>>I think Rob's right in that if there are a lot of lookups by ColumnName then Dictionary would be faster. OTOH is you wanted, say a list ofcolumns of int Types then IEnumerable would be better....
>
>In my case I output all columns for a table. I normally access column by column name. I already switched to List. Should I switch back to dictionary?
>
>My current classes:
>
>

> /// <summary>
>   /// Column schema - some properties
>   /// </summary>
>    public class ColumnSchema
>    {
>       public String ColumnName { get; set; }
>       public Type DataType { get; set; }
>       public SqlDbType DbType { get; set; }
>       public Int32 ColumnSize { get; set; }
>       public Int16 Precision { get; set; }
>       public Int16 Scale { get; set; }
>       public Boolean IsIdentity { get; set; }
>      
>       /// <summary>
>       /// default constructor
>       /// </summary>
>       public ColumnSchema()
>       {
>       }
>
>       public ColumnSchema(String columnName, Type dataType, SqlDbType dbType, Int32 columnSize, Int16 precision, Int16 scale, Boolean isIdentity)
>       {
>          ColumnName = columnName;
>          DataType = dataType;
>          DbType = dbType;
>          ColumnSize = columnSize;
>          Precision = precision;
>          Scale = scale;
>          IsIdentity = isIdentity;
>       }
>    }
>
>   /// <summary>
>   /// TableSchema class - has a list of columns
>   /// </summary>
>    public class TableSchema
>    {
>        internal String TableName;
>        internal List<ColumnSchema> Columns = new List<ColumnSchema>();
>
>        public TableSchema(String table, List<ColumnSchema> columnsList)
>        {
>            TableName = table;
>            Columns = columnsList;
>        }
>       /// <summary>
>       /// Returns column's schema
>       /// </summary>
>       /// <param name="columnName"></param>
>       /// <returns></returns>
>        internal ColumnSchema GetColumnSchema(String columnName)
>        {
>           var columnSchema = Columns.Where(x => x.ColumnName == columnName).FirstOrDefault();
>
>            return columnSchema;
>        }
>
>        internal Type GetColumnType(String columnName)
>        {
>           var columnSchema = GetColumnSchema(columnName);
>           if (columnSchema==null)
>                return null;
>
>            return columnSchema.DataType;
>        }
>
>        internal SqlDbType? GetColumnDbType(String columnName)
>        {
>           var columnSchema = GetColumnSchema(columnName);
>           if (columnSchema == null)
>              return null;
>
>           return columnSchema.DbType;
>        }
>    }

As per Rob, if you only need to access the collection via the ColumnName (and do it often) then Dictionary will be the best option.