I'm looking to write a library that uses the new optional parameters feature of C# 4.0, but I want to target it to the 3.5 version of the framework. Is this possible?
Are optional parameters syntactic sugar in the same way that extension methods are?
With VS2010 RC, I was able to create a .NET 3.5 application that uses optional parameters in C#.
So yes, it's all about syntactic sugar.
VS 2010 supports optional parameters in C# for .NET 3.5 solutions. One caveat however, is that the command-line compiler will report errors.
So, if you have automated builds in running, - using NANT or something else using the command-line compiler - your builds will fail.
Like Jon Skeet I was getting "Feature 'optional parameter' cannot be used because it is not part of the 3.0 C# language specification". However in the RTM version of Visual Studio you can select Language Version to "default" in Project Properties->Build->Advanced. That got it to work for me.
You can use optional and named parameters in a targeted framework of a previous version as long the assemblies are build within a development environment for v4 (f.e. VS2010). But you should be aware of runtime compiling or publishing websites in this case the compiler of the targeted framework will be used. And because the compilers of the older frameworks doesn't understand the syntax of optional and named params it will results in compiler errors.
Go to project properties -> Build Tab -> click button 'Advanced' -> set language version to 'default' -> save -> be happy ;)
I don't have VS2010 installed here to check, but I believe this would be purely a language feature, and therefore should be usable regardless of the framework being targeted.
Edit: Looking at this link (and a few others), it appears that optional parameters compile to method arguments with an [opt] attribute in the il. I don't know if this parameter existed in previous versions of the clr, but still my guess would be that it does.
VB.NET has optional parameters if you want to use optional parameters in .NET 3.5.
Related
I've inherited an app. The app has to use .NET 2.0. However, I would like to make use of a feature introduce in C# 4.0 (optional arguments). I understand that a framework is separate from a language. However, what I'm not sure of is, can I use this C# feature in the context of .NET 2.0?
The code compiles. I wasn't sure if this was legitimate, or if I just got lucky :).
Thank you for your insights.
Optional arguments/parameters have been supported in CLR since CLR 1.0. This is due to CLR support for VB.net.
This is why your code compiles. Other new 4.0 features may not work the same.
Other post-C# 2.0 features that will compile into a .NET 2.0 application include named arguments, lambda expressions, auto properties, & extension methods.
The build machine where I work still uses the .Net 2.0 compiler.
I've set up Visual Studio to target the .Net Framework 2.0, but when I use the keyword var, it's compiling (since the compiler automagically change the type). But it breaks on the build machine compiler.
Is there a way to setup Visual Studio to break on those things, or even force it to use the 2.0 compiler, so that I won't make the mistake to break the build by using "too new" functionalities?
If I remember correctly, each version of .Net was tied to a specific version of DevStudio and it isn't possible to change this. If you need to compile for .Net 2 then you need the VS2005 compiler. One way to solve this is to use makefiles rather than the IDE to build the application and specify explicitly which compiler is used for each file.
This is, probably, a very simple answer for someone. I have a method with an Optional Parameter like so;
public static Email From(string emailAddress, string name = "")
{
var email = new Email();
email.Message.From = new MailAddress(emailAddress, name);
return email;
}
Now, I must target .Net 3.5 and it was my understanding that Optional Parameters are part of .Net 4. However, my project builds and I double checked the Properties - Application page which states 3.5 as the target framework. Then I found a article on MSDN saying it's a feature of C#4 in VS2010. (MSDN Article --> Named and Optional Arguments)
Can someone help clarify this for me. C#4 does not require .Net4? What are Optional Parameters ACTUALLY a part of?
Thank you.
Optional parameters have been supported in the CLR since 1.0. Languages like VB.Net's have been using them since the start. While the first version of C# to support them is 4.0, it can still generate valid code for a 2.0 CLR and in fact does so. Hence you can use default parameters in 2010 if you are targeting the 3.5 CLR (or 2.0, 3.0, etc ...)
