我有一个以此开头的函数：
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，但是我收到错误，抱怨在 case 块中声明它之前我不能使用“命令”。我不明白为什么函数顶部的声明在 case 块中不可用。
但是没问题。如果它在 case 块中不可见，我可以声明它。我将案例块中的第一条语句更改为“OdbcCommand 命令...”。现在我收到一个错误，抱怨我不能声明已经在父块中声明的变量！我怎么都赢不了！
这里发生了什么？
当然，我可以只使用不同的 OdbcCommand 对象，这就是我现在要做的，但我想了解这一点。
==============================================
我的原始代码示例中似乎确实缺少某些东西，但我不知道是什么。这是一个应该显示相同错误但没有显示的小函数：
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引发错误：
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;
}
}
}