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In what ways does F# shine? (Cool real-world examples of its added value) [duplicate]

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I work for a tech company that does more prototyping than product shipment. I just got asked what's the difference between C# and F#, why did MS create F# and what scenarios would it be better than C#.
I've been using the language for a while now and I love it so I could easily go on about the great features of F# however I lack the experience in C# to say why we should use one over the other.
What's the benefits of using C# vs F# or F# vs C#?

General benefits of functional programming over imperative languages:
You can formulate many problems much easier, closer to their definition and more concise in a functional programming language like F# and your code is less error-prone (immutability, more powerful type system, intuitive recurive algorithms). You can code what you mean instead of what the computer wants you to say ;-) You will find many discussions like this when you google it or even search for it at SO.
Special F#-advantages:
Asynchronous programming is extremely easy and intuitive with async {}-expressions - Even with ParallelFX, the corresponding C#-code is much bigger
Very easy integration of compiler compilers and domain-specific languages
Extending the language as you need it: LOP
Units of measure
More flexible syntax
Often shorter and more elegant solutions
Take a look at this document
The advantages of C# are that it's often more accurate to "imperative"-applications (User-interface, imperative algorithms) than a functional programming language, that the .NET-Framework it uses is designed imperatively and that it's more widespread.
Furthermore you can have F# and C# together in one solution, so you can combine the benefits of both languages and use them where they're needed.

It's like asking what's the benefit of a hammer over a screwdriver. At an extremely high level, both do essentially the same thing, but at the implementation level it's important to select the optimal tool for what you're trying to accomplish. There are tasks that are difficult and time-consuming in c# but easy in f# - like trying to pound a nail with a screwdriver. You can do it, for sure - it's just not ideal.
Data manipulation is one example I can personally point to where f# really shines and c# can potentially be unwieldy. On the flip side, I'd say (generally speaking) complex stateful UI is easier in OO (c#) than functional (f#). (There would probably be some people who disagree with this since it's "cool" right now to "prove" how easy it is to do anything in F#, but I stand by it). There are countless others.

F# Has Better Performance than C# in Math
You could use F# projects in the same solution with C# (and call from one to another)
F# is really good for complex algorithmic programming, financial and scientific applications
F# logically is really good for the parallel execution (it is easier to make F# code execute on parallel cores, than C#)

To answer your question as I understand it: Why use C#? (You say you're already sold on F#.)
First off. It's not just "functional versus OO". It's "Functional+OO versus OO". C#'s functional features are pretty rudimentary. F#'s are not. Meanwhile, F# does almost all of C#'s OO features. For the most part, F# ends up as a superset of C#'s functionality.
However, there are a few cases where F# might not be the best choice:
Interop. There are plenty of libraries that just aren't going to be too comfortable from F#. Maybe they exploit certain C# OO things that F# doesn't do the same, or perhaps they rely on internals of the C# compiler. For example, Expression. While you can easily turn an F# quotation into an Expression, the result is not always exactly what C# would create. Certain libraries have a problem with this.
Yes, interop is a pretty big net and can result in a bit of friction with some libraries.
I consider interop to also include if you have a large existing codebase. It might not make sense to just start writing parts in F#.
Design tools. F# doesn't have any. Does not mean it couldn't have any, but just right now you can't whip up a WinForms app with F# codebehind. Even where it is supported, like in ASPX pages, you don't currently get IntelliSense. So, you need to carefully consider where your boundaries will be for generated code. On a really tiny project that almost exclusively uses the various designers, it might not be worth it to use F# for the "glue" or logic. On larger projects, this might become less of an issue.
This isn't an intrinsic problem. Unlike the Rex M's answer, I don't see anything intrinsic about C# or F# that make them better to do a UI with lots of mutable fields. Maybe he was referring to the extra overhead of having to write "mutable" and using <- instead of =.
Also depends on the library/designer used. We love using ASP.NET MVC with F# for all the controllers, then a C# web project to get the ASPX designers. We mix the actual ASPX "code inline" between C# and F#, depending on what we need on that page. (IntelliSense versus F# types.)
Other tools. They might just be expecting C# only and not know how to deal with F# projects or compiled code. Also, F#'s libraries don't ship as part of .NET, so you have a bit extra to ship around.
But the number one issue? People. If none of your developers want to learn F#, or worse, have severe difficulty comprehending certain aspects, then you're probably toast. (Although, I'd argue you're toast anyways in that case.) Oh, and if management says no, that might be an issue.
I wrote about this a while ago: Why NOT F#?

