How to invoke C++ DLL in ASP.NET?

It's called P/Invoke and uses System.Runtime.InteropServices and System.Runtime.InteropServices.DllImportAttribute.
You can find a simple code sample here: http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.aspx[^].

It may me fairly simple or quite tricky. To specify marshaling between native (unmanaged) and managed parameters you may also need to use the attribute System.Runtime.InteropServices.MarshalAsAttribute, see the code sample:
http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshalasattribute.aspx[^].

In more complex cases (in particular, when memory allocation is involved), you may need to use explicit marshaling methods with the class System.Runtime.InteropServices.Marshal, see http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.marshal.aspx[^].

In even more complex cases you may need to use custom marshaling. See, for example: http://msdn.microsoft.com/en-us/magazine/cc164085.aspx[^].

[EDIT]

After changes in Microsoft documentation, most adequate link to P/Invoke and C++/CLI implicit P/Invoke is this:
https://msdn.microsoft.com/en-us/library/ms235282.aspx[^].

—SA