Class ValueObjectTypeHandler<TSelf, TValue>

Namespace: AdCodicem.ValueObjects.Dapper
Assembly: AdCodicem.ValueObjects.Dapper.dll

Maps a value object to and from its underlying column value for Dapper.

public sealed class ValueObjectTypeHandler<TSelf, TValue> : SqlMapper.TypeHandler<TSelf>, SqlMapper.ITypeHandler where TSelf : struct, IValueObject<TSelf, TValue>

Type Parameters

TSelf

Value object type.

TValue

Underlying value type.

Inheritance

object ← SqlMapper.TypeHandler<TSelf> ← ValueObjectTypeHandler<TSelf, TValue>

Implements

SqlMapper.ITypeHandler

Inherited Members

SqlMapper.TypeHandler<TSelf>.SetValue(IDbDataParameter, TSelf), SqlMapper.TypeHandler<TSelf>.Parse(object), object.Equals(object?), object.Equals(object?, object?), object.GetHashCode(), object.GetType(), object.ReferenceEquals(object?, object?), object.ToString()

Remarks

Reading uses the trusted factory, on the same reasoning as the Entity Framework Core converter: the rows come from a database this application wrote through the validating factory, and read paths are hot. A column holding text where the underlying type is not text, or the reverse, is the exception: the value object did not write it. Text read into a value object whose underlying type is not is parsed, and so validated, the way the value object parses text. Anything else read into a value object over - a number from a numeric column, a from a uuid or uniqueidentifier one - is converted to text, then validated through TryCreate.

A parameter declares the column a value object over is mapped to when the value object says more than its underlying type, as the Entity Framework Core conventions map it: an entity identifier as fixed-length, non-Unicode text of its exact length, a value object declaring Unicode = false as non-Unicode text of its maximum length or, declaring none, of the value's own, and one declaring a maximum length alone as Unicode text of that length. SQL Server then compares the parameter with the column as it is, with no implicit conversion of the column to cost an index seek, and an Always Encrypted varchar column takes it. The size is never smaller than the value, which both SqlClient and Npgsql would otherwise truncate.

A SQL NULL reads as null into an optional value object, TSelf?, and is refused with a for a required one in a single-column query, as Dapper refuses it for an . The handler implements itself for that, since cannot return null for a struct.

For a member of a mapped type, or a parameter of the constructor it is mapped through, Dapper checks for a NULL before it calls the handler, and never calls it: a required value object is left uninitialized, default(TSelf), and nothing throws. A column that can be NULL belongs in a TSelf? member.

Every the handler throws for a value it refuses carries the code of the rule in under , where reads it: the rule's own code, for a value that cannot be converted to TValue, and for a NULL.

Methods

Parse(object)

Parse a database value back to a typed value

public override TSelf Parse(object value)

Parameters

value object

The value from the database

Returns

TSelf

The typed value

Exceptions

DataException

value is a SQL NULL, which a value object cannot hold, a value that cannot be converted to TValue, or text, or a value converted to text, that the value object refuses.

SetValue(IDbDataParameter, TSelf)

Assign the value of a parameter before a command executes

public override void SetValue(IDbDataParameter parameter, TSelf value)

Parameters

parameter IDbDataParameter

The parameter to configure

value TSelf

Parameter value

Remarks

The handler refuses to write a value the value object rejects, which a read would then trust: only an instance equal to the default can hold one, since any other went through Create, and over a value type, where a constructed zero equals the default too, validation tells a valid zero from a refused one. Dapper calls the handler with the value object alone, whether the parameter is a TSelf or a TSelf? holding one, so it cannot tell a column that takes a NULL from one that does not, and refuses in both cases: a NULL is written from a TSelf? that holds nothing.

Exceptions

DataException

value equals default(TSelf) and holds a value the value object rejects.