An auto-filter is attached to a range and holds a criterion per column.

Attaching one, and value lists

using unvell.ReoGrid.Core.Filtering;

AutoFilter filter = ws.CreateAutoFilter(RangePosition.Parse("A1:D100"));

// filter by a list of values (the checkbox style)
filter.SetColumnFilter(1, ["Tokyo", "Osaka"]);
filter.Apply();

bool active = filter.HasActiveFilter;
filter.ClearColumn(1);
filter.ClearAll();

ws.RemoveAutoFilter();

The range’s first row is the header row, and that is where the filter buttons are drawn.

What you pass to SetColumnFilter is a set of display strings (after number formatting has been applied). Passing null clears that column’s value-list filter.

Nothing changes on screen until you call Apply(). Set the criteria for every column first, then apply once.

Condition filters

// one condition
filter.SetColumnCondition(2, new FilterCondition(FilterOperator.GreaterThan, "1000"));

// two clauses joined by And / Or (Excel's "Custom AutoFilter")
filter.SetColumnCondition(3, new FilterCondition(
	new FilterClause(FilterOperator.GreaterOrEqual, "2026-01-01"),
	new FilterClause(FilterOperator.LessThan, "2027-01-01"),
	and: true));

filter.Apply();

There are ten FilterOperator values.

Operands are given as strings, but when one parses as a number or a date the comparison is typed.

• "> 5" → numeric comparison; text cells never match
• ">= 2026-02-01" → date comparison

The text operators (Contains and friends) match against the display string, case-insensitively.

An empty cell never matches (as in Excel).

Setting both a value list and a condition on the same column keeps rows satisfying both (AND).

Getting the candidate values

// what the dropdown offers (distinct display strings)
List<string> values = filter.GetColumnValues(1);
string title = filter.GetColumnTitle(1);

bool filtered = filter.IsColumnFiltered(1);

This is for building your own UI. The stock control builds its dropdown from exactly this.

Two kinds of hidden

Rows hidden by a filter are tracked separately from rows the user hid by hand.

They are kept apart so that clearing a filter does not bring back rows the user hid.

Relationship to sorting

Hidden rows are excluded from a sort. Sorting while filtered rearranges only the rows that are visible.

Range and buttons

The filter button is implemented as a general overlay that can be drawn on any cell (the XXXX │▽ shape), so a future table or pivot feature can reuse the same mechanism.

From the UI

Persistence

Round-trips through both reogrid-json and XLSX (<autoFilter>): the range, each column’s checked-value list (<filters>) and its condition (<customFilters>).

Excel has no “contains” operator, so the text operators are written as equality against a wildcard operand (*text*) and recognized again on the way back. Date-group trees and the color/icon/dynamic filters are out of scope — a column carrying only those loads unfiltered. The streaming reader (OpenVirtual) is excluded, because its rows load lazily.

What to read next

• Sorting
• Rows and Columns