Can I use C# 4.0 with the target framework set to .NET 3.5 to develop an extension for ArcMap 9.3? Or does it have to be C# 3.0 or earlier?
Short answer: In my experience, there should be absolutely no problem developing .NET 3.5-based code for ArcGIS 9.3 in Visual Studio 2010 (with C# language version 4), as long as you explicitly target the .NET Framework 3.5. The C# language version is mostly irrelevant here.
Longer answer: You need to distinguish between the C# language version, and the targeted Framework version.
You can think of the .NET Framework as being made up of two major parts: the CLR (Common Language Runtime), and the BCL (Base Class Library). The former is the "virtual machine", while the latter is the class library (containing all the types that you can look up on MSDN).
.NET Frameworks 2 until 3.5 all use the same CLR (version 2), that is, the execution environment hasn't really evolved. What has evolved, however, is the BCL. If you're running a .NET 3.5 application on a .NET 2 machine, the main problem will not be that the "bytecode" (CIL) will be incompatible (it won't), but that the application might refer to and use types that were not yet available in the .NET 2 BCL.
Now, when you tell Visual Studio 2010 to target the .NET Framework 3.5, it will make sure that you won't use BCL types from any later Framework version. It will also make sure that the code output by the C# compiler will not require features only available in the CLR version 4.
The C# language version has very little to do with all this. What the C# compiler really does it to take your source code and translate it to a much lower-level programming language called CIL (Common Intermediate Language). Certain C# language constructs will no longer be recognisable in CIL: For example,
To sum this up: The C# language version becomes irrelevant as soon as your code is compiled. What is important is...
One notable exception (in the sense that a C# language construct requires a particular version of the framework; this was last the case whwn generics were introduced IIRC) might be the C# keyword
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Keep in mind that VS2010 isnt a supported IDE for 9.3 development. You wont get any assembly references, help or template integration.
You can kind of make it work however: http://forums.arcgis.com/threads/29935-VS2010-with-version-9.3.1