Multitargeting in .NET

Question

Having gone through various blogs, I am quite confused about the terminology of "multitargeting" or side by side execution.

1. Some blogs say that, side by side execution means two versions of CLRs in a process. Some others claims that, its like .net 2.0 and .net 3.0 assembly executing side by side. I am extremely disappointed that I am unsure who is right who is wrong.

2. I also saw in many blogs like Scott Hanselman's blog etc (Which confuses a lot), that, any feature of .NET 4.5 will not work if the target framework is 4.0. I can agree to it. But I cannot agree or understand the fact that, a feature of 4.0 whose bug is fixed in 4.5, will go hidden if I build it using 4.5 and deploy in 4.0. Here I don't understand the term "hidden" and nobody dare to explain what actually it means. It means runtime error ? It means compile time error? It cant be this. It means inconsistent behavior ? Exception ? If this is the case, I wonder why MS has let this type of flexibility in development in VS. Does it serves ANY purpose ? I understand that, the first case is meaningful, but dont understand or agree with second case.

3. I also saw in Rick/Scott Hanselman's post that, Major changes means, complete upgrade including CLR. Then, I should see 3.0 as major upgrade but its not as it still uses .NET 2.0 CLR. Then why the naming terminology is 3.0.x.x/3.5.x.x ? Like the case in .NET 4.0.30319.x where CLR is also new, so I agree with this. I am surprised who is correct. Either these folks or MSDN as both contradicts their principles (Like MSDN says the formatting as Major.Minor.Build.Revision, and Hanselman or others say, Major means CLR upgrade and while it is not in .NET 3.0)

Ref: http://www.hanselman.com/blog/NETVersioningAndMultiTargetingNET45IsAnInplaceUpgradeToNET40.aspx

http://msdn.microsoft.com/en-us/library/bb822049(v=vs.110).aspx

Any thoughts on above two questions?

Answer 1

First of all you have to understand difference between the Multitargetting and Side-by-side execution.

• Multitargettting is the ability you to compile applications which would be run on the CLR runtime which different from that you are using for the developing the application (.NET 4.0 in case of VS2010 or .NET 4.5 in case of VS 2012). Multitargetting in the VS ensures that your application would not use any types or methods which was introduced in the CLR runtime and Base Class Library (BCL) in the versions which are newer then the version which you are targetted. VS doing so by having copy of the DLL's which are used in the each runtime which could be targetted.
• Side-by-side execution means that you could have more then one version of CLR runtime installed on you PC. As of CLR 4.0 is no longer correct. You now able to load CLR 4 inside same process together with older runtimes (CLR 1.1 or CLR 2.0). Please read http://msdn.microsoft.com/en-us/magazine/ee819091.aspx (Section "Overview of Behavior") for more details.

Regarding your questions:

1. Please read link from the MSDN article mentioned above.
2. Term hidden means that when you will debug application under .NET 4.5 you have versions of BCL with bugfixes which potentially hit you in the back after you deploy to .NET 4.0 since that bug was not fixed yet on that version of BCL. You will have runtime error which you could observe in the way which would depends on the type of bug which you reaching. That could be security issue, logical error in your application or runtime exception. If you want to catch these issues you could always test on the clean PC which has only .NET 4.0 installed.
3. Both links that you give does not interfer with each other in any way. Scott and MSDN both are correct in the explanation what version of .NET has which version of CLR runtime has installed.