Debugging ArcMap Add-In .Net 4.5 in Visual Studio 2013: Unable to start ArcMap.exe

In Visual Studio 2013, to fix this check Use Managed Compatibility mode in VS2013 > Options > Debugging > General (last option in the list).

Original answer by Preston McCormick here

In order to debug ArcMap 10.3 Add-in with VS2013 in .Net 4.5. I had to attach VS2013 to ArcMap.exe process. This is kinda annoying but it is the only way I got it to work for now. I'm still confused on why this is happening.

How to attach process

1. Start ArcMap
2. In VS2013, go to menu "DEBUG" -> "Attach to Process"
3. Select "ArcMap.exe" and click on "Attach" button
4. In ArcMap, make sure the Add-In is installed.

If the Add-in is not installed whe you attach the process, you can try one of these things :

• Stop debugging and reattach process until Add-Inn is installed
• Install Add-in manually from your add-in debug folder.

Here is how I resolve this issue and it works pretty much every time it comes up.

1. Restart VS2013
2. Right click on project -> Debug -> Start new instance (this will prevent it from building first)

If above doesn't work then:

1. Restart VS2013
2. Use Select some project as Startup then click Start in the toolbar.

If that doesn't work this works every time:

1. delete all DLLs from output directory (we have one output directory for all DLLs/EXEs that we're building)
2. rebuild all projects.
3. restart vs2013