重写ExpressionTree -参数'x'不在范围内

如果我在以下代码中犯了任何错误/输入错误，请不要生气，只需在这里添加注释，我会立即修复-谢谢

目标

将Expression<TDelegate>从一个EntityA重新映射到一个EntityB。

我怀疑这种事情以前已经做过了，但我还没有发现任何特别有用的链接，所以请随时为我指出正确的方向。

到目前为止，我所拥有的是一个类的选择，它们组合在一起，允许在两个给定类的实体成员之间创建映射。例如，表面API可能具有以下签名:

public void AddMemberBinding<TEntityA, TEntityB, TMember>(Func<TEntityA, TMember> entityAMemberSelector, Func<TEntityB, TMember> entityBMemberSelector)
{
    // does some magic, eventually storing the necessary MemberInfo details required to
    // "remap" MemberExpressions (MemberAccess) from TEntityA to TEntityB
}

给定下列类…

public class EntityA
{
    public long Id { get; set; }
    public string Name { get; set ;}
}
public class EntityB
{
    public long MyId { get; set; }
    public string MyName { get; set; }
}

您将能够创建绑定的东西沿着一行…

public static void AddBindings()
{
    AddMemberBinding((EntityA n) => n.Id, (EntityB n) => n.MyId);
    AddMemberBinding((EntityA n) => n.Name, (EntityB n) => n.MyName);
}

在我的情况下，我有Assembly1知道EntityA是什么，但不知道EntityB。我有Assembly2，它知道EntityA和EntityB是什么，并且对Assembly1可见。Assembly2为Assembly1提供了一个方法，可能如下所示:

public static IEnumerable<EntityA> GetData<TResult>(Expression<Func<EntityA, bool>> criteria, Expression<Func<EntityA, TResult>> selector)
{
    // EntityB's are stored in a database, I could do one of two things here...
    // 1) Return all EntitieB's and then apply criteria and selector through the IEnumerable extensions
    //    this would be sub-optimal - particularly if there are millions of EntityB's!
    // 2) "Transmute" (for lack of a better word) the expressions provided, using the keymappings
    //    specified earlier, to derive expressions that can be passed through to the QueryableProvider
    // ... as you might have guessed, I opted for #2
}

我使用的是ExpressionTree Visitor的派生版本，带有以下重写的方法:

protected override Expression VisitLambda(LambdaExpression lambda)
{
    Type targetParameterType = lambda.Parameters[0].Type;
    Type targetExpressionType = lambda.Type;
    If (lambda.Parameters.Count = 1 && lambda.Parameters(0).Type == EntityA)
    {
        targetParameterType = EntityB;
        // the `GetResultType` method called gets the TResult type from Func<T, TResult>
        Type targetExpressionResultType = GetResultType(lambda);
        targetExpressionType = gettype(Func<EntityB, targetExpressionResultType>) 
    }
    // this is probably wrong, but maintains the current (last) parameter instance
    // I started doing this after reading about a similar issue to mine found:
    // https://stackoverflow.com/questions/411738/expression-or-the-parameter-item-is-not-in-scope
    this.CurrentLambdaParameters = lambda.Parameters.Select(x => Expression.Parameter(targetParameterType, x.Name));
    Expression body = this.Visit(lambda.Body);
    If (body != lambda.Body)
    {
        return Expression.Lambda(targetExpressionType, body, this.CurrentLambdaParameters);
    }
    return lambda;
}
protected override Expression VisitMemberAccess(MemberExpression m)
{
    // at this point I go off and look at key mappings, fetch the mapping required, etc
    // the entity I retrieve has a `TargetMemberInfo` property which is a `MemberInfo` for the
    // member on the target entity - speaks for itself I guess...
    return Expression.MakeMemberAccess(this.CurrentParameters.Single(), myMappingClassThing.TargetMemberInfo);
}

问题

说了这么多，做了这么多，当我用测试用例运行代码时，我在标题中得到了错误…我可以从描述中看到这是一个参数问题，但是我读过一个类似的问题，我希望我通过使用我在修改根lambda表达式时创建的参数来解决VisitMemberAccess方法中的问题- ParameterExpression的相同实例修复了我认为的链接问题?

似乎我没有很好地理解这部分流程。问题是，我哪里做错了!?我需要用这些参数表达式做什么才能使它们在范围内?

提前感谢你的回答，如果你读到这里，向你致敬!!

protected override Expression VisitMemberAccess(MemberExpression m) 
{ 
    return Expression.MakeMemberAccess(Visit(m.Expression), myMappingClassThing.TargetMemberInfo); 
}