Uses and misuses of implicit typing

Article
04/20/2011

One of the most controversial features we've ever added was implicitly typed local variables, aka "var". Even now, years later, I still see articles debating the pros and cons of the feature. I'm often asked what my opinion is, so here you go.

Let's first establish what the purpose of code is in the first place. For this article, the purpose of code is to create value by solving a business problem.

Now, sure, that's not the purpose of all code. The purpose of the assembler I wrote for my CS 242 assignment all those years ago was not to solve any business problem; rather, its purpose was to teach me how assemblers work; it was pedagogic code. The purpose of the code I write to solve Project Euler problems is not to solve any business problem; it's for my own enjoyment. I'm sure there are people who write code just for the aesthetic experience, as an art form. There are lots of reasons to write code, but for the sake of this article I'm going to make the reasonable assumption that people who want to know whether they should use "var" or not are asking in their capacity as professional programmers working on complex business problems on large teams.

Note that by "business" problems I don't necessarily mean accounting problems; if analyzing the human genome in exchange for National Science Foundation grant money is your business, then writing software to recognize strings in a sequence is solving a business problem. If making a fun video game, giving it away for free and selling ads around it is your business, then making the aliens blow up convincingly is solving a business problem. And so on. I'm not putting any limits here on what sort of software solves business problems, or what the business model is.

Second, let's establish what decision we're talking about here. The decision we are talking about is whether it is better to write a local variable declaration as:

TheType theVariable = theInitializer;

var theVariable = theInitializer;

where "TheType" is the compile-time type of theInitializer. That is, I am interested in the question of whether to use "var" in scenarios where doing so does not introduce a semantic change. I am explicitly not interested in the question of whether

IFoo myFoo = new FooStruct();

is better or worse than

var myFoo = new FooStruct();

because those two statements do different things, so it is not a fair comparison. Similarly I am not interested in discussing bizarre and unlikely corner cases like "what if there is a struct named var in scope?" and so on.

In this same vein, I'm interested in discussing the pros and cons when there is a choice. If you have already decided to use anonymous types then the choice of whether to use implicit typing has already been made:

var query = from c in customers select new {c.Name, c.Age};

The question of whether it is better to use nominal or anonymous types is a separate discussion; if you've decided that anonymous types are worthwhile then you are almost certainly going to be using "var" because there is no good alternative.

Given that the overarching purpose is assumed to be solving business problems, what makes good code? Obviously that's a huge topic but three relevant factors come to mind. Good code:

works correctly according to its specification to actually solve the stated problem
communicates its meaning to the reader who needs to understand its operation
allows for relatively low-cost modification to solve new problems as the business environment changes.

In evaluating whether or not to use "var" we can dismiss the first concern; I'm only interested in pros and cons of cases where using var does not change the meaning of a program, only its textual representation. If we change the text without changing its semantics then by definition we have not changed its correctness. Similarly, using or not using "var" does not change other observable characteristics of the program, such as its performance. The question of whether or not to use "var" hinges upon its effect on the human readers and maintainers of the code, not on its effect upon the compiled artefact.

What then is the effect of this abstraction on the reader of the code?

All code is of course an abstraction; that's the whole reason why we have high-level languages rather than getting out our voltmeters and programming at the circuit level. Code abstractions necessarily emphasize some aspects of the solution while "abstracting away" other aspects. A good abstraction hides what is irrelevant and makes salient what is important. You might know that on x86 chips C# code will typically put the value returned by a method in EAX and typically put the "this" reference in ECX, but you don't need to know any of that to write C# programs; that fact has been abstracted away completely.

It is clearly not the case that more information in the code is always better. Consider that query I referred to earlier. What is easier to understand, that query, or to choose to use a nominal type and no query comprehension:

IEnumerable<NameAndAge> query = Enumerable.Select<Customers, NameAndAge>(customers, NameAndAgeExtractor);

along with the implementations of the NameAndAge class, and the NameAndAgeExtractor method? Clearly the query syntax is much more abstract and hides a lot of irrelevant or redundant information, while emphasizing what we wish to be the salient details: that we are creating a query which selects the name and age of a table of customers. The query emphasizes the business purpose of the code; the expansion of the query emphasizes the mechanisms used to implement that purpose.

