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Gets or sets a value determining how the widths of table columns are calculated. The default is Fixed.
public TableSizingModeEnum ColumnSizingMode { get; set; }
To make a table with automatically calculated columns' widths, set this property to Auto, and the Width on the RenderTable to Auto, e.g.:
RenderTable rt = new RenderTable();
rt.ColumnSizingMode = TableSizingModeEnum.Auto;
rt.Width = Unit.Auto;