Allow C# application built with .NET 2.0 to run on .NET 4.0/4.5

Question

We have a C# DLL (let's call it myapp.exe) built with .NET 2.0 Framework (VS2005) and we found out that our application won't work on machines where only .NET 4.0 or above is installed. To let our application work on .NET 4.0, I added the following some lines to myapp.exe.config by following this article Installing .NET Framework V4.0 and Running .NET 2.0/3.0/3.5/3.5Sp1 Applications

<startup>
  <supportedRuntime version="v4.0.30319"/>    
</startup>

And it is working.

Then I saw that this article also mentioned the following (especially second paragraph):

Now, I knew that you can’t just take a 3.5 Service Pack 1 application and run it on the V4.0 CLR. It needs a V2.0 CLR or reconfiguring with a tag in order to bend the application to run on the V4.0 CLR and that bending might be something that you don’t want to do.

What I hadn’t realised though was that installing .NET 4.0 wouldn’t install the bits that you need for a 2.0/3.0/3.5/3.5Sp1 application. It would only install the V4.0 CLR and the V4.0 assemblies and not additionally install the equivalent of .NET Framework V3.5 Sp1. So, you’d need to install (e.g.) .NET Framework V3.5 Sp1 yourself along with .NET 4.0

From my testing it would mean that I could run my 2.0 C# application on .NET 4.0 with .NET 4.0 framework (4.0 assembly/libraries) which is contradicting to what the articles said.

Or am I missing something here? It could be helpful if someone could clarify on this. Microsoft doesn't really make this clear at all.

Solution 1

From my testing it would mean that I could run my 2.0 C# application on .NET 4.0 with .NET 4.0 framework (4.0 assembly/libraries) which is contradicting to what the articles said.

You can run your application on .NET 4 using the .NET 4 assemblies. There is always the possibility that there may be a slight change in runtime behavior, however, as you won't be using the same runtime and framework which you used for development. I suspect the article is trying to suggest that you won't get the exact same behavior by just installing 4.0, though, as you've seen, it should work.

Doing this is likely fine, though I would recommend doing thorough testing of your application if this is going to be a standard deployment option.

Solution 2

And I quote:

"The .NET Framework 4 is backward-compatible with applications that were built with the .NET Framework versions 1.1, 2.0, 3.0, and 3.5. In other words, applications and components built with previous versions of the .NET Framework will work on the .NET Framework 4."

Taken from Version Compatibility in the .NET Framework

You have the right idea with the App.config file, but your really limiting yourself with the one line.
Might I suggest a more liberal use of supportedRuntime lines?

For example:

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v2.0.50727"/>
    <!-- 
    <supportedRuntime version="v3.5"/>  "The .NET Framework version 3.0 and 3.5 use version 2.0.50727 of the CLR."  
    -->
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0,Profile=Client" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.1,Profile=Client" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.1" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.2,Profile=Client" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.2" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.3,Profile=Client" />
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0.3" /> 
    <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
  </startup>
</configuration>

Why is supportedRuntime version="v3.5" commented out? Remember, this configuration identifies which versions of the Common Language Runtime (CLR) your application is compatible with. There is no 3.0 or 3.5 version of the CLR. Refer to .NET Framework Versions and Dependencies

Solution 3

Just to be a little more concise. In the App.config you are showing support for the CLR. From .Net v2 and on there are only 2 version of the CLR. So the following will give you support of .Net v2 and on. Make sure to include your exe.config file that is built with your project otherwise you are likely to still see the popup asking to install .Net.

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <startup useLegacyV2RuntimeActivationPolicy="true">
    <supportedRuntime version="v2.0.50727"/>
    <supportedRuntime version="v4.0"/>
  </startup>
</configuration>

For support for .Net v2 to v3.5 use:

<supportedRuntime version="v2.0.50727"/>

For support for .Net v4.0 to 4.6 use:

<supportedRuntime version="v4.0"/>

More information can be found for Configure an App to Support .NET Framework

Solution 4

From: microsoft docs:

you can choose any App.config file setting.

View more solutions

Comments

[Hichem]

.net 4.0 can not be installed alone without older version (3.5 sp1/3.5 )

[Hichem]

1st this is not a web application 2nd the purpose is to keep the assembly in .net 2.0 and make it working on all other version ..

[Reed Copsey]

@K3rnel31 It can be - you just need to configure your app to force it to use CLR 4...

[Hichem]

i mean on the machine .net 4.0 cant be installed alone without above versions because .net 4.0/4.5 it is just an update for 3.5sp1/sp2 ..

[Reed Copsey]

@K3rnel31 That's not true. You can install .NET 4 without having 3.5sp1 - Windows 8 does this by default, for example. .NET 4 uses a separate framework/CLR, and wasn't an update - it's a new framework isntall.

[Reed Copsey]

@K3rnel31 Windows 8 comes without .NET 3.5. You don't have to install .NET 3.5 on XP or Vista (it was not pre-installed, though .NET 2 was), but you can remove 3.5 from XP/Vista/Windows 7, and still install .NET 4. I've run into this with customers in the wild.

[Hichem]

@BenVoigt neither, smart is a gift it is not copy past from research is inventing and art ! xD and you are not

[user2736158]

Yup. Understood. All of the above still applies. You still need to re-compile your app (winforms app or whatever you have) against a 4.0 runtime. Changing the app.config doesn't ensure anything. To assume that it will work otherwise is a big risk - it MAY - but a lot depends on what 3rd party assemblies are being leveraged (nHibernate, log4net etc...) Unless you upgrade ALL of those to their correct (4.0) versions and recompile against them, you are set for failure. Too bad you accepted an answer that is contrary to real world experience (it only works in very LIMITED cases)...

[Moradnejad]

Can you explain that why did you commented the v3.5 line?

[Scott Solmer]

@ananda Because, it is not necessary. Updated my answer to address this.

[AdvApp]

This did not work for me. I used this exact entry in my app.config. I built it with target set to NET Framework 2.0 and running it on a Win10 system with only NET Framework 4, it prompted to install "NET Framework 3.5". I built it with target set to NET Framework 4.6.1 and running it on a system with only NET Framework 3.5, it prompted "You must first install one of the following versions of .NET Framework" and only listed v4.0.30319.