The question then of whether "var" makes code better or worse for the reader comes down to two linked questions:

1) is ensuring salience of the variable's type important to the understanding of the code? and,
2) if yes, is stating the type in the declaration necessary to ensure salience?

Let's consider the first question first. Under what circumstances is it necessary for a variable's type to be clearly understood when reading the code? Only when the mechanism of the code -- the "how it works" -- is more important to the reader than the semantics -- the "what its for".

In a high-level language used to solve business problems, I like the mechanisms to be abstracted away and the salient features of the code to be the business domain logic. That's not always the case of course; sometimes you really do care that this thing is a uint, it has got to be a uint, we are taking advantage of the fact that it is a uint, and if we turned it into a ulong or a short, or whatever, then the mechanism would break.

For example, suppose you did something like this:

var distributionLists = MyEmailStore.Contacts(ContactKind.DistributionList);

Suppose the elided type is DataTable. Is it important to the reader to know that this is a DataTable? That's the key question. Maybe it is. Maybe the correctness and understandability of the rest of the method depends completely on the reader understanding that distributionLists is a DataTable, and not a List<Contact> or an IQueryable<Contact> or something else.

But hopefully it is not. Hopefully the rest of the method is perfectly understandable with only the semantic understanding, that distributionLists represents a collection of distribution lists fetched from a storage containing email contacts.

Now, to the crowd who says that of course it is better to always know the type, because knowing the type is important for the reader, I would ask a pointed question. Consider this code:

decimal rate = 0.0525m;
decimal principal = 200000.00m;
decimal annualFees = 100.00m;
decimal closingCosts = 1000.00m;
decimal firstPayment = principal * (rate / 12) + annualFees / 12 + closingCosts;

Let's suppose that you believe that it is important for all the types to be stated so that the code is more understandable. Why then is it not important for the types of all those subexpressions to be stated? There are at least four subexpressions in that last statement where the types are not stated. If it is important for the reader to know that 'rate' is of type decimal, then why is it not also important for them to know that (rate / 12) is of type decimal, and not, say, int or double?

The simple fact is that the compiler does huge amounts of type analysis on your behalf already, types which never appear in the source code, because for the most part those types would be distracting noise rather than helpful information. Sometimes the declared type of a variable is distracting noise too.

Now consider the second question. Suppose for the sake of argument it is necessary for the reader to understand the storage type. Is it necessary to state it? Often it is not:

var prices = new Dictionary<string, List<decimal>>();

It might be necessary for the reader to understand that prices is a dictionary mapping strings to lists of decimals, but that does not mean that you have to say

Dictionary<string, List<decimal>> prices = new Dictionary<string, List<decimal>>();

Clearly use of "var" does not preclude that understanding.

So far I've been talking about reading code. What about maintaining code? Again, var can sometimes hurt maintainability and sometimes help it. I have many times written code something like:

var attributes = ParseAttributeList();
foreach(var attribute in attributes)
{
if (attribute.ShortName == "Obsolete") ...

Now suppose I, maintaining this code, change ParseAttributeList to return a ReadOnlyCollection<AttributeSyntax> instead of List<AttributeSyntax>. With "var" I don't have to change anything else; all the code that used to work still works. Using implicitly typed variables helps make refactorings that do not change semantics succeed with minimal edits. (And if refactoring changes semantics, then you'll have to edit the code's consumers regardless of whether you used var or not.)

Sometimes critics of implicitly typed locals come up with elaborate scenarios in which it allegedly becomes difficult to understand and maintain code:

var square = new Shape();
var round = new Hole();
... hundreds of lines later ...
bool b = CanIPutThisPegInThisHole(square, round); // Works!

Which then later gets "refactored" to:

var square = new BandLeader("Lawrence Welk");
var round = new Ammunition();
... hundreds of lines later ...
bool b = CanIPutThisPegInThisHole(square, round); // Fails!

