IWMDMStorage3：：GetMetadata 方法 (mswmdm.h)

项目
03/13/2024

GetMetadata 方法检索与存储关联的元数据。

语法

HRESULT GetMetadata(
  [out] IWMDMMetaData **ppMetadata
);

参数

[out] ppMetadata

指向与存储关联的 IWMDMMetaData 指针的指针。调用方负责在此接口上调用 Release ，并在使用完该接口时调用所有分配的值，如发现设备格式功能中的“清除分配的内存”中所述。

返回值

该方法返回 HRESULT。 Windows Media 设备管理器中的所有接口方法都可以返回以下任一类错误代码：

标准 COM 错误代码
转换为 HRESULT 值的 Windows 错误代码
Windows Media 设备管理器错误代码

有关可能错误代码的广泛列表，请参阅错误代码。

注解

此方法检索与存储关联的所有元数据。如果应用程序正在查找特定元数据，则调用 IWMDMStorage4：：GetSpecifiedMetadata 可能会更有效。

从 Windows 可移植设备 (WPD) 设备检索数据时，数据以二进制形式返回。应用程序应取消序列化此数据以获取实际属性值。

示例

以下 C++ 函数检索与存储关联的所有元数据。


// Function to print out all the metadata associated with a storage.
HRESULT CWMDMController::GetMetadata(IWMDMStorage *pStorage)
{
   HRESULT hr = S_OK;

   // A dummy loop to handle unrecoverable errors. When we hit an error we
   // can't handle or don't like, we just use a 'break' statement.
   // The custom BREAK_HR macro checks for failed HRESULT values and does this.
   do
   {
      CComPtr<IWMDMStorage3> pStorage3;
      CComPtr<IWMDMMetaData> pMetadata;
      hr = pStorage->QueryInterface(__uuidof(IWMDMStorage3), (void**)&pStorage3);
      BREAK_HR(hr, "Got an IWMDMStorage3 interface in GetMetadata.", "Couldn't get an IWMDMStorage3 interface in GetMetadata.");

      hr = pStorage3->GetMetadata(&pMetadata);
      BREAK_HR(hr, "Got an IWMDMMetaData interface in GetMetadata.", "Couldn't get an IWMDMMetaData interface in GetMetadata.");

      //
      // Loop through all metadata properties, and print out the value of each.
      //
      BYTE* value;
      WMDM_TAG_DATATYPE type;
      UINT len = 0;
      UINT count = 0;
      WCHAR* name;
      // Get the number of metadata items.
      hr = pMetadata->GetItemCount(&count);

      BREAK_HR(hr, "Got a metadata count in GetMetadata.", "Couldn't get a metadata count in GetMetadata.");
      for(;count > 0; count--)
      {
         // Get the metadata property by index.
         WCHAR* name;
         hr = pMetadata->QueryByIndex(count-1, &name, &type, &value, &len);
         if (SUCCEEDED(hr))
         {
            // TODO: Display the property name.
            CoTaskMemFree(name);

            // Print out the value of the property, according to the value type.
            switch (type)
            {
            case WMDM_TYPE_QWORD:
            case WMDM_TYPE_DWORD:
            case WMDM_TYPE_WORD:
               // TODO: Display the value.
               break;
            case WMDM_TYPE_STRING:
               // TODO: Display the value.
               // Release the method-allocated property value memory.
                    if (SUCCEEDED(hr))
                        CoTaskMemFree(value);
               break;
            case WMDM_TYPE_BOOL:
               // TODO: Display the value.
               break;
            case WMDM_TYPE_BINARY:
               // TODO: Display the value.
               break;
            case WMDM_TYPE_DATE:
               {
                  WMDMDATETIME *val = (WMDMDATETIME*)value;
                        // TODO: Display the month, day, and year.
               }
               break;
            case WMDM_TYPE_GUID:
               {
                  WCHAR strGuid[64];
                  StringFromGUID2(reinterpret_cast<GUID&>(value),(LPOLESTR)strGuid, 64);
                  // TODO: Display the GUID value.
               }
               break;
            default:
               // TODO: Display the message: "Could not understand the returned value type
            }
         }
         else // Couldn't get the metadata property at index count - 1.
            // TODO: Display the message: 
            // "Couldn't get a value for index " 
            // followed by the current index value.
      }

      // Now get a specific property by name.
      // If this property isn't supported, the method returns E_INVALIDARG.
      hr = pMetadata->QueryByName(g_wszWMDMFileName, &type, &value, &len);
      if (hr == S_OK) 
      {
         wstring wstr((wchar_t*)value, len / 2); // Create a string from the name.
            // TODO: Display the file name.
            CoTaskMemFree(value);
      }

      // See if file is DRM-protected.
      hr = pMetadata->QueryByName(g_wszWMDMIsProtected, &type, &value, &len);
      if (hr == S_OK)
      {
         // TODO: Display a message that the object is DRM protected.
      }


   }while(FALSE);// End of dummy loop.

   // Clean up and return.
   return hr;
}