How to refresh a combobox in c# form app when save button in another form is click?

// This goes in your Main form
private void BtnAddNewCustomer_Click(object sender, EventArgs e)
{ // When the user clicks on the button
    CreateUserForm form = new CreateUserForm();
    // TODO: do more stuff with the form
    if(form.ShowDialog() == DialogResult.OK)
    { // User clicked ok on the form and user is now saved
        // TODO: Reload your combo box
    }
    // Safe to close form now
    form.Close();
    form.Dispose();
}


// This goes in CreateUserForm.cs code
private void BtnCreateUser_Click(object sender, EventArgs e)
{
    // TODO: Validate/Save your data here
    DialogResult = DialogResult.OK;
    /* Do not close from this form, 
     * otherwise everything gets disposed of
     * and you will loose data
     */
    Hide();
}

This could serve you for some more scenarios.

Encapsulate you customer data in one class (eg. MyCustomerData) with all the properties and fields needed for holding the information which is relevant to your application (MyCustomerData.cs).

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ComboBoxUpdate
{
    /// <summary>
    /// Customer Data Encapsulation
    /// </summary>
    public class MyCustomerData
    {
        /// <summary>
        /// Field for holding the Name
        /// </summary>
        private string m_Name;

        /// <summary>
        /// Gets or Sets the customer Name
        /// </summary>
        public string Name
        {
            get { return m_Name; }
            set { m_Name = value; }
        }


        /// <summary>
        /// Contructor
        /// </summary>
        /// <param name="Name">The Name of the Customer</param>
        public MyCustomerData(string Name)
        {
            this.Name = Name;
        }
    }
}

Create one (let's say) MyCustomerDataEditor form with one constructor that takes one instance from your MyCustomerData class. Of course you need to add all the user input validation logic that ensures data correctness, enables|disables the 'Accept' button after changes validation (etc.).
MyCustomerDataEditorFrm.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace ComboBoxUpdate
{
    public partial class MyCustomerDataEditorFrm : Form
    {
        private MyCustomerData m_Customer;


        public MyCustomerData Customer
        {
            get { return m_Customer; }
            set 
            {
                m_Customer = value;
                ValidateEditors();
            }
        }


        public MyCustomerDataEditorFrm( MyCustomerData Customer )
        {
            m_Customer = Customer;

            InitializeComponent();

            this.Customer = Customer;
        }


        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            ValidateEditors();
        }


        private void ValidateEditors()
        {
            // Simple validation, only accept not empty name.
            button1.Enabled = !string.IsNullOrEmpty(textBox1.Text);
        }


        private void button1_Click(object sender, EventArgs e)
        {
            // Apply changes;
            m_Customer.Name = textBox1.Text;
        }
    }
}

Declare some kind of list for holding the MyCustomerData instances list in your main form:

List<mycustomerdata> m_MyCustomers = new List<mycustomerdata>()</mycustomerdata></mycustomerdata>

In your main form populate the list with valid data from your database. Let's suppose you have one method called GetMeTheDataFromTheDataBase()hat retrieves that data.

m_MyCustomers.AddRange( GetMeTheDataFromTheDataBase())

You may configure the ComboBox so the text it displays is the CustomerName property (provided that you declared it in the MyCustomerData class).

The main form could end up like this:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace ComboBoxUpdate
{
    public partial class Form1 : Form
    {
        private List<mycustomerdata> m_Customers;

        public Form1()
        {
            m_Customers = new List<mycustomerdata>();

            InitializeComponent();

            comboBox1.DisplayMember = "Name";
            LoadCustomers();
            UpdateCustomersComboBox();
        }


        /// <summary>
        /// Update the combobox items from the customers list.
        /// </summary>
        private void UpdateCustomersComboBox()
        {
            foreach (MyCustomerData Customer in m_Customers) 
            {
                if (comboBox1.Items.IndexOf(Customer) < 0) 
                    comboBox1.Items.Add(Customer);
            }

            UpdateUIControls();
        }


        /// <summary>
        /// Populate the local copy of the customers list
        /// </summary>
        private void LoadCustomers()
        {
            m_Customers.AddRange( new MyCustomerData[] {
                new MyCustomerData( "John Doe" ),
                new MyCustomerData( "Jane Doe" )
            });
        }


        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            UpdateUIControls();
        }


        /// <summary>
        /// Update the state of all relevant controls
        /// </summary>
        private void UpdateUIControls()
        {
            button2.Enabled = (comboBox1.SelectedIndex >= 0);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // Simply add some data. Here you should do 
            MyCustomerDataEditorFrm Dlg = new MyCustomerDataEditorFrm(new MyCustomerData( "Joe's Brother!" ));
            if (Dlg.ShowDialog(this) == DialogResult.OK)
            {
                m_Customers.Add(Dlg.Customer);
                UpdateCustomersComboBox();
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            MyCustomerData Customer = comboBox1.SelectedItem as MyCustomerData;

            if (DialogResult.Yes != MessageBox.Show(this, "Sure to remove " + Customer.Name, "Remove Customer", MessageBoxButtons.YesNo, MessageBoxIcon.Warning))
                return;

            m_Customers.Remove(Customer);
            comboBox1.Items.Remove(Customer);

            UpdateUIControls();
        }
    }
}
</mycustomerdata></mycustomerdata>

One more thing, the components in the main form are the combobox (combobox1), one "Add New" button (button1) and one "Remove" button (button2).

I didn't see the way to attach the sample project/code, but if you need it i can e-mail it to you.

Hope it helps you some how.

In Adding Customer Name Button Clear The Combobox like CmbName.items.clear(),
After Inserting A new name to data base reload all names from data base to combobox