In practice these sorts of contrived situations do not arise often, and the problem is actually more due to the bad naming conventions and unlikely mixtures of business domains than the lack of explicit typing.

Summing up, my advice is:

Use var when you have to; when you are using anonymous types.
Use var when the type of the declaration is obvious from the initializer, especially if it is an object creation. This eliminates redundancy.
Consider using var if the code emphasizes the semantic "business purpose" of the variable and downplays the "mechanical" details of its storage.
Use explicit types if doing so is necessary for the code to be correctly understood and maintained.
Use descriptive variable names regardless of whether you use "var". Variable names should represent the semantics of the variable, not details of its storage; "decimalRate" is bad; "interestRate" is good.

Comments

Anonymous
April 20, 2011
Eric, I like your analysis, but believe you left out one case (at least explicitly) that has been important in actual projects I work on. That is when you want the code to BREAK in the event of a type change. Consider a control system. Many elements have On() and Off() methods. there are many cases where there is no relationship between the types (i.e. no common base classes or interfaces), there is only the similarity that both have methods with those signatures. Now I write code: var thing = SomeFactory.GetThing() // Returns something that is safe to turn off... thing.Off(). Then later a change is made to the Factory and that method now returns something completely different, which happens to have severe consequences if it is arbitrarily turned off [having such a design is debatable for many reasons - but they are outside the defined scope of your post]. By using var, the previous code will compile without compliant. Even though the return type may have changed from "ReadingLamp" to "LifeSupportSystem". I believe (based on my experiences as a "traveling consultant") that there are more time when there is the possibility of an "unintended side-effect" caused by a change in the type than there are times where the change in type has no bearing on the code that consumes it. As a result, I very rarely use var. Even when the return type is obvious (such as the LHS of a "new"), I find it easier to be consistent.
Anonymous
April 20, 2011
I'll deal with it if the team I'm on dictates it, but as for my personal preference, you'll have to pry explicitness from my cold dead hands. Within reason, of course, as I concede the point on anonymous typing, even enumerable query results in general, and (of course) sub-expressions. While I am rationally aware that the static typing is the same as always, it just reminds my irrational brain too much of weak typing in scripting languages and VB and makes me cringe.
Anonymous
April 20, 2011
@David - in your example I would have to imagine your factory is responsible for creating similar objects given the business domain. If your factory could return a "ReadingLamp" or "LifeSupportSystem", then I would assume your factory is called "ThingsThatTurnOffFactory" and might involve revisiting a design decision.
Anonymous
April 20, 2011
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Anonymous
April 20, 2011
Easily the most clear, succinct, and relevant analysis of the pros and cons of var I have ever read. @David V. Corbin, That is exactly the kind of contrived scenario that Eric was referring to as being due to bad design rather than the use of implicit typing. If you really have a method that may realistically change from returning a lamp to returning a life support system, you have MUCH bigger problems than whether or not to use var.
Anonymous
April 20, 2011
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Anonymous
April 20, 2011
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Anonymous
April 20, 2011
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Anonymous
April 20, 2011
I know that not all development happens in Visual Studio, but most does. If you REALLY need to know the type of a variable, you can hover your mouse over it and VS will tell you what type it is. AutoComplete also readily shows what methods are available. I get the warm fuzzies knowing that C# is strongly typed and that prevents a whole class of errors but it is rare that needing to know the type of something is ever that important.
Anonymous
April 20, 2011
@David: I believe in those cases you should abstract the methods to an interface definition, so that you can explicitly tell the type, also change the concrete implementation if the semantics are preserved. But overall I prefer using var to explicitly declaring types. If there is one more thing compiler can do for me, and one less I have to care about, it's a nice feature.
Anonymous
April 20, 2011
Eclipse has a nice feature that highlights (and lets you refactor) code that uses types more derived than necessary. If all you do is enumerate over the return value of ParseAttributeList() then you should type it to IEnumerable (or its generic version). That way your code explicitly states that you are free to change the actual type to anything you want as long as it implements IEnumerable. Using var does not show that. In my opinion it makes the code more difficult to read because it is not clear about what the return type is expected to be. There's nothing to say that if you are going to change it you must implement IEnumerable, unless of course you scan all the code that uses the return value. In your example it's right there, in a real world code it may not be. It may be enumerated in a different method we're passing the return value to. That said, anonymous types, and maybe even things such as collection iterators, are a reasonable place to use var. But other than that I believe it leads to code that is easier to write but difficult to read.
Anonymous
April 20, 2011
I've more or less switched over to using "var" for almost everything, but I've found one unexpected situation where it's caused problems - and that's with the "dynamic" type. I've got a little helper library that implements json as "dynamic" so you can do "var json = Json.NewObject(); json.x = "y";" and have json.ToString() return {x: "y"}. The 'var' in this case evaluates to 'dynamic' - so far, so good. Then you have a method that takes one of these as a parameter - say, IThingy CreateThingyFromJson(dynamic json). Then you write code like this: var myThingy = CreateThingyFromJson(json); ... and 'var' evaluates to 'dynamic' rather than to 'IThingy', even when there's only one CreateThingyFromJson method in scope and it literally takes a 'dynamic' as its parameter. Oops. That was unexpected. I sort of want a way to declare that I'm using a 'dynamic' undynamically...
Anonymous
April 20, 2011
I've been using var almost exclusively since the betas. One thing that has not been mentioned, and which figures strongly into my motivations for using var, is its effect on design of code. Var, by its nature, can act as a forcing function toward simplicity of the code. That is: If your source cannot be reasonably understood using implicit typing, then you should consider rewriting it. Doing so helps keep me honest about maintainability. @Stuart: That feels like a compiler bug to me, not an issue with implicit typing in itself.
Anonymous
April 20, 2011
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Anonymous
April 20, 2011
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Anonymous
April 20, 2011
@Alex Stockton, the compiler will error out on a cast that is determined at compile-time to be impossible. Just as you cannot "Car a = new Elephant()", you cannot "var a = (Car)new Elephant()".
Anonymous
April 20, 2011
@Ben. Right, but your examples don't negate the point that a cast can easily change something that you want to be a compile time failure in to a run time failure. C# casts should not be used for documentation purposes nor to assert things that you want to ensure at compile time. The following are not equivalent:

