Hi Amol111,

Is your question for window application ?

If no refer the below article.

How to edit GridView Row using AJAX Modal Popup Extender in ASP.Net

Else refer the below code.

Form1.cs

public static string customerId;
public static string contactName;
public static string country;

public Form1()
{
    InitializeComponent();
    BindGrid();
}

private void BindGrid()
{
    string constring = @"Data Source=.;Initial Catalog=Northwind;User id = sa;password=test";
    using (SqlConnection con = new SqlConnection(constring))
    {
        using (SqlCommand cmd = new SqlCommand("SELECT * FROM Customers", con))
        {
            cmd.CommandType = CommandType.Text;
            using (SqlDataAdapter sda = new SqlDataAdapter(cmd))
            {
                using (DataSet ds = new DataSet())
                {
                    sda.Fill(ds);
                    //Set AutoGenerateColumns False
                    dataGridView1.AutoGenerateColumns = false;

                    //Set Columns Count
                    dataGridView1.ColumnCount = 3;

                    //Add Columns
                    dataGridView1.Columns[0].Name = "CustomerId";
                    dataGridView1.Columns[0].HeaderText = "Customer Id";
                    dataGridView1.Columns[0].DataPropertyName = "CustomerID";

                    dataGridView1.Columns[1].HeaderText = "Contact Name";
                    dataGridView1.Columns[1].Name = "Name";
                    dataGridView1.Columns[1].DataPropertyName = "ContactName";

                    dataGridView1.Columns[2].Name = "Country";
                    dataGridView1.Columns[2].HeaderText = "Country";
                    dataGridView1.Columns[2].DataPropertyName = "Country";
                    dataGridView1.DataSource = ds.Tables[0];
                }
            }
        }
    }
}

private void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e)
{
    customerId = dataGridView1.CurrentRow.Cells[0].Value.ToString();
    contactName = dataGridView1.CurrentRow.Cells[1].Value.ToString();
    country = dataGridView1.CurrentRow.Cells[2].Value.ToString();
    Popup pop = new Popup();
    pop.Show();
}

Popup.cs

public Popup()
{
    InitializeComponent();
}

private void Popup_Load(object sender, EventArgs e)
{
    lblCustomerId.Text = Form1.customerId;
    lblContactName.Text = Form1.contactName;
    lblCountry.Text = Form1.country;
}

Screenshot