Compiling .NET 3.5 project using .NET 4.0 compiler

I have a solution that contains several projects. One of the test projects is using .NET 4.0 due to a dependency. The deploy-able project I want to build is targeted against the .NET 3.5 compiler. Our current build script is not granular enough to target different .NET compilers on a project by project basis. Basically I can only use one compiler for every project in the solution.

Is the .NET 4.0 compiler smart enough to compile 3.5 code?

Solution Structure

Project 1 .NET 3.5 (deploy-able)
Project 2 .NET 4.0 (used for testing in automated build cycle)

Note: I can not just ignore Project 2, because our automated build server runs the unit tests in project 2. All tests must pass before a build is successful.

Edit:

I am not compiling this code within Visual Studio. We have a custom build script that is run external to visual studio. It is only smart enough to use 1 .NET compiler. It uses this compiler to compile all of the projects in a given solution. The build script does not care whether or not the projects are defined to be compiled against 3.5 or 4.0. Instead it compiles them with the defined compiler.

Project 1 does not reference project 2. Project 2 is a project containing unit tests. Our automated build tool compiles project 2 only to run tests.

Nope, I mean we have a build script that compiles all projects in a solution. The build script can only target 1 .NET Compiler. So in the case it is targeting the .NET 4.0 compile for compilation of every project in the solution. At this point assemblies haven't been created.

sgmeyer 2010-10-27 19:29:56

@sgmeyer, i'm still confused. Are you compiling everything twice?

JaredPar 2010-10-27 19:31:21

@JaredPar no, I am not compiling everything twice. My build script runs through it once. I am asking if I can compile a 3.5 project with a 4.0 compiler. Not compile a project referencing a 3.5 assembly.

sgmeyer 2010-10-27 19:33:15

@sgmeyer, then the answer is yes. This is very much doable.

JaredPar 2010-10-27 19:36:27

@JaredPar Awesome! Thanks very much.

sgmeyer 2010-10-27 19:47:35

I am not compiling with Visual Studio. The first part does nothing for me. Instead I am compiling with a build script external to Visual Studio. This is controlled by our build team. I am wondering if this is possible without modifying the build script.

sgmeyer 2010-10-27 19:35:07

Are you developing in Visual Studio? If so do what I said, it will update your project file. Then use MSBuild to compile your solution file, it will pick up what framework version to build each project with based on their setting from the project file.

Bob 2010-10-27 19:40:16

I am developing in VS2010. It is already targeted at 3.5. I can compile everything just fine in VS. The problem is with with the build server and the scripts it uses to build the projects. That is why I wanted to know if .NET 4.0 C# compiler will compiler .NET 3.5 projects.

sgmeyer 2010-10-27 19:44:40

Technically none of the Microsoft C# compilers can compile any .NET project file (.csproj). The C# Compiler can compile C# source code files (.cs) and they can target different versions of the .NET framework for the output binary. MSBuild can compile project files (.csproj) and solution files (.sln). So the answer is No. You can do what you want however by using MSBuild and specifing the target framework in the project properties.

Bob 2010-10-27 19:47:12

+1 Thanks for the clarification on MSBuild vs .NET compiler.

sgmeyer 2010-10-27 20:00:52

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Compiling .NET 3.5 project using .NET 4.0 compiler

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