Маппинг на основе выражения доступа к свойству

author	version	line-number	content
		1	{{code language="c#"}}
		2	public class ExpressionMapper2<TEntity, TValue>
		3	{
		4	private readonly Dictionary<
		5	string,
		6	TValue
		7	> Mapping;
		8
		9	private readonly Func<IEnumerable<Expression>, string> KeySelector;
		10	private static readonly Func<IEnumerable<Expression>, string> DefaultKeySelector =
		11	(e) =>
		12	{
		13	var expression = e.First(e2 => e2 is MemberExpression) as MemberExpression;
		14	return expression.Member.Name;
		15	};
		16
		17	public ExpressionMapper2(
		18	IEnumerable<(string key, TValue value)> source,
		19	Func<IEnumerable<Expression>, string>? keySelector = null
		20	)
		21	{
		22	KeySelector = keySelector ?? DefaultKeySelector;
		23	Mapping = source.ToDictionary(
		24	e => e.key,
		25	e => e.value
		26	);
		27	}
		28	public ExpressionMapper2(
		29	IEnumerable<(Expression<Func<TEntity, object>> keyExpression, TValue value)> source,
		30	Func<IEnumerable<Expression>, string>? keySelector = null
		31	)
		32	{
		33	KeySelector = keySelector ?? DefaultKeySelector;
		34	Mapping = source.ToDictionary(
		35	e => GetKey(e.keyExpression),
		36	e => e.value
		37	);
		38	}
		39
		40
		41	private string GetKey(Expression expression)
		42	{
		43	Visitor visitor = new Visitor();
		44	visitor.Visit(expression);
		45	var key = KeySelector(visitor.ExpressionList);
		46	return key;
		47	}
		48
		49	public bool TryGetValue<T>(
		50	Expression<Func<TEntity, T>> expression,
		51	out TValue value
		52	)
		53	{
		54	var key = GetKey(expression);
		55	return Mapping.TryGetValue(
		56	key,
		57	out value
		58	);
		59	}
		60
		61	public TValue GetValue<T>(
		62	Expression<Func<TEntity, T>> expression
		63	)
		64	{
		65	if (TryGetValue(expression, out var value))
		66	{
		67	return value;
		68	}
		69	else
		70	{
		71	throw new KeyNotFoundException();
		72	}
		73	}
		74
		75	// Можно сделать более эффективную и строгую реализацию
		76	// для примера наиболее простая и общая
		77	private sealed class Visitor
		78	: ExpressionVisitor
		79	{
		80	public List<Expression> ExpressionList { get; private set; }
		81	= new List<Expression>();
		82
		83
		84	[return: NotNullIfNotNull("node")]
		85	public override Expression? Visit(
		86	Expression? node
		87	)
		88	{
		89	ExpressionList.Add(node);
		90	return base.Visit(node);
		91	}
		92	}
		93
		94	public static Builder CreateBuilder()
		95	{
		96	return new Builder();
		97	}
		98
		99	public class Builder
		100	{
		101	private Func<IEnumerable<Expression>, string> KeySelector = DefaultKeySelector;
		102	private List<(Expression, TValue)> Rules = new List<(Expression, TValue)>();
		103
		104
		105	public Builder SetKeySelector(
		106	Func<IEnumerable<Expression>, string> keySelector
		107	)
		108	{
		109	KeySelector = keySelector;
		110	return this;
		111	}
		112
		113	public Builder AddRule<T>(
		114	Expression<Func<TEntity, T>> rule,
		115	TValue value
		116	)
		117	{
		118	Rules.Add((rule, value));
		119	return this;
		120	}
		121
		122
		123	public ExpressionMapper2<TEntity, TValue> Build()
		124	{
		125	(string key, TValue value)[] rules = new (string key, TValue value)[Rules.Count];
		126	{
		127	Visitor visitor = new Visitor();
		128	int i = 0;
		129	foreach (var elem in Rules)
		130	{
		131	visitor.Visit(elem.Item1);
		132	var key = KeySelector(visitor.ExpressionList);
		133	visitor.ExpressionList.Clear();
		134
		135	rules[i] = (key, elem.Item2);
		136	i++;
		137	}
		138	}
		139
		140	return new ExpressionMapper2<TEntity, TValue>(
		141	rules,
		142	KeySelector
		143	);
		144	}
		145	}
		146	}
		147	{{/code}}