Why use DllImport Attribute as apposed to adding a reference?

Question

I've seen a couple of examples such as this:

[DllImport("user32.dll")]
static extern bool TranslateMessage([In] ref Message lpMsg);

[DllImport("user32.dll")]
static extern IntPtr DispatchMessage([In] ref Message lpmsg);

But, what I don't understand is why someone would do that as apposed to just referencing the DLL like they do other libraries? The MSDN states: "The DllImport attribute is very useful when reusing existing unmanaged code in a managed application. For instance, your managed application might need to make calls to the unmanaged WIN32 API." But, is that saying it is not useful to reference an unmanaged dll or impossible otherwise?

Answer 1

Some libraries such as user32.dll are unmanaged code. Basically this means they do not have the required metadata to allow .Net to talk to them by reference (there is much more that goes into it but hopefully that gives you enough of a head start.)

• Using the DllImport Attribute
• DllImportAttribute
• Win32/COM
• PInvoke.Net <= more information about other unmanaged libraries for Windows
• Managed vs. Unmanaged Development

Answer 2

"But, is that saying it is not useful to reference an unmanaged dll or impossible otherwise?"

Yes, exactly so. What you're thinking of as 'referencing a DLL' is actually 'referencing a .NET assembly' - it just so happens that the most common way of packaging the kind of assemblies one tends to reference is in a DLL.

DLLImport is entirely about importing 'traditional DLLs' - i.e. ones which export all their methods using the original Windows DLL export mechanism.

Think of DLLImport as actually being called 'UnmanagedImport', and things might be clearer.

Answer 3

The .NET platform code compiles into Managed Code and it is stored using Assemblies, this assemblies are .DLL files BUT NOT ALL .DLL files are assemblies containing Managed Code. You only can use Managed Code with 'Add Reference' style.

Other languages and development techniques generates .DLL files with unmanaged code, actually you even can interoperate (call methods) with them but you need the DLLImport attribute