DataTable dt = new DataTable();

// first add your columns
for (int i = 0; i < count; i++)
{
    dt.Columns.Add(fromFileFields[i]);
}

// and then add your rows
for (int i = 0; i < count; i++)
{
    var row = dt.NewRow();
    // Set values for columns with row[i] = xy
    dt.Rows.Add(row);
}

datagridview.DataSource = dt;