Header Checkbox

Try this:

private CheckBox headerCheckBox;

private void PersonelEkle_Load(object sender, EventArgs e)
{
    // Create the header CheckBox
    headerCheckBox = new CheckBox();
    headerCheckBox.Size = new Size(15, 15);
    headerCheckBox.CheckedChanged += CheckBox1_CheckedChanged;

    // Add the header CheckBox to the DataGridView's column header
    dataGridView1.Controls.Add(headerCheckBox);
    dataGridView1.Columns[0].HeaderCell.Value = "Select All";

    // Adjust the position of the CheckBox to align with the column header
    Point headerCellLocation = dataGridView1.GetCellDisplayRectangle(0, -1, true).Location;
    headerCheckBox.Location = new Point(headerCellLocation.X + 8, headerCellLocation.Y + 2);
}

private void CheckBox1_CheckedChanged(object sender, EventArgs e)
{
    // Update the state of all CheckBox cells in the DataGridView
    foreach (DataGridViewRow row in dataGridView1.Rows)
    {
        DataGridViewCheckBoxCell checkBoxCell = (DataGridViewCheckBoxCell)row.Cells[0];
        checkBoxCell.Value = headerCheckBox.Checked;
    }
}