What .NET Framework and C# version should I target with my class library?

.net, build, c#, dll

Solution

We target multiple runtime versions concurrently (.NET 1.1, .NET 2.0, and .NET 3.5) for some products.

We handle this in several ways:

- Separate Solution and Project files and for each of .NET 1.1, 2.0, and 3.5 SP1, but referencing the same source files.

eg:

 \ProductFoo_1_1.sln (.NET 1.1 solution, VS 2003)
 \ProductFoo_2_0.sln (.NET 2.0 solution, VS 2008)
 \ProductFoo_3_5.sln (.NET 3.5 solution, VS 2008)

 \FooLibrary\FooLibrary_1_1.csproj (.NET 1.1 Project, VS 2003) 
 \FooLibrary\FooLibrary_2_0.csproj (.NET 2.0 Project, VS 2008) 
 \FooLibrary\FooLibrary_3_5.csproj (.NET 3.5 Project, VS 2008) 

 \FooLibrary\FooClass.cs (shared amongst all Projects)
 \FooLibrary\FooHelpers_1_1.cs (only referenced by the .NET 1.1 project)

 \FooService\FooService_3.5.csproj (.NET 3.5 Project, VS 2008)
 \FooService\FooService.cs

Defining `NET_X_X` symbols in each of the solutions

For .NET Framework specific code, we use preprocessor instructions such as this:

public void SomeMethod(int param)
{
#ifdef NET_1_1
 // Need to use Helper to Get Foo under .NET 1.1
  Foo foo = Helper.GetFooByParam(param);
#elseif NET_2_0 || NET_3_5
 // .NET 2.0 and above can use preferred  method. 
  var foo =  new Foo { Prop = param }; 
  foo.LoadByParam();  
#endif 
  foo.Bar();
}

#ifdef NET_3_5
// A method that is only available under .NET 3.5 
public int[] GetWithFilter(Func Filter)
{ 
  // some code here
}
#endif

For clarification, the above lines starting with # are preprocessor commands. When you compile a solution, the C# Compiler (csc) pre-processes the source files. If you have an `#ifdef` statement, then csc will evaluate to determine if that symbol is defined - and if so, include the lines within that segment when compiling the project.

It's a way to mark up code to compile in certain conditions - we also use it to include more intensive debugging information in specific verbose debug builds, like so:

#if DEBUG_VERBOSE
  Logging.Log("Web service Called with parameters: param = " + param);
  Logging.Log("Web service Response: " + response); 
  Logging.Log("Current Cache Size (bytes): " + cache.TotalBytes); 
  // etc. 
#endif

- We then have NAnt scripts which automate the production of a release for each .NET version. We happen to control all this through TeamCity, but we can trigger the NAnt scripts manually too.

It does make things more complicated, so we only tend to do it where we need to maintain a legacy .NET 1.1 or 2.0 instance (eg where a customer can't/won't upgrade).

I imagine that when .NET 4.0 rolls around, we'll do the same thing and just add a NET_4_0 symbol.

Problem

I'm building a DLL class library - I want to make it usable by as many people as possible. Which version of the .NET Framework and which C# version should I use? Is it possible to produce a backwards-compatible DLL or different DLLs for different versions? Or does Windows automatically update the .NET framework so I should just use the latest version? Any guidance appreciated!

Original source