You're asking for a comparison between a procedural language and a functional language so I feel your question can be answered here: What is the difference between procedural programming and functional programming?
As to why MS created F# the answer is simply: Creating a functional language with access to the .Net library simply expanded their market base. And seeing how the syntax is nearly identical to OCaml, it really didn't require much effort on their part.

F# is not yet-another-programming-language if you are comparing it to C#, C++, VB.
C#, C, VB are all imperative or procedural programming languages. F# is a functional programming language.
Two main benefits of functional programming languages (compared to imperative languages) are 1. that they don't have side-effects. This makes mathematical reasoning about properties of your program a lot easier.
2. that functions are first class citizens. You can pass functions as parameters to another functions just as easily as you can other values.
Both imperative and functional programming languages have their uses. Although I have not done any serious work in F# yet, we are currently implementing a scheduling component in one of our products based on C# and are going to do an experiment by coding the same scheduler in F# as well to see if the correctness of the implementation can be validated more easily than with the C# equivalent.

F# is essentially the C++ of functional programming languages. They kept almost everything from Objective Caml, including the really stupid parts, and threw it on top of the .NET runtime in such a way that it brings in all the bad things from .NET as well.
For example, with Objective Caml you get one type of null, the option<T>. With F# you get three types of null, option<T>, Nullable<T>, and reference nulls. This means if you have an option you need to first check to see if it is "None", then you need to check if it is "Some(null)".
F# is like the old Java clone J#, just a bastardized language just to attract attention. Some people will love it, a few of those will even use it, but in the end it is still a 20-year-old language tacked onto the CLR.

One of the aspects of .NET I like the most are generics. Even if you write procedural code in F#, you will still benefit from type inference. It makes writing generic code easy.
In C#, you write concrete code by default, and you have to put in some extra work to write generic code.
In F#, you write generic code by default. After spending over a year of programming in both F# and C#, I find that library code I write in F# is both more concise and more generic than the code I write in C#, and is therefore also more reusable. I miss many opportunities to write generic code in C#, probably because I'm blinded by the mandatory type annotations.
There are however situations where using C# is preferable, depending on one's taste and programming style.
C# does not impose an order of declaration among types, and it's not sensitive to the order in which files are compiled.
C# has some implicit conversions that F# cannot afford because of type inference.

Visual C++ vs Visual C# , which is the best to learn? [closed]

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Closed 10 years ago.
I've done my C++ classes and practices after which I started learning Visual C++ using book Ivor Horton's Visual C++. The problem is that I am unable to understand the language of this book and badly trying to understand the codes. I want to learn Visual C++ for Windows application development and making my future in that (also because I already know C++).
Some of my friends told me to switch to C# since it has many library function to create GUI etc and told me it's hard to code Windows applications in Visual C++.
Now in these holidays I am going to join classes, can you help me which language I should stick to, and which one will be easy?
From very beginning I want to learn Visual C++ and sometimes I think it's hard when I don't understand concept in reference books etc.
Any help will be very appreciated, thanks a lot for taking your precious time.

Visual C++ and Visual C# are not languages nor GUI frameworks; they are Integrated Development Environments - IDEs. This means they are text editors tailored to the task of development.
Visual C++ lets you code in C++, Visual C# lets you code in C#. Also, both let you create GUIs in a point-and-click manner.
Sounds like what you really want to do is create GUIs. In that case, without having further information, I recommend you go for C#. It is a much cleaner language than C++, it has fewer ways to shoot yourself in the foot, and it provides access to the immensely useful .NET framework.
C# features that C++ doesn't have:
Fully automatic memory management
Lambda functions¹
Type inference¹
Reflection
Remoting
Automatic serialization
True entity types
Properties
Database integration via LINQ
Convenient functional-style programming via LINQ
No header files
No undefined behavior
Direct interoperability with many languages
Compile once, run everywhere
¹ these features have been added to C++ in the C++11 standard.
C++ features that C# doesn't have
Template metaprogramming
Typedefs
Zero-overhead principle
Means to enforce const-correctness
Mature compilers that produce extremely optimized code nowadays
Much wider platform support

can you help me which language i should stick to , and which one will be easy.
In general, if you're goal is to develop Windows Applications, you'll probably find many, many more simple examples to learn from in C# than in C++. There is a huge community around C# for Windows GUI development.
That being said, many of the concepts you'll need are the same in any language you learn. You can't really go wrong - at some point, you'll probably want to learn both langauges if you're going to program professionally (as well as others).
i think it's hard when i don't understand concept in ref books etc.
You should get a beginning programming book, not reference books, and go through it step-by-step. If your goal is just GUI development, I'd recommend a good, simple C# book, and just step through it form beginning to end, and do all of the examples. Learning from a "reference" book won't work - it's reference material, and not meant to teach concepts.

