public class RowDataCollection : IList<RowData> {
  private List<RowData> rowList;

  internal RowDataCollection(List<RowData> data) {
    rowList = data;
  }
  // ...
}

public RowDataCollection Rows;

public RowDataCollection Select(string colName, object value) {
  List<RowData> rowList = from item in Rows
         where item[colName].Value == value
         select item;
  return new RowDataCollection(rowList);
}

public RowDataCollection SelectRow(string colName, object value) {
  if (!String.IsNullOrEmpty(colName) && (value != null) && (0 < Rows.Count)) {
    switch (Rows[0][colName].GetValueType()) {
      case TableDataType.Boolean:
        return new RowDataCollection(Rows.Where(r => (bool)r[colName].Value == (bool)value).ToList());
      case TableDataType.Character:
        return new RowDataCollection(Rows.Where(r => (char)r[colName].Value == (char)value).ToList());
      case TableDataType.DateTime:
        return new RowDataCollection(Rows.Where(r => (DateTime)r[colName].Value == (DateTime)value).ToList());
      case TableDataType.Decimal:
        return new RowDataCollection(Rows.Where(r => (Decimal)r[colName].Value == (Decimal)value).ToList());
      case TableDataType.Integer:
        return new RowDataCollection(Rows.Where(r => (int)r[colName].Value == (int)value).ToList());
      case TableDataType.String:
        return new RowDataCollection(Rows.Where(r => r[colName].Value.ToString() == value.ToString()).ToList());
    }
  }
  return null;
}

public RowDataCollection SelectRow(string colName, object value) {
  List<RowData> rowList = Rows.Where(r => r[colName].Value.Equals(value)).ToList();
  return new RowDataCollection(rowList);
}

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

List<RowData> rowList = Rows.Where(item => item[colName].Value.Equals(value))
                            .ToList();

if (string.IsNullOrEmpty(colName))
{
    throw new ArgumentException("colName");
}

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

List<RowData> rowList = (from item in Rows
                         where item[colName].Value == value
                         select item).ToList();

if (colName == null)
{
    throw new ArgumentNullException("colName");
}
else if (!Rows.All(row => row.HasColumn(colName)))
{
    throw new ArgumentException("No such column " + colName, "colName");
}

...
// note the change to Any()
else if (!Rows.Any(row => row.HasColumn(colName))
{
    throw new ArgumentException("No such column " + colName, "colName");
}

List<RowData> rowList = (from item in Rows
                         where item.HasColumn(colName)
                            && item[colName].Value == value
                         select item).ToList();

public RowDataCollection Select(string colName, object value) {
      List<RowData> rowList = from item in Rows
             where item[colName].Value == value
             select item;
      return new RowDataCollection(rowList.ToList());
    }