c# - 从范围 'x.Sub' 引用类型为 'SubType' 的变量 '' 但未定义错误

Question

检查这个 fiddle 的错误:https://dotnetfiddle.net/tlz4Qg

我有两个这样的类:

public class ParentType{
    private ParentType(){}

    public int Id { get; protected set; }
    public SubType Sub { get; protected set; }
}

public class SubType{
    private SubType(){}

    public int Id { get; protected set; }
}

我要将多级匿名表达式转换为多级非匿名表达式。为了实现这一点，我有一个类似于下面提到的表达式:

x => new
{
   x.Id,
   Sub = new
   {
      x.Sub.Id
   }
}

为了实现这个目标，我将其转换为如下表达式:

x => new ParentType()
{
   Id = x.Id,
   Sub = new SubType()
   {
      Id = x.Sub.Id
   },
 }

但是当我调用 Compile() 方法时，出现以下错误:

Variable 'x.Sub' of type 'SubType' referenced from scope '' but it is not defined

这是我的访客类:

public class ReturnTypeVisitor<TIn, TOut> : ExpressionVisitor
{
    private readonly Type funcToReplace;
    private ParameterExpression currentParameter;
    private ParameterExpression defaultParameter;
    private Type currentType;

    public ReturnTypeVisitor() => funcToReplace = typeof(Func<,>).MakeGenericType(typeof(TIn), typeof(object));

    protected override Expression VisitNew(NewExpression node)
    {
        if (!node.Type.IsAnonymousType())
            return base.VisitNew(node);

        if (currentType == null)
            currentType = typeof(TOut);

        var ctor = currentType.GetPrivateConstructor();
        if (ctor == null)
            return base.VisitNew(node);

        NewExpression expr = Expression.New(ctor);
        IEnumerable<MemberBinding> bindings = node.Members.Select(x =>
        {
            var mi = currentType.GetProperty(x.Name);

 //if the type is anonymous then I need to transform its body
                if (((PropertyInfo)x).PropertyType.IsAnonymousType())
                {
 //This section is became unnecessary complex!
 //
                    var property = (PropertyInfo)x;

                    var parentType = currentType;
                    var parentParameter = currentParameter;

                    currentType = currentType.GetProperty(property.Name).PropertyType;

                    currentParameter = Expression.Parameter(currentType, currentParameter.Name + "." + property.Name);

 //I pass the inner anonymous expression to VisitNew and make the non-anonymous expression from it
                    var xOriginal = VisitNew(node.Arguments.FirstOrDefault(a => a.Type == property.PropertyType) as NewExpression);

                    currentType = parentType;
                    currentParameter = parentParameter;

                    return (MemberBinding)Expression.Bind(mi, xOriginal);
                }
                else//if type is not anonymous then simple find the property and make the memberbinding
                {
                    var xOriginal = Expression.PropertyOrField(currentParameter, x.Name);
                    return (MemberBinding)Expression.Bind(mi, xOriginal);
                }
        });

        return Expression.MemberInit(expr, bindings);
    }

    protected override Expression VisitLambda<T>(Expression<T> node)
    {
        if (typeof(T) != funcToReplace)
            return base.VisitLambda(node);

        defaultParameter = node.Parameters.First();

        currentParameter = defaultParameter;
        var body = Visit(node.Body);

        return Expression.Lambda<Func<TIn, TOut>>(body, currentParameter);
    }
}

然后像这样使用它:

public static Expression<Func<Tin, Tout>> Transform<Tin, Tout>(this Expression<Func<Tin, object>> source)
    {
        var visitor = new ReturnTypeVisitor<Tin, Tout>();
        var result = (Expression<Func<Tin, Tout>>)visitor.Visit(source);
        return result;// result.Compile() throw the aforementioned error
    }

这是在我的 Visitor 类中使用的扩展方法:

public static ConstructorInfo GetPrivateConstructor(this Type type) =>
            type.GetConstructor(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic, null, Type.EmptyTypes, null);

// this hack taken from https://stackoverflow.com/a/2483054/4685428
// and https://stackoverflow.com/a/1650895/4685428
public static bool IsAnonymousType(this Type type)
{
 var markedWithAttribute = type.GetCustomAttributes(typeof(CompilerGeneratedAttribute), inherit: false).Any();
 var typeName = type.Name;

 return markedWithAttribute
               && (typeName.StartsWith("<>") || type.Name.StartsWith("VB$"))
               && typeName.Contains("AnonymousType");
}

更新

这是问题的 .Net Fiddle 链接:https://dotnetfiddle.net/tlz4Qg

更新

我已经删除了似乎超出问题范围的额外代码。

Answer 1

问题的原因是线路

currentParameter = Expression.Parameter(currentType, currentParameter.Name + "." + property.Name);

在 VisitNew 方法中。

对于您的示例，它会创建一个名为“x.Sub”的新参数，因此如果我们用 {} 标记参数，实际结果是

Sub = new SubType()
{
    Id = {x.Sub}.Id
}, 

超出预期

Sub = new SubType()
{
    Id = {x}.Sub.Id
},

一般情况下，除非重新映射 lambda 表达式，否则不应创建新的 ParameterExpression。并且所有新创建的参数都应传递给 Expression.Lambda 调用，否则它们将被视为“未定义”。

另请注意，访问者代码有一些一般情况下不成立的假设。例如

var xOriginal = Expression.PropertyOrField(currentParameter, x.Name);

不会在嵌套的 new 中工作，因为在那里您需要访问 x 参数的成员，例如 x.Sub.Id而不是 x.Id。这基本上是来自 NewExpression.Arguments 的对应表达式。

使用表达式访问者处理嵌套的 lambda 表达式或集合类型成员和 LINQ 方法需要更多的状态控制。虽然像示例中那样转换简单的嵌套匿名 new 表达式甚至不需要 ExpressionVisitor，因为它可以通过像这样的简单递归方法轻松实现:

public static Expression<Func<Tin, Tout>> Transform<Tin, Tout>(this Expression<Func<Tin, object>> source)
{
    return Expression.Lambda<Func<Tin, Tout>>(
        Transform(source.Body, typeof(Tout)),
        source.Parameters);
}

static Expression Transform(Expression source, Type type)
{
    if (source.Type != type && source is NewExpression newExpr && newExpr.Members.Count > 0)
    {
        return Expression.MemberInit(Expression.New(type), newExpr.Members
            .Select(m => type.GetProperty(m.Name))
            .Zip(newExpr.Arguments, (m, e) => Expression.Bind(m, Transform(e, m.PropertyType))));
    }
    return source;
}