开关语句中的 C# 变量作用域

我有一个以这个开头的函数：

    void RecordLoadPosition()
    {
        OdbcCommand command;
        if (_hasPlantGenie)
        {
            command = new OdbcCommand();
            string query;
            switch (ItemType)
            {
                case ItemTypeEnum.COIL:
                    // We're only going to add records to the coils_pg table when the coil gets moved.  After
                    // a record is added, we'll update it.
                   command = new OdbcCommand("select * from coils_pg where coil_id = " + _itemName, _db);                       

当我编译它时，我在 if 块的第一行没有收到错误，但我收到错误，抱怨在案例块内声明之前我无法使用"命令"。 我不明白为什么函数顶部的声明在案例块中不可用。

不过还好。 如果它在案例块中不可见，我可以声明它。 我将案例块中的第一个语句更改为"OdbcCommand command..."。 现在我收到一个错误，抱怨我无法声明已经在父块中声明的变量！ 无论哪种方式，我都赢不了！

这是怎么回事？

当然，我只能使用不同的 OdbcCommand 对象，这就是我现在要做的，但我想了解这一点。

====

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我的原始代码示例中似乎确实缺少某些内容，但我不知道是什么。 这是一个小函数，它应该显示相同的错误，但没有：

    void ScopeTest()
    {
        OdbcCommand command = null;
        if (_hasPlantGenie)
        {
            command = new OdbcCommand();
            switch (ItemType)
            {
                case ItemTypeEnum.COIL:
                    // We're only going to add records to the coils_pg table when the coil gets moved.  After
                    // a record is added, we'll update it.
                    command = new OdbcCommand();
                    break;
            }
        }
    }

====

===================================================

而且，因为人们要求它，这里是完整的原始函数（包括在 if 块顶部额外创建一个 OdbcCommand 对象，只是为了证明它不会抛出错误：

    void RecordLoadPosition()
    {
        OdbcCommand command = null;
        if (_hasPlantGenie)
        {
            command = new OdbcCommand();
            string query;
            switch (ItemType)
            {
                case ItemTypeEnum.COIL:
                    // We're only going to add records to the coils_pg table when the coil gets moved.  After
                    // a record is added, we'll update it.
                    command = new OdbcCommand("select * from coils_pg where coil_id = " + _itemName, _db);
                    OdbcDataReader reader = command.ExecuteReader();
                    if (reader.Read())
                    {
                        query = "update plant_genie.coils_pg set x_coordinate = " +
                                XPosCurrent.ToString() +
                                ", y_coordinate = " +
                                YPosCurrent.ToString() +
                                " where coil_id = '" +
                                _itemName + "'";
                        reader.Close();
                        command.CommandText = query;
                        command.ExecuteNonQuery();
                    }
                    else
                    {
                        query = "insert into plant_genie.coils_pg(coil_id, x_coordinate, y_coordinate) values (" +
                                XPosCurrent.ToString() + YPosCurrent.ToString() +
                                "'" + _itemName + "')";
                        reader.Close();
                        command.CommandText = query;
                        command.ExecuteNonQuery();
                    }
                    break;
                case ItemTypeEnum.INNER_COVER:
                    // The inner_cover_pg table will be pre-built.  We can assume that it has records for
                    // each inner cover.
                    query = "select set_inner_cover_down(" +
                            XPosCurrent.ToString() +
                            ", " +
                            YPosCurrent.ToString() + 
                            ", '" +
                            _itemName +
                            "')";
                    OdbcCommand command = new OdbcCommand(query, _db);
                    command.ExecuteNonQuery();
                    // See if the cover has been set down in a storage location.  If it has
                    break;
            }
        }
    }

OdbcCommand command = new OdbcCommand(query, _db);

command = new OdbcCommand(query, _db);