapa8y ago
In your Insert Method, Instead of writing code in catch block.
Please write the code inside try block.

Hope it will fix your error.
Thanks.
Rajneesh Chaubey8y ago
hi,
It is clearly visible that you are missing constructor in your ContactClass
Just add below lines to your code:-
public ContactClass()
{

}
love code8y ago
Got a NullReferenceException and System.TypeInitializationException in this updated C# crud project.
ContactClass c = new ContactClass(); generates these errors.
What are the solutions for this errors and hy are they there? See code or .zip

ContactClass.java
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Rcontact.RcontactClasses
{
    class ContactClass
    {
        // Getter Setter properties
        // Acts as a data carrirer in the application
        public int ContactID { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Phone { get; set; }
        public string Email { get; set; }
        public string Note { get; set; }
        public string Status { get; set; }

        static string myconnstrng = ConfigurationManager.ConnectionStrings["connstrng"].ConnectionString;

        // Selecting data from database
        public DataTable Select()
        {
            // Step 1 Database connection
            SqlConnection conn = new SqlConnection(myconnstrng);
            DataTable dt = new DataTable();

            try
            {

                // Step 2 Wrting SQL query
                string sql = "SELECT * FROM tbl_contact";
                // creating cmd using swql and conn
                SqlCommand cmd = new SqlCommand(sql, conn);

                // Creating SQL DataAdapater using cmd
                SqlDataAdapter adapter = new SqlDataAdapter(cmd);
                conn.Open();
                adapter.Fill(dt);

            }
            catch (Exception ex)
            {

            }

            finally
            {
                conn.Close();
            }

            return dt;
        }
        // inserting data into database
        public bool Insert(ContactClass c)
        {
            // Creating a default return type and setting its value to false
            bool isSuccess = false;

            // Step 1: Connect database
            SqlConnection conn = new SqlConnection(myconnstrng);
            try

            {

            }
            catch (Exception ex)

            {
                // Step 2  Creating a SQL Quary to insert data
                string sql = "INSERT INTO tbl_contact(FirstName, LastName, Phone, Email, Note, Status) " +
                                                    "(@FirstName, @LastName, @Phone, @Email, @Note, @Status";
                // Creatigin SQL command using sql and conn
                SqlCommand cmd = new SqlCommand(sql, conn);
                // Create Parameters tot add data
                cmd.Parameters.AddWithValue("@FirstName", c.FirstName);
                cmd.Parameters.AddWithValue("@LasttName", c.LastName);
                cmd.Parameters.AddWithValue("@Phone", c.Phone);
                cmd.Parameters.AddWithValue("@Email", c.Email);
                cmd.Parameters.AddWithValue("@Note", c.Note);
                cmd.Parameters.AddWithValue("@Status", c.Status);

                // Connection open here
                conn.Open();
                int rows = cmd.ExecuteNonQuery();
                // If the query runs succesfully then the value of rows will be > 0 else its value will be 0
                if (rows > 0)
                {
                    isSuccess = true;


                }
                else
                {
                    isSuccess = false;
                }

            }

            finally
            {
                // close connection
                conn.Close();
            }

            return isSuccess;
        }

        // Method to update data in database from our application
        public bool Update(ContactClass c)
        {
            // Create a default returntype and set its default value to false
            bool isSuccess = false;
            SqlConnection conn = new SqlConnection(myconnstrng);
            try
            {
                // SQL to update data in the database
                string sql = "UPDATE tbl_contact SET FirstName=@FirstName, LastName=@LastName, Phone=@Phone, Email=@Email, Note=@Note, Status=@Status WHERE ContactID=@ContactID";

                // Creating sql command
                SqlCommand cmd = new SqlCommand(sql, conn);

                // Create Parameteres to add value
                cmd.Parameters.AddWithValue("@FirstName", c.FirstName);
                cmd.Parameters.AddWithValue("@LastName", c.LastName);
                cmd.Parameters.AddWithValue("@Phone", c.Phone);
                cmd.Parameters.AddWithValue("@Email", c.Email);
                cmd.Parameters.AddWithValue("@Note", c.Note);
                cmd.Parameters.AddWithValue("@Status", c.Status);
                cmd.Parameters.AddWithValue("@ContactID", c.ContactID);
                // Open database connection
                conn.Open();

                int rows = cmd.ExecuteNonQuery();
                // If the query rund succesfully then the value of rows will be > 0 else its value will be 0
                if (rows > 0)
                {
                    isSuccess = true;
                }

                else
                {
                    isSuccess = false;
                }
            }

            catch (Exception ex)
            {

            }
            finally
            {

            }
                return isSuccess;

        }
        // Method to delete data from  database
        public bool Delete(ContactClass c)
        {
            // Create a default return value and set its value to false
            bool isSuccess = false;
            // Create SQL Connection
            SqlConnection conn = new SqlConnection(myconnstrng);
            try
            {
                // SQL to delete data
                string sql = "DELETE FROM tbl_contact WHERE ContactID=@ContactID";

                // Creating SQL command
                SqlCommand cmd = new SqlCommand(sql, conn);
                cmd.Parameters.AddWithValue("@ContactID", c.ContactID);
                // Open SQL connection
                conn.Open();
                int rows = cmd.ExecuteNonQuery();
                // If the query run succesfully then the value of rows is > 0 else its value is 0
                if (rows > 0)
                {
                    isSuccess = true;

                }

                else
                {
                    isSuccess = false;
                }
            }

            catch (Exception ex)
            {

            }

            finally
            {
                // Close database connection
                conn.Close();
            }

            return isSuccess;
        }

    }

    }
RContact.java
namespace Rcontact
{
public partial class Rcontact : Form
{
public Rcontact()
{
InitializeComponent();
}

// gets the errrors how to fix it and why are they there?
ContactClass c = new ContactClass();

private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
{

}

private void Label6_Click(object sender, EventArgs e)
{

}

private void BtnAdd_Click(object sender, EventArgs e)
{
// Get the value from the input fields
c.FirstName = tbFirstName.Text;
c.LastName = tbLastName.Text;
c.Phone = tbPhone.Text;
c.Email = tbEmail.Text;
c.Note = tbNote.Text;
c.Status = tbNote.Text;

// Inserting data into database using method created in step 3
bool succes = c.Insert(c);
if (succes == true)
{
// Succesfolly inserted
MessageBox.Show("Now contact succesfully inserted");
}
else
{
// faild to add contact
MessageBox.Show("Failed to add new contact. Try again. ");
}

}
}
}
Manav Pandya8y ago
Hello

I think problem is with connection string , if you are using password and username than provide it also

And its suggested to always use Provider name at the end of connection string

Try and debug solution and observe inner exception for that

Than you will get to know where problem occur

Thanks
Rajneesh Chaubey8y ago
When you are assigning result on button click, you are missing 'Note'.
In Insert method you are passing parameter
cmd.Parameters.AddWithValue("@Note", c.Note);
But you are not assigning it on button click.
This is where proble occuring.
Just add this to your insert method:
c.Note = txtBoxNote.Text;
love code8y ago
Except this one; Faild to add ... No othere errors
That's the reason i uploaded the soursefiles.
Rajneesh Chaubey8y ago
Have you debug you code?
What is the exact error you are getting?