You are apparently referencing the DLL incorrectly. I am not familiar with the QuickBooks SDK, but if you notice your csc.exe line, no link to any QuickBooks library is provided.
Have you added the right link in every project in your solution?
:. Interop.QBFC .NET ... ? , . , DLL , . DLL .
: COM- Interop.XXX.dll. QBFC8 COM, , "". , using Interop.QBFC8; using QBFC8;. , , , , csc.exe . , COM-:
C:\Windows\Microsoft.NET\Framework\v3.5\Csc.exe /noconfig /nowarn:1701,1702 errorreport:prompt /warn:4 /define:DEBUG;TRACE /reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll"
/reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Data.DataSetExtensions.dll"
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Data.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Deployment.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Drawing.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Windows.Forms.dll
/reference:C:\Windows\Microsoft.NET\Framework\v2.0.50727\System.Xml.dll
/reference:"C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\v3.5\System.Xml.Linq.dll"
/reference:obj\Debug\Interop.COMSVCSLib.dll
/debug+ /debug:full /filealign:512 /optimize-
/out:obj\Debug\WindowsFormsApplication1.exe
/resource:obj\Debug\WindowsFormsApplication1.Form1.resources
/resource:obj\Debug\WindowsFormsApplication1.Properties.Resources.resources /target:winexe
Form1.cs Form1.Designer.cs Program.cs Properties\AssemblyInfo.cs Properties\Resources.Designer.cs Properties\Settings.Designer.cs
/reference:obj\Debug\Interop.XXX.dll, .