X 不能在此范围内声明 + X 不能在声明之前使用

Question

我正在关注 Microsoft tutorial 使用 Azure Spatial Anchors 创建一个新的 HoloLens Unity 应用程序并且给定的代码有一些错误。

错误
'distance' cannot be declared in this scope because that name is used in an enclosing local scope 是第一个遇到的错误。我试图通过在distance前面评论float来解决它，但后来我得到了Cannot use local variable 'distance' before it is declared + Cannot infer the type of implicitly-typed deconstruction variable 'distance'.

private bool IsAnchorNearby(Vector3 position, out GameObject anchorGameObject)
{
    anchorGameObject = null;

    if (_foundOrCreatedAnchorGameObjects.Count <= 0)
    {
        return false;
    }

    //Iterate over existing anchor gameobjects to find the nearest
    var (distance, closestObject) = _foundOrCreatedAnchorGameObjects.Aggregate(
        new Tuple<float, GameObject>(Mathf.Infinity, null),
        (minPair, gameobject) =>
        {
            Vector3 gameObjectPosition = gameobject.transform.position;
            float distance = (position - gameObjectPosition).magnitude;
            return distance < minPair.Item1 ? new Tuple<float, GameObject>(distance, gameobject) : minPair;
        });

    if (distance <= 0.15f)
    {
        //Found an anchor within 15cm
        anchorGameObject = closestObject;
        return true;
    }
    else
    {
        return false;
    }
}

本教程的代码有什么问题？

Answer 1

t看起来错误是由在Aggregate方法中的lambda表达式内外使用变量名distance引起的。在 lambda 表达式中，距离变量被声明为局部变量，这意味着它不能在 lambda 表达式之外使用。

修复此错误的一种方法是将 lambda 表达式中的距离变量重命名为其他名称，如下所示：

var (distance, closestObject) = _foundOrCreatedAnchorGameObjects.Aggregate(
    new Tuple<float, GameObject>(Mathf.Infinity, null),
    (minPair, gameobject) =>
    {
        Vector3 gameObjectPosition = gameobject.transform.position;
        float dist = (position - gameObjectPosition).magnitude;
        return dist < minPair.Item1 ? new Tuple<float, GameObject>(dist, gameobject) : minPair;
    });

或者，您可以删除 lambda 表达式中距离变量的声明，如下所示：

var (distance, closestObject) = _foundOrCreatedAnchorGameObjects.Aggregate(
    new Tuple<float, GameObject>(Mathf.Infinity, null),
    (minPair, gameobject) =>
    {
        Vector3 gameObjectPosition = gameobject.transform.position;
        return (position - gameObjectPosition).magnitude < minPair.Item1 ? 
               new Tuple<float, GameObject>((position - gameObjectPosition).magnitude, gameobject) : minPair;
    });

在这种情况下，distance 变量将被赋予 position 和 gameObjectPosition 之间的距离值，而不是被声明为局部变量。