SQL passing many values (vector) through one parameter for the query/command

Question

First of all I want to say I'm using Firebird and do not know if this is problem with other database servers.

What I want to do is pass part of command string as parameter.

I have code like this:

using (FbConnection con = new FbConnection(M.Baza.connectionString))
{
    con.Open();
    using (FbCommand cmd = new FbCommand("DELETE FROM STAVKA WHERE VRDOK = @VRDOK AND BRDOK = @BRDOK AND ROBAID IN (@ROBAID)", con))
    {
        cmd.Parameters.AddWithValue("@VRDOK", vrDok);
        cmd.Parameters.AddWithValue("@BRDOK", brDok);
        cmd.Parameters.AddWithValue("@ROBAID", robaId); //robaId is string which looks like this '15, 18, 16, 14, 28'

        cmd.ExecuteNonQuery();

        if (showMessage)
            MessageBox.Show("Stavke uspesno obrisane!");
    }
}

The code above is not working and it drops error

unable to cast object of type 'system.Int32` to type 'system.iconvertible'

But when I write code like this:

using (FbConnection con = new FbConnection(M.Baza.connectionString))
{
    con.Open();
    using (FbCommand cmd = new FbCommand("DELETE FROM STAVKA WHERE VRDOK = @VRDOK AND BRDOK = @BRDOK AND ROBAID IN (" + robaId + ")", con))
    {
        cmd.Parameters.AddWithValue("@VRDOK", vrDok);
        cmd.Parameters.AddWithValue("@BRDOK", brDok);

        cmd.ExecuteNonQuery();

        if (showMessage)
            MessageBox.Show("Stavke uspesno obrisane!");
    }
}

It works.

Why is this happening?

Let's say I have command like this "SELECT " + columns + " FROM TABLE" where columns is type of string which looks like this column1, column2, column3 and it is working. How I can make same code look like this "SELECT @COLUMNS FROM TABLE" and pass that @COLUMNS just like I am passing parameters with cmd.Parameters.AddWithValue("@COLUMNS", columns);