C++ vs. C# - a Checklist from a C++ Programmers Point of View
http://www.thinkingparallel.com/2007/03/06/c-vs-c-a-checklist-from-a-c-programmers-point-of-view/

C# is the premeir .net language for developing applications on the .net framework.
Native C++ is more cross platform and low level. It's used for writing applications for windows, mac, linux, embedded systems, etc.
C++.net is C++ adapted to the .net platform, and is primarily used for writing performance critical parts of an application (the rest of which is written in C#, VB.net, or something else as they integrate virtually seamlessly).
Visual C++ is microsoft's windows compiler for C++ that can be used to write native and C++.net applications.
Writing .net applications solely in C++ in much harder than writing them in C# as you have to deal with more low level issues. If you want to write .net applications, I recommend using C# and C++.net if you need it.

with C++, you could be able to touch the 'low level' near 'assembly things' part of programming
c# is never meant to be created to beat c++, with c# you get the whole .net framework utilities, faster cleaner form generations, with the lack of 'low level' part

C# is largely intended for Business Applications so as Java. Visual C# is Microsoft's implementation of the C# programming language specification, included in the Microsoft Visual Studio suite of products.
C# is much slower than C++, since the code passes through CLR. VC++.net has the same drawback.
VC++ is very hard to learn, so as MFC and windows programming, even though you know C++. But is it a prestigious and efficient language.
It is easy to switch from VC++ to VC#. But the reverse is harder.
If you go for VC++, MFC and Windows programming, try this.

Maybe consider what you are likely to want to do in the future, as a professional or a hobbyist. Programming is a wide field, in which both languages have their place. If its too early for you to make that decision, C# is probably most likely to do you good in the future.
It's worth remembering that they are both only languages - and just as writing a good book is about more than correct spelling & grammar, writing good code is about more than the details of a particular languages syntax.

I would learn both.
Having said that, I would create GUI/Application projects using C#, and use VC++ for any COM or device communication.
I would check out Which platform should I use : native C++ or C# ?

What features should Java 7 onwards have to encourage switching from C#? [closed]

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Closed 11 years ago.
C# has a good momentum at the moment. What are the features that you would need to have in order to switch (or return) to Java?
It would also be quite useful if people posted workarounds for these for the current Java versions, e.g. Nullables being wrapped around custom classes, to make this a much more interesting wiki.

As a .NET/C# developer here are the missing features that annoy me. This list in no particular order - just as thoughts come to mind:
The Java library is too small. For common things I have to choose between 5 competing open source products because the base library is lacking in so many ways.
This is an Object Oriented programming language right? Why in the heck do primitive types not inherit from "object"?
Pointers
Lambdas
Closures
Partial Classes and to a lesser extent partial Methods
Real support for Generics
Using statements and Dispose - this was a real WTF for me. You really have to explicitly close connections in try/catch blocks. Poor Java guys!
Yield return would be nice
Unsigned integers - again WTF? I have to use number types larger than I need for what purpose again?
In Java you can return from final blocks of try/catch. A co-worker confused the hell out of me for hours by introducing a bug this way. This behavior should be prohibited as in C#.
I rarely have to use Java and when I do, I have all sorts of WTF moments.
Edit: I Removed for-each comment based on the fact it is no longer a missing Java feature since 1.5.