This type of support is not limited to default parameters. Many new C# features can be used on older version of the framework because they do not rely on CLR changes. Here are a few more which are supported on CLR versions 2.0 and above
Named Arguments: Added C# 4.0
Lambda Expressions: Added C# 3.0
Auto Properties: Added C# 3.0
Extension Methods: Added C# 3.0
Co/Contra Variance: Added C# 4.0
If you compile that up and examine the output using a tool like ILDASM you'll see that the optional parameter is simply implemented by adding an attribute to the metadata that describes the method's formal parameters. As long as that attribute class is available on the target platform there should be no problem with using the emitted code on a downlevel platform.
The language version is independent of the framework version. For C# they happen to run mostly in parallel, i.e. C# 4 and framework 4.0 came with Visual Studio 2010. (However there is no 3.5 version of C#.)
With VB the version numbers are different, as VB 7 was the first VB.NET version. So, VB 10 comes at the same time as framework 4.0.
The optional parameters is a language feature introduced in C# 4. When you use VS 2010 you use C# 4, even if you target framework 2.0, so you can use all C# 4 features.
C# 4.0 is included with Visual Studio 2010, and the C# compiler understands optional parameters. So yes, the C# 4.0 language definition is something different than .NET 4.0. I guess a method with optional parameters compiled for .NET 3.5 will show overloaded methods when opening with eg. VS 2008
I created new project in Visual Studio with target framework 2.0. But even if I left somewhere var keyword Visual Studio successfully compiles project. Is this the correct behavior as var is 3.0 feature? Is there any settings to prevent code with var to be compiled?
var is a feature of C# 3.0, but it doesn't require any framework features. In other words, it's absolutely fine to use within a project targeting .NET 2.0. The same is true of many other features - anonymous types, automatic properties, lambda expressions etc.
See my versions article for more information. (I need to update it for C# 4 at some point...)
If you want to restrict yourself to C# 2.0, you can specify the language version by clicking on "Advanced" in the Build tab of the project properties, IIRC. (It's definitely there somewhere, but I'd rather have a cup of coffee than check for the exact location right now.)
var is purely a compile-time feature, once the assembly is compiled, the compiler inserts the actual type and the fact that you had used var is "lost".
So a project that's targetting version 2.0 of the framework can still make use of the var feature, because it doesn't actually affect the outputted assembly in any way.
As long as your project will always be compiled with Visual Studio 2008 or newer, you're safe to use C# 3.0 features. The .NET 2.0 target only restricts what libraries you can use, not what language features.
Either do full text replace of var by 1var1 and manually replace them or use pre 3.0 versions of compiler.
What can be the issues if I build a solution where all the projets are under targetFrameworkVersion=2.0 but one with targetFrameworkVersion=3.5 and
None of the 3.5 features are used
Some of the 3.5 features are used but the classes calling the 3.5 code are never instanciated
Some of the 3.5 features are used in some classes, the classes are instanciated but the code in 3.5 never called
the 3.5 code is called
It depends on what you mean by "features". There are compile-time features like the var keyword and lambda expressions and there are run-time features like LINQ or WCF that require libraries in the .NET 3.x runtime.
I assume you're using Visual Studio 2008, which will handle all the compile-time features. If all you're using is the compile-time features, then everything will work fine in all cases. I do this rather often on my current project.
If you're using run-time features then I believe this is how it will shake out:
Things will just work.
I think this will just work also.
It depends on when static functions are JITted and if you have any 3.5 library referencing code in static functions.
Probably MissingMethodException when a function containing a 3.5 library feature is called.
Rather than worry about all of this, if you're planning on using run-time features, I would simply add a key to the App.config that the 3.5 runtime is required and it'll check on startup and bomb if it isn't present. Then you don't need to figure out all these permutations and your application will be more stable.
First of all, you need to be aware that what you are targeting is actually .NET 2.0 SP1.
How are your projects related? Do you have projects that are built under .NET 2.0 which reference the .NET 3.5 project (or vice versa)?