VProj is a simple tool that will copy an existing VS solution and convert it to build under the .NET 3.5 framework.

If you are building a re-usable library and you wish to target multiple .NET Framework versions (4.0 and 3.5) then you might find it useful.

Many projects that target multiple framework versions use a different approach, in that they add conditional elements in the MSBUILD project file. I have experimented with this alternative approach and prefer it for the following reasons:

1. Minor changes to the Visual Studio projects are required, and they can be performed from Visual Studio.
2. Less confusion to Visual Studio, as there are no custom edits to the project file.
3. Plays nicer with nuget, as there are no conditional elements for assembly references.

That last point is an important one for me. I find that if I have hand-edited <Reference> elements in my project file, it can confuse nuget during updates.

• Only knows about C# projects. It will ignore everything else. (I only use C#, but it would not be difficult to upgrade for VB, F#, etc).
• Does not convert Web projects. I find that I only want to use this for re-usable assemblies, I never want to target a web application for multiple frameworks.
• Only works for .NET 4.0 at the moment. In the near future I will probably have to enhance it to handle .NET 4.5, 4.0 and 3.5. (I don't intend to include .NET 3.0 or 2.0 at the moment).

Open your solution and decide which projects you wish to target for multiple frameworks. For each project, ensure that the compile constant NET40 is defined. It is important to define NET40 for every configuration in each project (eg Debug and Release for each project). If you do not define NET40 for every configuration in a project, then that project will be ignored.

Run the vproj tool to create a copy of your solution file and assocated project files.

To build for multiple framework targets, build your original (.NET 4.0) solution, and then build your copied (.NET 3.5) solution.

If you have code that needs conditional compilation, use the NET40 and NET35 conditional compilation constants:

  #if NET40
      // ... code that works under .NET 4.0
  #endif
  
  #if NET35
      // ... code that works under .NET 3.5
  #endif

  C:\>vproj --help
  Usage: vproj [ options ]
  Convert a VS project from .NET 4.0 to .NET 3.5.
  
  Options:
    -p, --project=VALUE        * Project file
    -o, --output=VALUE           Output file
    -n, --nochange               Show proposed changes, but do not write files
        --clean                  Remove files created from previous operation
        --noprompt               Do not prompt to remove files, just do it
    -v, --verbose                Log additional debug messages
    -q, --quiet                  Quiet operation, log errors and warnings only
    -?, --help                   Show this help message
  (Items marked * are mandatory)

The following steps outline how this utility converts the solution from .NET 4.0 to .NET 3.5

Changes to the solution file (eg MyProject.sln):

1. Load the solution file and discard any projects that are not C# projects, and any C# projects that do not have the NET40 compile constant defined for all configurations.

2. Rename each project by adding ".Net35" to the end, eg MyProject becomes MyProject.Net35.

3. Rename each project file. So src\MyProject.csproj would become src\MyProject.Net35.csproj.

4. Generate a new Project Guid for the project.

5. Update project dependencies to reflect project guid changes.

6. Save solution file to new file. So MyProject.sln would be saved to MyProject.Net35.sln.

Changes to each project file:

1. Name changed and Guid changed as per above.

2. Remove compile constants NET40, NET40_CLIENT.

3. Add compile constant NET35

4. Remove reference assemblies that are .NET 4.0 only. Currently this is Microsoft.CSharp, but there may be others.

5. Change project references to reflect change of Guids and change of names.

6. Change assembly references. This uses the nuget convention. If the assembly hint contains net40 or net40-client, then look for a matching assembly in the associated net35 directory.

7. Change the target framework version to v3.5.

8. Clear the target framework profile (ie if it is Client then it is changed to be empty).