In my experience, Java vs. .Net is more of a business decision than a technical one. Shops with MS experience trend towards .Net and shops with Java experience trend towards Java & OSS. I've seen little evidence of people switching based upon language features. On the other hand, I've seen shops heavily tilt towards one platform b/c of one or two key employees they wanted to hire being knowledgeable in that area.

in no particular order:
function pointers (delegates); the whole passing interfaces around thing is stupid
real generics; what's the point of having type safe generics if the compiler can't even hold the meta data through a unit's compilation?
ui speed; all the self-drawing ui libraries are very slow compared to native controls wrapped in "managed" libraries, not to mention that microsoft's self-drawn ui is hardware accelerated through direct3d
an yield return construct; c# is so user friendly in this it's crazy
operator overloading
linq; just cuz its so addicting
properties; more synctatic sugar you get addicted to
better interop with native code; c#'s p/invoke and native com support makes interop so easy compared to jni -.-
first class value types; this goes hand in hand with real generics, having generic lists that never box/unbox in c# is part of why c# is faster than java.
and i'd say this is the most important one:
a responsive, informative debugger; nothing can even come close to visual studio right now

Just going off the cuff, I think the biggest thing Java 7 needs that everyone is lamenting the lack of is closures.

I'm a C# programmer and about one year ago I had to participate in the development of an application in Java. What I missed the most was:
Visual Studio (Eclipse is nice, but
VS is VS, especially Team Edition)
Comparing strings with ==
Properties
Basic types as first-class objects
(e.g. not "int" type vs "Integer"
class)
Anonymous methods (a workaround can
be made by using anonymous classes
but it is not the same)
LINQ
And there was one single Java feature that I missed when I went back to C#: explicit exception declaration in method signatures.
DISCLAIMER: I am speaking about a somewhat old project, I don't know if some of these features are present in current versions of Java.

Less heinous XML parsing and manipulation tools. Doing anything with XML in Java sucks.

I'll also give one answer the other way around. C# needs an enumeration implementation like Java has. Java's enumerations rock!
Let me add anonymous overrides to this list also. Sorry to go off topic, but C# needs the ability to anonymously override methods. I have been switching back and forth between Java and C#, and I have to say for unit testing legacy code, there is nothing better than anonymous class overriding.

I would need function pointers of some sort. The ability to use delegates in C# is so useful. There are so many times in Java where I want to create a map of delegates or function pointers as the solution, but I can't. I know you can simulate a similar thing in Java, but having the abstraction as part of the language is a huge plus!

Momentum. I have done a lot of development on both platforms. I'm enjoying the .NET side more because of the way the C# language is evolving. Java's evolution seems to be more a reaction to C# than an actual vision. So I think the best thing for Java to do is ignore .NET and create something new of its own.

Another one I would like is some equivalent to LINQ. I noticed how much I actually depend on the ability to use LINQ when I tried to do a top coder competition and realized they only support .NET 2.0. Once you get used to using LINQ to solve problem and make it part of your regular programming vocabulary it is very difficult to not see problems in that light. It is akin to using generics and then not being able to use generic.

I think, choice between Java and C# is not a question of language features, but a question of platform and ecosystem choice.
So, I doubt that any new syntactic sugar in Java or C# can lead to a significant amount of switchers between the platforms.
After all, JVM world has Scala and many language-sensitive developers are using it in some way.

I think, you should consider swithcing this holywar from "Java vs C#" to "JVM vs CLR", because JVM is (in the last years — mostly) not only Java, but also Scala, Groovy, Clojure, JRuby, Jython, and dozen of JVM-languages.

Properties!
Anonymous objects are nice too

Continuations, like Scala (on top of the rest) would be good too, for agents development.

Support to run over CLR (and vice versa perhaps for JVM) without IKVM and other such layers.

Let's be clear that we need to distinguish between Java and the JVM. I actually switched from C# to Java, but I admit it was not because of Java's amazing language features! In my very humble opinion, C# is the better language, and CLR is the perhaps a more elegant VM. However, even with Mono, you're not writing applications that run everywhere!
I think the greatest argument for Java is the amazing community it has. This is where the cutting edge technology is being developed, not at Sun (now Oracle). The Java community has consistently been the leader in developing Enterprise technologies. For instance, how long did it take Microsoft to provide a DI Framework? Where's AOP in .NET? When I start a .NET project, my foundation is Spring.NET, a Java port. When I need ORM, it's NHibernate. Need a testing framework? NUnit. I realize there are other OSS projects for the .NET platform, but their numbers and support from Microsoft are laughable compared to the Java community.

