Remove row from datatable gives error table does not have primary key

Replacing your foreach loop with the below loop to identify the specific row and then delete it should do the trick.

for(int i = dt.Rows.Count-1; i >= 0; i--)
{
DataRow dr = dt.Rows[i];
if (dr["Data"] == ((ComboBox)sender).Text)
dr.Delete();
}

The rows are identified and compared via a primary key. You're asking if the set of rows contains a row with the primary key selected.

Also, you should never make changes to an iterator within the iteration. This will throw an error also.

Try this instead:

private void dropDownDemo_Load(object sender, EventArgs e)
        {
 
            dt.Columns.Add("Data");
            dt.Rows.Add("None");
            dt.Rows.Add("1st");
            dt.Rows.Add("2nd");
            dt.Rows.Add("3rd");
        }
public void removeSelected(string selected)
        {
            var dtTemp = new DataTable;
            dtTemp .Columns.Add("Data");
            if (selected != "None")
            {
                foreach (DataRow row in dt.Rows)
                {
                    if (!dt.Rows["Data"].Contains(selected))   
                    {
                        dtTemp.Rows.Add(row)
                    }
                }
            }
            dt = dtTemp;
        }

Another way would be to add a Break; after you remove the row, but this is pretty clean

None of the other solutions seem to have picked up on the fact you have not added a primary key to your datatable...Contains will not work without a primary key (Reference DataColumnCollection.Contains Method (String) (System.Data)[^] )

private void dropDownDemo_Load(object sender, EventArgs e)
{
 
            dt.Columns.Add("Data");
            dt.Rows.Add("None");
            dt.Rows.Add("1st");
            dt.Rows.Add("2nd");
            dt.Rows.Add("3rd");

            dt.PrimaryKey = new DataColumn[] { dt.Columns["Data"] };
}

You can then use Contains (NB untested)

public void removeSelected(string selected)
{
    if (selected != "None")
    {
        while (dt.Rows.Contains(selected))
        {
            DataRow foundRow = table.Rows.Find(selected);
            if(foundRow != null)
            {
                dt.Rows.Remove(foundrow);
            }
        }
    }
}