C++/CLI 中的 System::Object 到 int 或 double

Question

我正在从数据库中获取一些信息，我想在计算中使用它。但由于我写的内容，我无法将其转换为数字。我收到 System::Object^。这是代码部分：

OleDbConnection ^ cnNwind = gcnew OleDbConnection();

cnNwind-> ConnectionString = 
L"Provider = Microsoft.Jet.OLEDB.4.0;" 
L"Data Source = C:\\temp\\A.mdb";

try
{
    // Open the database
    cnNwind->Open();
    Console::WriteLine(L"Connected to database successfully!");

    // Count the customers
    OleDbCommand ^ cmProducts = gcnew OleDbCommand();
    cmProducts->CommandText = L"SELECT ID FROM Table1";
    cmProducts->CommandType = CommandType::Text;
    cmProducts->Connection = cnNwind;

    // Print the result
    Object ^ numberOfProducts = cmProducts->ExecuteScalar();
    Console::Write(L"Number of products: ");
    Console::WriteLine(numberOfProducts);
}
catch (OleDbException ^ pe)
{
    Console::Write(L"Error occurred: ");
    Console::WriteLine(pe->Message);
}
// Close the connection
if (cnNwind->State != ConnectionState::Closed)
{
    cnNwind->Close();
}

Console::WriteLine(L"The database connection is closed...");

我想使用 numberOfProducts 作为数字。我的意思是输入double 或integer。我该如何改造它？

Answer 1

只需使用 safe_cast 即可投射 Object^ 为适当的类型。本页对此进行了详细介绍：如何：在 C++/CLI 中使用 safe_cast< /a>

Object^ numberOfProductsObj = cmProducts->ExecuteScalar();
// IIF the underlying type is System::Int32
int numberOfProducts = safe_cast<int>(numberOfProductsObj);
// or, IIF the underlying type is System::Double
double numberOfProducts = safe_cast<double>(numberOfProductsObj);

由于只能有一种基础类型（而且我不知道你的情况是什么），因此只有其中一种可以工作——另一种会抛出异常。重点是，第一步是确定实际的基础类型（大概是 double、float 或 int）。