public class Kategoria { public int Id { get; set; } public string Nev { get; set; } = ""; }
public class Feladat { public string Cim { get; set; } = ""; public int KategoriaId { get; set; } = 1; public bool Kesz { get; set; } }

private BindingList<Kategoria> kategoriak = new BindingList<Kategoria>();
private BindingList<Feladat> feladatok = new BindingList<Feladat>();
private BindingSource bsKat = new BindingSource();

private void Form1_Load(object sender, EventArgs e)
{
    kategoriak.Add(new Kategoria { Id = 1, Nev = "Iskola" });
    kategoriak.Add(new Kategoria { Id = 2, Nev = "Otthon" });
    kategoriak.Add(new Kategoria { Id = 3, Nev = "Hobbi" });

    cmbUjKat.DropDownStyle = ComboBoxStyle.DropDownList;
    cmbUjKat.DisplayMember = "Nev";
    cmbUjKat.ValueMember = "Id";
    cmbUjKat.DataSource = kategoriak;

    bsKat.DataSource = kategoriak;
    lstKategoria.DisplayMember = "Nev";
    lstKategoria.ValueMember = "Id";
    lstKategoria.FormattingEnabled = true;
    lstKategoria.Format += lstKategoria_Format;
    lstKategoria.DataSource = bsKat;

    TablaBeallit();
    dgv.DataError += dgv_DataError;
}

private void TablaBeallit()
{
    dgv.AutoGenerateColumns = false;
    dgv.Columns.Add(new DataGridViewTextBoxColumn { Name = "colCim", HeaderText = "Feladat", DataPropertyName = "Cim", AutoSizeMode = DataGridViewAutoSizeColumnMode.Fill });

    DataGridViewComboBoxColumn kat = new DataGridViewComboBoxColumn { Name = "colKat", HeaderText = "Kategória", DataPropertyName = "KategoriaId", DataSource = kategoriak, DisplayMember = "Nev", ValueMember = "Id", Width = 140 };
    dgv.Columns.Add(kat);

    dgv.Columns.Add(new DataGridViewCheckBoxColumn { Name = "colKesz", HeaderText = "Kész", DataPropertyName = "Kesz", Width = 60 });

    dgv.AllowUserToAddRows = false;
    dgv.DataSource = feladatok;
    dgv.CellValueChanged += (s, ev) => bsKat.ResetBindings(false);   // a kategóriaváltás a számokat is érinti
}

private int Darab(int kategoriaId)
{
    int db = 0;

    foreach (Feladat f in feladatok)
    {
        if (f.KategoriaId == kategoriaId) { db++; }
    }

    return db;
}

private void lstKategoria_Format(object sender, ListControlConvertEventArgs e)
{
    Kategoria k = e.ListItem as Kategoria;

    if (k != null)
    {
        e.Value = k.Nev + " (" + Darab(k.Id) + " feladat)";
    }
}

private void UjFeladat()
{
    if (txtCim.Text.Trim().Length == 0 || !(cmbUjKat.SelectedValue is int)) { return; }

    feladatok.Add(new Feladat { Cim = txtCim.Text.Trim(), KategoriaId = (int)cmbUjKat.SelectedValue });
    txtCim.Clear();
    txtCim.Focus();
    bsKat.ResetBindings(false);
}

private void btnUjFeladat_Click(object sender, EventArgs e) { UjFeladat(); }

private void txtCim_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Enter) { e.SuppressKeyPress = true; UjFeladat(); }
}

private void btnUjKat_Click(object sender, EventArgs e)
{
    string nev = txtUjKat.Text.Trim();

    if (nev.Length == 0) { return; }

    foreach (Kategoria k in kategoriak)
    {
        if (k.Nev.Equals(nev, StringComparison.CurrentCultureIgnoreCase))
        {
            lblInfo.Text = "Ilyen kategória már van.";
            return;
        }
    }

    int max = 0;

    foreach (Kategoria k in kategoriak) { max = Math.Max(max, k.Id); }

    kategoriak.Add(new Kategoria { Id = max + 1, Nev = nev });
    txtUjKat.Clear();
}

private void btnTorolKat_Click(object sender, EventArgs e)
{
    Kategoria k = bsKat.Current as Kategoria;

    if (k == null) { return; }

    if (Darab(k.Id) > 0)
    {
        MessageBox.Show(this, "A kategóriához feladatok tartoznak, ezért nem törölhető.", "Törlés", MessageBoxButtons.OK, MessageBoxIcon.Warning);
        return;
    }

    bsKat.RemoveCurrent();   // a ComboBoxok és az oszlop is követi
}

private void dgv_DataError(object sender, DataGridViewDataErrorEventArgs e)
{
    e.ThrowException = false;
    lblInfo.Text = "Érvénytelen kategória a(z) " + (e.RowIndex + 1) + ". sorban.";
}