A validation rule is attached to a range and is evaluated only at the moment a value is committed to a cell. It never runs during rendering, so a validated sheet scrolls exactly as fast as one without rules.

Lists

using unvell.ReoGrid.Core;
using unvell.ReoGrid.Core.Validation;

// options given inline
ws.AddValidation(RangePosition.Parse("C2:C100"), new ListValidationRule
{
	Options = ["Tokyo", "Osaka", "Nagoya"],
});

// options read from cells (this sheet, another sheet, or a defined name)
ws.AddValidation(RangePosition.Parse("D2:D100"), new ListValidationRule
{
	Source = "=Master!$A$1:$A$20",
});

Source accepts any of these:

Options wins when both are given. Blank cells in the source are left out of the list.

ShowDropdown = false hides the arrow but keeps the rule in force — the same as clearing Excel’s “In-cell dropdown” checkbox.

Numbers, dates, times and lengths

// whole numbers from 1 to 100
ws.AddValidation(RangePosition.Parse("B2:B100"), new ComparisonValidationRule
{
	Kind = ComparisonKind.Whole,
	Operator = ValidationOperator.Between,
	Value1 = ValidationValue.Num(1),
	Value2 = ValidationValue.Num(100),
});

// today or later — a bound may be a formula
ws.AddValidation(RangePosition.Parse("E2:E100"), new ComparisonValidationRule
{
	Kind = ComparisonKind.Date,
	Operator = ValidationOperator.GreaterThanOrEqual,
	Value1 = ValidationValue.Str("=TODAY()"),
});

// at most eight characters
ws.AddValidation(RangePosition.Parse("F2:F100"), new ComparisonValidationRule
{
	Kind = ComparisonKind.TextLength,
	Operator = ValidationOperator.LessThanOrEqual,
	Value1 = ValidationValue.Num(8),
});

There are five ComparisonKinds:

ValidationOperator has Excel’s eight members — Between, NotBetween, Equal, NotEqual, GreaterThan, LessThan, GreaterThanOrEqual, LessThanOrEqual. Value2 matters only for Between / NotBetween.

A bound may be a number, a formula (a string starting with =), or a literal the kind can read ("2026-04-01", "09:00"). A formula bound may reference cells, so the rule follows them.

Custom formulas

// The formula is written for the range's top-left cell; relative references
// shift by each cell's offset inside the range.
ws.AddValidation(RangePosition.Parse("A2:A100"), new CustomValidationRule
{
	Formula = "=COUNTIF($A$2:$A$100, A2)=1",   // no duplicates
});

A truthy result passes. References shift exactly the way a conditional-format expression rule’s do, and $-anchored references stay put. The formula runs on the sheet’s own engine, so cross-sheet references and defined names work.

Messages and alert styles

ws.AddValidation(RangePosition.Parse("B2:B100"), new ComparisonValidationRule
{
	Kind = ComparisonKind.Decimal,
	Operator = ValidationOperator.GreaterThanOrEqual,
	Value1 = ValidationValue.Num(0),

	// shown while the cell is selected
	ShowInputMessage = true,
	InputTitle = "Amount",
	InputMessage = "Enter zero or more.",

	// shown when an entry is rejected; only Stop blocks the write
	AlertStyle = ValidationAlertStyle.Warning,
	ErrorTitle = "Negative amount",
	ErrorMessage = "That amount is negative.",

	IgnoreBlank = true,
});

With IgnoreBlank (default true), empty input always passes. ShowErrorMessage = false suppresses the alert without changing what a Stop rule does.

When rules run

• On an in-cell commit and on a formula-bar entry — those two only.
• A formula entry (=…) is not checked. Its result is unknown at entry time; Excel behaves the same way.
• Paste, auto-fill and SetValue are not checked either, again matching Excel. A rule guards what a person types.

Checking it yourself

// the dropdown options (list rules only; empty when the arrow is suppressed)
IReadOnlyList<string> options = ws.GetValidationListOptions(1, 2);

// check an entry yourself — this is what the controls call on commit
ValidationResult result = ws.ValidateInput(1, 1, "-5");
if (!result.IsValid && result.Blocks)
{
	string? title = result.Title;
	string? message = result.Message;
	_ = (title, message);
}

ValidationRule? rule = ws.GetValidation(1, 1);
_ = (options, rule);

Removing rules

string id = ws.AddValidation(RangePosition.Parse("B2:B100"), new AnyValidationRule());

// drop one rule by id
ws.RemoveValidation(id);

// clear a range: a rule reaching outside it is split, so the rest survives
ws.ClearValidations(RangePosition.Parse("B10:B20"));

bool any = ws.HasValidations;
_ = any;

AnyValidationRule accepts anything. Paired with ShowInputMessage it gives you a cell that only prompts.

What the controls do

• When the selected cell carries a list rule, the dropdown arrow is drawn just outside the cell’s right edge — Excel’s position, so it never covers the cell’s own text. Click it, or press Alt+Down, to pick.
• An ShowInputMessage prompt appears in a bubble under the cell.
• Neither is printed or exported to PDF (GridViewport.ShowValidationUI is off there).
• A rejected entry raises CellValidationFailed first. Set Handled to substitute your own dialog, and Reject to overrule the decision. Left alone, WinForms and WPF show Excel’s message box; Avalonia has no built-in modal dialog and draws a transient in-grid message instead.

Undo and I/O

• Adding and clearing rules records an undo step through RangeFacets.Validations, and undoing a row or column change restores each rule’s range.
• reogrid-json: stored as the sheet’s validations, the same shape reogrid-web uses.
• XLSX: read and written as <dataValidations>. A sqref naming several ranges expands to one entry per range. Excel’s showDropDown is an inverted attribute — it hides the arrow — and that difference is absorbed here.

Trying it in Studio

Data ▸ Data Validation… opens the dialog and applies to the selection. Data ▸ Clear Validation clears just the selected cells.