Data format feature requires the following namespace.
using unvell.ReoGrid.DataFormat;
To specify data format, use SetRangeDataFormat method of worksheet.
var sheet = reoGridControl.CurrentWorksheet;
sheet.SetRangeDataFormat(range, formatFlag, [Optional] object argument = null)
For example:
sheet.SetRangeDataFormat("A1:B2", CellDataFormatFlag.Number,
  new NumberDataFormatter.NumberFormatArgs()
  {
    // decimal digit places, e.g. 0.1234
    DecimalPlaces = 4,
                    
    // negative number style, e.g. -123 -> (123) 
    NegativeStyle = NumberDataFormatter.NumberNegativeStyle.RedBrackets,
 
    // use separator, e.g.123,456
    UseSeparator = true,
  });
Or use action. Operation done by using action can be undone by call Undo method or press Ctrl+Z:
reoGridControl.DoAction(new SetRangeDataFormatAction(range, formatFlag, [Optional] object dataFormatArgs = null));
Test:
sheet["B2"] = 12345;
sheet["C2"] = 12345.67890;
sheet["B3"] = -1234;
sheet["C3"] = -1234.56789;

Available Data Format
| Type | CellDataFormatFlag | Argument | 
|---|---|---|
| Number | CellDataFormatFlag.Number | NumberDataFormatter.NumberFormatArgs | 
| DateTime | CellDataFormatFlag.DateTime | DateTimeDataFormatter.DateTimeFormatArgs | 
| Percent | CellDataFormatFlag.Percent | PercentDataFormatter.PercentFormatArgs | 
| Currency | CellDataFormatFlag.Currency | CurrencyDataFormatter.CurrencyFormatArgs | 
| Text | CellDataFormatFlag.Text | None | 
| Custom Extension | CellDataFormatFlag.Custom | Any object | 
Number
Number format provides the generic number format with specifiable separator, number of digits for decimal place and negative number styles.

Example:

Code to set cell as number format:
sheet.SetRangeDataFormat("A1:E7", CellDataFormatFlag.Number,
  new NumberDataFormatter.NumberFormatArgs()
  {
    // decimal digit places 0.1234
    DecimalPlaces = 4,
    // use separator: 123,456
    UseSeparator = true,
  });
Negative number style
Negative number styles defined in enum NumberNegativeStyle.
ReoGrid provides the following styles to format a negative number.
| Negative number style | Example | Enum Value | 
|---|---|---|
| No style | -123,456.789 | Minus | 
| Red | 123,456.789 | Red | 
| Red Minus | -123,456.789 | Red |Minus | 
| Bracketed | (123,456.789) | Brackets | 
| Bracketed Red | (123,456.789) | Brackets |Red | 
| Bracketed Red Minus | (-123,456.789) | Brackets |Red |Minus | 
| Japanese Prefix | ▲ 123,456,789 | Prefix_Sankaku | 
| Japanese Prefix (ReoGrid Pro) | ▲ 123,456,789 | Prefix_Sankaku |Red | 
Example:

var sheet = grid.CurrentWorksheet;
sheet.SetRangeDataFormat("B2", CellDataFormatFlag.Number, 
  new NumberDataFormatter.NumberFormatArgs
  {
    DecimalPlaces = 3,
    NegativeStyle = NumberDataFormatter.NumberNegativeStyle.RedBrackets,
    UseSeparator = true,
  });
sheet["B2"] = -12345.67;
DateTime

sheet.SetRangeDataFormat(RangePosition.EntireRange, CellDataFormatFlag.DateTime,
  new DateTimeDataFormatter.DateTimeFormatArgs
  {
    // culture
    CultureName = "en-US",
    // pattern
    Format = "yyyy/MM/dd",
  });
Percent

sheet.SetRangeDataFormat(RangePosition.EntireRange, CellDataFormatFlag.Percent,
  new NumberDataFormatter.NumberFormatArgs
  {
    // decimal digit places
    DecimalPlaces = 2,
  });
Currency
Currency format extended from number format, it is possible to specify the currency symbol before or after currency number. For example:

var sheet = grid.CurrentWorksheet;
// add test code here
sheet.SetRangeDataFormat("B2", CellDataFormatFlag.Currency,
  new CurrencyDataFormatter.CurrencyFormatArgs
  {
    CultureEnglishName = "en-US",
    DecimalPlaces = 2,
    UseSeparator = true,
    PrefixSymbol = "$"
  });
sheet["B2"] = 1234;
Put space before symbol and number
It is possible to put a space after dollar symbol, for example:

sheet.SetRangeDataFormat("B2", CellDataFormatFlag.Currency,
  new CurrencyDataFormatter.CurrencyFormatArgs
  {
    CultureEnglishName = "en-US",
    DecimalPlaces = 2,
    UseSeparator = true,
    PrefixSymbol = **"$ "**
  });
Example: Set postfix symbol

sheet.SetRangeDataFormat("B2", CellDataFormatFlag.Currency,
  new CurrencyDataFormatter.CurrencyFormatArgs
  {
    CultureEnglishName = "en-US",
    DecimalPlaces = 2,
    UseSeparator = true,
    PostfixSymbol = " USD"
  });
Text
Set data format as Text to prevent data format.

sheet.SetRangeDataFormat(RangePosition.EntireRange, CellDataFormatFlag.Text, null);
Custom cell data formatter
To add custom cell data formatter, see Custom Data Format.