on the other side, has .net got enterprise open projects like Terracotta (semi commerical), Infinispan, Compass ? no. ncache (commerical), lucene.net are far behind them. Especially Terracotta is unique, it can improve some of your app x100, it's simply perfect and free (partitioning is commerical). If we implement a high load app in both .net with anything and java with terracotta + hibernate + terracotta-hibernate-integration, java app will probably far more performant than .net one. Ther're some ports to .net like nhibernate, log4net, lucene.net, but all of them are trying to catch java versions. And entity framework is a disaster, they have to start with nhibernate as the base or get some lessons from them.
.net is only working on windows (mono is far from enterprise, there isn't any enterprises using it), how much money does myspace.com spent for their 4000 windows web server licences ? 1 million ?
You can't install a simple plugin to VS (and can't do some other things) if don't buy professional edition,
Some windows instance types in amazon web services are nearly x2 price of the linux ones.
if you look at ohloh.net open source java project counts (with language comparison tools), you'll see that java has x5 more volume than c#.
also Java has %20 share on worldwide while c# has %4.x (source:tiobe.com)
Look at the top web sites; only microsoft and myspace are using .net. google, amazon, ebay, linkedin, alibaba, twitter (switched to scala from rails)... many of them are using java and many others are using php, ruby (facebook, yahoo,..)
As a language Java is far behind c#, but Scala (runs on jvm) is as good as c# and it's performance is nearly same as java and also it can use all jdk and other java code as his library.
I'm not saying java is better, but I'm saying java is as valuable as .net, too.

A native 'decimal' type for Java replacing the BigDecimal class would be nice. But Java thread-safe collections are nice.
Perhaps the question is not so much what Java 7 needs to persuade developers to use it, but more a case of what makes developers want to move from Enterprise/server-side java language to a different C# desktop Windows-only oriented world?
For most developers, the language isn't difficult to pick up be it Java or C#. I develop in Java, but Linq didn't take long to understand and use.
I think the choice of Java or C# depends on what motivates you personally -perhaps money? in which case either language will do whether they have certain features or not.
Ste

They should start by fixing the Calendar/Date related classes, even that seems too much to ask.

A simple way to map values in your model to the UI (like bind in Java/FX)

I think it depends on bussiness decisions, not the languages themselves.
But I really really really love C# lambdas and curry :D :D
http://mikeomatic.net/?p=82

a map({codeblock}) which runs over anything iterable would be nice. And filter too. And being able to return multiple values easily from a method.
(a,b,c) = getThreeValues();
(would assign the individual variables a, b and c).
Actually I just think they should have Haskell as a supported scripting language on the JVM :D

linq, lambda, anonymous types

I don't see any point in "luring" anyone anywhere. They solve different problems and you should use whichever one suits you.
Java has less "Language" and less structures to trip up new people, it's platform independent and it doesn't change too fast allowing old code to stick around for a LONG time (Good for some large companies).
C# has tight desktop integration and a slew of nice features that make it more fun to program. It has .net integration. It has pointers, closures, etc which make it harder for n00bs (a valid language target, hence Basic) but more fun for experienced programmers--I haven't convinced myself that these features make you more productive, but in some situations they can make your code a lot nicer! Also if you need pointers, you need pointers.
I don't see a whole lot of overlap in target audiences. Why change a language to attract programmers when it's healthier for the entire industry to have two healthy languages each targeted at solving different problems attracting different developers?

IronPython vs. C# for small-scale projects

I currently use Python for most of my programming projects (mainly rapid development of small programs and prototypes). I'd like to invest time in learning a language that gives me the flexibility to use various Microsoft tools and APIs whenever the opportunity arises. I'm trying to decide between IronPython and C#. Since Python is my favorite programming language (mainly because of its conciseness and clean syntax), IronPython sounds like the ideal option. Yet after reading about it a little bit I have several questions.
For those of you who have used IronPython, does it ever become unclear where classic Python ends and .NET begins? For example, there appears to be significant overlap in functionality between the .NET libraries and the Python standard library, so when I need to do string operations or parse XML, I'm unclear which library I'm supposed to use. Also, I'm unclear when I'm supposed to use Python versus .NET data types in my code. For example, which of the following would I be using in my code?
d = {}
or
d = System.Collections.Hashtable()
(By the way, it seems that if I do a lot of things like the latter I might lose some of the conciseness, which is why I favor Python in the first place.)
Another issue is that a number of Microsoft's developer tools, such as .NET CF and Xbox XNA, are not available in IronPython. Are there more situations where IronPython wouldn't give me the full reach of C#?

