Supported data types

.NET value	Column	Stored as
string	VARCHAR(n), CHAR(n), TEXT	as written (CHAR(n) pads with spaces when it stores and drops them when it reads)
byte / short / int / long	TINYINT / SMALLINT / INT / BIGINT	100 / 32000 / 2000000000 / 9000000000000
uint	INT UNSIGNED	4000000000
decimal	DECIMAL(p, s)	12345.678901
float / double	FLOAT / DOUBLE	3.25 / 2.718281828459045 (a FLOAT keeps about six significant digits)
bool	BOOLEAN (TINYINT(1))	1
Guid	CHAR(36)	6f9619ff-8b86-d011-b42d-00c04fc964ff (lower case); to a BINARY(16) column as its 16 bytes when the schema is preloaded
DateTimeOffset	TIMESTAMP(6)	the moment, stored as UTC: 22:30 +08:00 is 14:30:00.000000 (the offset isn't kept)
DateTime	DATETIME(6), DATETIME	as written: 2026-11-07 09:00:00.000000 (a plain DATETIME rounds to the second)
DateOnly / TimeOnly	DATE / TIME(6)	2026-11-07 / 17:45:30.000000
byte[], ArxBytea	BLOB, LONGBLOB, BINARY(n)	0xCAFE
ArxJsonB	JSON	MySQL's own form of the document (invalid JSON: ARX4003)
ArxEntityReference	the lookup column	the referenced id
ArxSelectionSetValue	the set's column (an ENUM or SET column included)	the option's code
null	any	NULL
sbyte / ushort / uint / ulong	converted to SMALLINT / INT / BIGINT / DECIMAL	-100 / 65000 / 4000000000 / 18446744073709551615
char	text	X
a plain enum	refused (ARX1007)	write (int)value or value.ToString(), or use a selection set, which stores the code its set declares
DateTime with Kind Utc	TIMESTAMP(6)	the moment
DateTime with Kind Local	DATETIME(6), TIMESTAMP(6)	converted to UTC first

Refused before anything is sent: any other type (ARX1007 UnsupportedValueType: write uri.ToString() rather than a Uri); arrays (ARX8001: MySQL has no array columns, so store a list as delimited text, JSON or rows of its own); NaN and the infinities (ARX4001). A TIME column also holds durations, up to ±838 hours (a TimeSpan).

With the column types preloaded (ArxPreLoadMode.DataTypes), a number with a fraction written to a whole-number column is refused before MySQL (ARX4001 ValueOutOfRange) on a create, an update and a write by a filter, where MySQL would round it (7.9 stored as 8); a condition in a read (qty < 7.5) isn't refused. A NaN or an infinity written to a declared decimal column is refused the same way.

MySQL converts some values itself: "42" into an INT, 2.5 into an INT by rounding (half away from zero), a year-first string such as '2026-11-07' into a DATE. Text that isn't a number, and a date string in any other order, are errors (ARX4003). A preload with column types (ArxPreLoadMode.DataTypes) refuses a value of the wrong type before the database. RealCase2_EveryDataType writes every row of this table and prints what MySQL stored.

You have	Use	Column
A moment (an order was placed)	DateTimeOffset, or DateTime with Kind Utc	TIMESTAMP(6) (1970 to 2038)
A moment, past 2038	DateTime with Kind Utc	DATETIME(6) (store UTC, read it back as UTC)
A wall-clock time with no zone	DateTime with Kind Unspecified	DATETIME(6)
A calendar date	DateOnly	DATE
A time of day	TimeOnly	TIME(6)
A duration	a number (seconds, minutes), or a TimeSpan up to 838 hours	INT, BIGINT, TIME

A TIMESTAMP is converted from the session's time zone to UTC when stored and back when read: ARX's sessions are UTC, so the instant is stored and read as it is. A DATETIME stores the clock time it is given: a DateTime with Kind Unspecified or Utc as it is, and one with Kind Local (DateTime.Now) converted to UTC first, as every ARX write does with a local time. It reads back with no zone, so use Kind Unspecified for a wall-clock time and Utc for a moment, never Local. Fractional seconds are kept only when the column declares them ((6)); MySQL rounds to the column's precision. Refused: default(DateTime) (ARX1013: it means "not set"; use null), and a date string MySQL can't read (ARX4003: ARX never parses strings as dates). Full rules: Data types.

MySQL column	Reads back as	Notes
VARCHAR(n), CHAR(n), TEXT	string	CHAR(n) drops its padding spaces when read; "" stays ""
TINYINT / SMALLINT / INT / BIGINT	sbyte / short / int / long	TINYINT UNSIGNED is a byte
INT UNSIGNED, BIGINT UNSIGNED	uint, ulong
DECIMAL(p, s)	decimal	More than 28 significant digits: ARX4001 ValueOutOfRange
FLOAT / DOUBLE	float / double
BOOLEAN (TINYINT(1))	bool
CHAR(36)	Guid	Text that isn't a Guid: ARX5003 SchemaMismatch
BINARY(16)	byte[]; a Guid for a declared Guid id
TIMESTAMP	DateTime, Kind Utc	The instant, read in ARX's UTC session (on your own connection, in its session's zone)
DATETIME	DateTime, Kind Unspecified	The clock time as stored: MySQL keeps no zone
DATE / TIME	DateOnly / TimeOnly	A TIME past 24 hours or below zero is a TimeSpan
BLOB, LONGBLOB, BINARY(n)	byte[]
JSON	string	MySQL's own form of the document
ENUM, SET	string	The label; a SET's labels comma-separated
A declared lookup (.Lookup(...))	ArxEntityReference	Name from the referenced table's .DisplayName(...)
A selection set column	Its stored code	entity.GetSelectionSet(column, set, catalog) gives the option
NULL	null	The attribute is present, with a null value

RealCase1_EveryDataTypeReadBack writes one row of every type with Create and prints each value's .NET type as it reads back.