For documentation, samble, SDKs and whatever else, your starting point should be 'Autodesk Developer Network Open'.
For your specific questions:
In you want to develop plug-ins (internal application extensions) for AutoCAD, you will have a complete and well documented access, but you wouldn't be able to have a single binary working on every version. AutoCAD breaks binary compatibility every 3 years, if not less. So, you would be able to have a single DLL working for 2010-2011-2012 version, one for 2013-2014 and one for 2015. Basically you need to change the NET library version. I'm writing out of memory, I'm sure you'll find better references.
If you want to develop a stand-alone application you may choose your compiler/language/NET library, but you'll still need COM interfaces.
For easier development you may prefer to reference a specific AutoCAD Type Library (2012, 2013, 2014, 2015, ecc.) but this will tie your application to a specific version, thus falling back to the plugin problem.
You may however develop a stand alone application without referencing a specific type library. This will free your application, but would be extremely more complicated to develop.
For example, this is the code for generic access to an AutoCAD application and get the list of Documents:
public static List<String> GetOpenFiles()
{
List<String> result = new List<String>();
try
{
// get Application COM Object
Object acadApp = Marshal.GetActiveObject("AutoCAD.Application");
if (acadApp == null) return null;
// Get current document list
object objDoc = acadApp.GetType().InvokeMember("Documents", BindingFlags.IgnoreCase | BindingFlags.GetProperty, null, acadApp, null);
if (objDoc == null) return null;
Int64 iCount = Convert.ToInt64(objDoc.GetType().InvokeMember("Count", BindingFlags.IgnoreCase | BindingFlags.GetProperty, null, objDoc, null));
for (int i = 0; i < iCount; i++)
{
object[] dataArray = new object[1];
dataArray[0] = i;
object obj = objDoc.GetType().InvokeMember("Item", BindingFlags.IgnoreCase | BindingFlags.InvokeMethod, null, objDoc, dataArray);
if (obj != null)
{
object objName = obj.GetType().InvokeMember("FullName", BindingFlags.IgnoreCase | BindingFlags.GetProperty, null, obj, null);
String szName = objName.ToString();
object objRead = obj.GetType().InvokeMember("ReadOnly", BindingFlags.IgnoreCase | BindingFlags.GetProperty, null, obj, null);
bool bReadOnly = Convert.ToBoolean(objRead);
if (szName.Length > 0) result.Add((bReadOnly ? "R": "W") + szName);
}
}
}
catch (System.Exception ex)
{
result = null;
}
return result;
}
Given the added complexity, I'd suggest to go for a multiple plugin versions. The difference between AutoCAD versions are minimal, mainly the NET framework version. The Autoloader mechanism is easy enough to allow to distribute a single bundle folder for every AutoCAD version you may decide to support.
I usually maintain and develop against the latest version, and if needed, I'll port the changes on older versions projects. However, I do not support versions older than 2012, since it was the first version to support the autoloader feature with .bundle folders.