I've built a large-scale application in IronPython bound with C#.
It's almost completely seamless. The only things missing in IronPython from the true "python" feel are the C-based libraries (gotta use .NET for those) and IDLE.
The language interacts with other .NET languages like a dream... Specifically if you embed the interpreter and bind variables by reference.
By the way, a hash in IronPython is declared:
d = {}
Just be aware that it's actually an IronPython.Dict object, and not a C# dictionary. That said, the conversions often work invisibly if you pass it to a .NET class, and if you need to convert explicitly, there are built-ins that do it just fine.
All in all, an awesome language to use with .NET, if you have reason to.
Just a word of advice: Avoid the Visual Studio IronPython IDE like the plague. I found the automatic line completions screwed up on indentation, between spaces and tabs. Now -that- is a difficult-to-trace bug inserted into code.

I'd suggest taking a look at Boo [http://boo.codehaus.org/], a .NET-based language with a syntax inspired by Python, but which provides the full range of .NET 3.5 functionality.
IronPython is great for using .NET-centric libraries -- but it isn't well-suited to creating them due to underlying differences in how the languages do typing. As Boo does inference-based typing at compile time except where duck typing is explicitly requested (or a specific type is given by the user), it lets you build .NET-centric libraries easily usable from C# (and other languages') code, which IronPython isn't suitable for; also, as it has to do less introspection at runtime, Boo compiles to faster code.

Why should I learn C#? [closed]

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I want to know why I should learn C#?
What features does C# have that Java does not?

C# is an object-oriented high-level language which can be useful to develop applications.
It supports getter/setter constructs, delegates and true generics, which Java does not.
C# usually feels more "native" on Windows, since Windows.Forms and WPF are closer to native code than Swing.
However, Java is more cross-platform than C#(even though project "Mono" is changing that)

There's a whole lot of differences: http://en.wikipedia.org/wiki/Comparison_of_C_Sharp_and_Java
Whether you should use one over the other depends on your priorities. What do you want to write in them?

C# (and other the CLR languages) have aspects of functional programming (such as delegates and closures) that make some types of development and design patterns easier. See this post by Steve Yegge for more. Delegates can make code much more expressive and concise.

You should learn both, and use each of them as appropriate. I don't feel like it's a "which one should I learn?" kind of question. (Or a "I know Java, why should I learn C#?" question.) That comparison that Welbog posted will help you decide when to use one over the other. But, myself, I like having both in my toolkit.

Syntax:
They are both reasonable languages--they support pretty much the same high level structures and designs (OO, encapsulation, ...) and are fairly equal up until you get down to the code level (which overall isn't that big a deal. C# beats java in features, Java beats C# in simplicity I guess).
Portability:
My biggest factor has been that lately I've worked on apps for a spectrum analyzer and upcoming cable systems. Neither would have been remotely programmable in C# (you are completely reliant on microsoft to port the runtime to your platform of interest. Java is open source (there may still be one or two little pieces that aren't, but they are sincerely working on that and it's enough that you can port it and use it just about anywhere.
If you are only ever interested in Windows and if you'd like to take advantage of tight, simple integration with windows, then C# is the platform for you.
VM
Both languages include a "Runtime" that can support other languages--and those other languages generally have access to the (very rich) libraries provided in each environment.
Microsoft can run a few (a dozen) languages on it's VM, none as impressive as C# (although Ruby and Python could be good--are they fully available yet)?
The JVM has a few hundred including some cutting-edge experimental languages that often run as well as or better than their native counterparts. Some are considered the next generation in programming, as different from Java/C# as they were from C. I looked into Scala, but I must be getting old, it just hurt my head, but I admit there are some very advanced concepts available in there. Groovy rocks.
System integration/tools:
C# wins hands down since Java can't commit to any one platform. C# is also very well integrated with databases and other external dependencies, Java is getting better integrated, but the tools are a little more mismatched. C# the entire toolset tends to be very integrated since it all tends to come from a single vendor.
Number of platforms using it (number of potential customers):
Last I heard, Java wins hands down. WAY down. Every try to figure out how many phones, microwaves, TVs and toasters might be running Java? The number dwarfs every other language combined as I understand it...

the only reason you should is because you want to
C# is a
multi-paradigm programming language that encompasses functional, imperative, generic, object-oriented (class-based), and component-oriented programming disciplines
so you get to learn a lot of tricks from different paradigms, all in a single language

There are lots of questions that people have asked on this topic already. Read up, they've got good infos there. :)

