How to Resolve mscorlib Related Issues in Visual Studio/MSBuild

For many .NET developers, mscorlib is something miserable as they never pay much attention to it. In fact, they should learn about it, especially when Visual Studio and MSBuild report an error about it.

What is mscorlib?

We might go to Mono’s source code to see what is mscorlib, as that’s better than using a decompiler.

OK. A glance can show you how many namespaces are included (please ignore Mono.* which are Mono specific). So when we write in C# using System; or using System.IO;, usually the consumed classes come from mscorlib, not System.dll. All .NET assemblies have mscorlib.dll as a reference, so it is part of the foundation of Base Class Library.

How It Is Referenced By Others Normally?

If you create a new C# project in Visual Studio (such as Windows Console applications), you perhaps don’t see a reference to mscorlib.dll. Why? Well, that’s because of an MSBuild/CSC trick Microsoft implements.

First, most csproj files do not have <NoStdLib></NoStdLib> defined under <PropertyGroup>. In this way MSBuild uses the default value of false in its execution. This finally translates to a switch of csc.exe called /nostdlib-.

Second, csc.exe automatically picks up the proper mscorlib.dll (the details on that resolution itself are worth another post).

Finally, csc.exe generates the assembly for your project, and the reference to mscorlib.dll is written into that file.

The above knowledge can be easily gained by turning on MSBuild logging via /verbose switch.

Specially Case

Normally you never need to worry about the reference to mscorlib.dll if you read above. Now let’s visit the most common exceptional cases where you should care about mscorlib.dll.

Because of the fact that .NET targets multiple platforms, some project types requires a special mscorlib.dll to be added as reference. The very first example was .NET Compact Framework, where its mscorlib.dll contains a reduced set of classes. Then more platforms follow, such as Xamarin.iOS, Xamarin.Android, Windows RT, and Windows Phone.

So if you happen to have a CF project file open at hand like this one, you can see how VS is smart enough to add <NoStdLib>true</NoStdLib> for us, and also insert a reference to mscorlib.dll to CE version, under C:\Program Files\Microsoft.NET\SDK\CompactFramework\v3.5\WindowsCE\mscorlib.dll.

Note that in this case when MSBuild translates the csc.exe command line, a /nostdlib+ is added.

OK, so if you compile any project and meet the following errors, make sure review what I typed above and soon locate the problem in your project file,

• “Predefined type ‘System.Object’ is not defined or imported”
• “A reference to ‘mscorlib’ could not be added. This component is already automatically referenced by the build system.”

Happy hacking.