What's the general way to remove "The name xxx does not exist" error when we recompile unity3d C# dll?

Question

As we know there are multiple ways to decompile Unity3d DLL files to reverse engineering the C# source code via ( dotpeek, ILSpy and Reflector )

Example: [Target File] Unity3D_Game_RootPath/Managed/Assembly-CSharp.dll

1. Assembly-CSharp.dll => (one of the above tools) => Export/Save to .sln
2. C# project file extention .sln => open in visualStudio
3. Switch to "Solution Explorer" to check and add all necessary References (libraries to be included, like System, UnityEngine.dll, UnityEngineUI.dll, etc....)
4. Passing Step 3 will remove most of the reference Errors (red hightlights in IDE)

However, usually the decompiled source code still contains a lot of error patterns below:

Example: iTween.<>f__switch$mapA == null

error type I : the name 'xxx' does not exit (CS1513)

error type II : Unexpected character '$'

would like to know what is the cause of the above general errors through the decompile process mentioned above and what would be a general way to remove them.

The Final Goal of the above process is try removing recompile errors and successfully recompile the source code generated from C# decompile Tools ( dotpeek, ILSpy and Reflector ) into runable dll to replace the old one.