The biggest reason, imho, why is because (for windows desktop apps at least) Java UI is godawful ugly. Fugly ugly. Vomitously ugly. Ugly like the kid you'd have if you impregnated your sister.
If you are required to run on any OS, Java is your best option (tho Mono is catching up!). But if you're programming for the Windows environment, C# is a dream to code and looks like any other native application.
Other than that, I'd say Linq. And the ability to fail fast on exceptions rather than having to handle exceptions everywhere (like we actually know how to handle each and every exception). Oh, and Linq again. Linq is so beautiful... to me... Can't you see?
Crap, forgot about lambdas. Damn, I love lambdas and all their closure goodness.

Simple answer -- C# is the primary language for developing Micorosft applications at this point. Learning it opens up a new world of potential opportunities to work on Microsoft-based applications.
Also, it's not really that hard to learn. Just a little harder than VB was, but with lots of OO power.
Some people would say C# is Microsoft's Java -- actually they look almost identical, just minor differences but different class libraries. If you already know Java, picking up C# shouldn't be too hard.

I can't believe nobody has posted this yet..
Because, it is a fun language. Personally I think Java is a rather nasty language (still can be fun), but not as enjoyable as C#.

Do you need to learn C#?
Is your job situation going to improve?
Do you want to learn something new?
Is the development environment better in C#, is the tooling better?
Do you think you can do something in C#, that you can't do in Java?
Maybe a generic language for desktop, web and mobile - but Java has this as well.
Just a few questions you could use to decide.
I'm C# myself, started out in Java when I was introduced to programming. I think Java did a fantastic job for me in understanding programming, mostly because there were no magic IntelliSense, or form designer for Windows apps - we had to write the whole damn thing our selves. Which was good, when you're learning. But I think C# and the whole .NET/Visual Studio eco-system enables you to be more productive.

More important than the feature of the language are the run-time environment and the libraries.
C# and Java have a different set of libraries, frameworks, operating system interactions, etc.

Based on the question seems like you are not interested in learning it for the sake of knowing more about programming and clearly looking for practical reasons.
That is why I don't think you should bother learning it, since it will be just another tool to accomplish same thing in your toolbox.

I like what Welbog wrote. I would add that it would also depend on where you're at professionally, your interests, and your job prospects /interests.
If you're you've just started to learn Java, then picking up a little C# might help in terms of broadening your programming horizons. The languages are similar (roughly speaking), but of course there are nuances to say the least in programming in one versus another. If you've been programming Java for awhile, then perhaps you'd be better off either expanding your J2EE area of expertise or maybe looking at other supporting areas and/or languages.
In terms of job prospects, you might find companies in your area hire more .NET developers than Java, or vice-versa. You might also want to look at companies you want to work for and make sure your skill set matches. For example, if you've dreamed at working at a particular firm who uses JSP, just as an example, then sticking with Java would be better.

It is not C# that you should want to learn but .NET Framework and related technologies.
Think rather J2EE vs .NET.
Why would you want to learn .NET? It's up to you to decide. Like Java it's one of the few major development platforms. It is functional, very actively developed and supported with a huge community knowledge base.

To me the choice is as simple as I need to use .NET libraries, so I'll use C# or I need to use Java libraries so I'll use Java. If I'm working on something on a Windows client I'll use C#, if I'm working on something that needs to run against the P6 Integration API I'll use Java.
You shouldn't feel like you you are missing something or should use one or the other. Look at what you have to do and pick the best tool for doing that.
It's tough to list on a post here all the features of the .NET Framework, just like it would be tough to list all the features contained in the Java SDK Libraries.

Learning C# because it may or may not have features that Java doesn't have seems like a very odd reason in opinion (it does have a number of features, that Java doesn't but so does F#, LISP, Haskel, Ruby, Lua and even COBOL).
You should learn C# because
You need to (to keep or get a job, stay in school, whatever)
You want to
It is fun to expand your horizon
Any or all of the above

There are some syntactic sugar advantages in .NET such as LINQ or Properties. (Yes, you don't have properties - not to mention auto-properties - in Java).

Strategically, it helps to know several languages as demonstrated by a little research. Currently, PHP, Java, and ASP.NET (C#) are the most widely used server-side programming technologies (see https://w3techs.com/technologies/overview/programming_language/all). Therefore, knowing all three empowers your backend engineering potential.