c# - 记录 'Lenses' - with 表达式的表达式树

Question

有没有办法为新的 with 构建表达式树运算符？

我正在尝试为记录实现“Lens”功能，该功能只需要一个选择器并自动生成更改器(mutator)

我的目标是从“选择器”进行转换:

Expression<Func<T, TMember>> expression (即employee => employee.Name)

致“变异者”:

(employee, newName) => employee with { Name = newName }

我确实设法对上面的简单情况执行此操作，请参阅下面的答案，但这不适用于嵌套情况，即:

record Employee(string Name, int Age);
record Manager(String Name, Employee Employee);

这里我想改变 ie

manager => manager.Employee.Name

至

(manager, newEmployeeName) => manager with { Employee = manager.Employee with { Name = newEmployeeName}}

有什么帮助吗？

Answer 1

可以处理嵌套属性的

CalcMutator 方法看起来像这样

static Func<T, TMember, T> CalcMutator(Expression<Func<T, TMember>> expression)
{
    var typeParam = expression.Parameters.First();
    var valueParam = Expression.Parameter(typeof(TMember), "v");

    var variables = new List<ParameterExpression>();
    var blockExpressions = new List<Expression>();

    var property = (MemberExpression)expression.Body;
    Expression currentValue = valueParam;
    var index = 0;

    while (property != null)
    {
        var variable = Expression.Variable(property.Expression.Type, $"v_{index}");
        variables.Add(variable);

        var cloneMethod = property.Expression.Type.GetMethod("<Clone>$");
        if (cloneMethod is null) throw new Exception($"CalcMutatorNo Clone method on {typeof(T)}");
        var cloneCall = Expression.Call(property.Expression, cloneMethod);

        var assignClonedToVariable = Expression.Assign(variable, cloneCall);
        
        var accessVariableProperty = Expression.MakeMemberAccess(variable, property.Member);
        var assignVariablePropertyValue = Expression.Assign(accessVariableProperty, currentValue);

        blockExpressions.Add(assignClonedToVariable);
        blockExpressions.Add(assignVariablePropertyValue);

        property = property.Expression as MemberExpression;
        currentValue = variable;
        index++;
    }

    // Return root object
    blockExpressions.Add(currentValue);

    var block = Expression.Block(variables, blockExpressions);
    var assignLambda = (Expression<Func<T, TMember, T>>)Expression.Lambda(block, typeParam, valueParam);
    return assignLambda.Compile();
}

请记住，使用 ImmutableDictionary 实现的 Cache 不是线程安全的。如果您想确保缓存的表达式可以安全地在多线程环境中使用，最好使用 ConcurrentDictionary 进行缓存，或者在 ImmutableDictionary 周围应用一些同步原语.