var x = (Car)CreateTaxi();
Car x = CreateTaxi();
var x = CreateTaxi(); Suppose the return type of CreateTaxi was originally Car then CreateTaxi gets updated so that the return type becomes Vehicle. A cast from Vehicle to Car is possible, so 1 will still compile, and will result in a runtime cast that can fail. Suppose the object returned from the new version of CreateTaxi is actually of type Boat. In this situation 1 will fail at run time, 2 will fail at compile time, and 3 will not fail at all. Obviously you have to decide for yourself what behavior you want now and how you want your code to behave when the code around it changes, but compile time failures are preferable to run time failures, so don't cast unless you have to (and you don't have to if what you're trying to do is either fix or document the compile time type of an expression).

Anonymous
April 20, 2011
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Anonymous
April 20, 2011
The var keyword is nothing like the Variant datatype. There is no type coercion. The compiler correctly determines the static type of the variable.
Anonymous
April 20, 2011
Less dependencies means code that is easier to maintain, and without var you're dependent on a typename. I try to eliminate dependencies from my code as much as possible. Yes there is a disadvantage sometimes, but it greatly outweighs the advantage. Yes birds can hit wind turbines, but regular power plants produce too much pollution. Your choice :)
Anonymous
April 20, 2011
Having the name of the type isn't really a dependency, there are refactoring tools if the name changes and if the return type of the function could change you should probably be using an interface anyways. I would also have to argue against var making code more maintainable, at least on a project the size I am on. It isn't so much that it makes is "harder" to maintain per say, more just frustrating. Sure you can read tooltips to figure out what they are, but trying to keep them straight is annoying and sort of a momentum killer (at least to me)...plus nothing like getting really focused on debugging a difficult bug and then having to stop and switch your brain into "what is this variable? All i can tell from the code is it has a property named Value that is an object or numeric data type." Actually after I joined, I had to debug some year+ old code by someone no longer on the team riddled with vars nested inside foreach loops of vars, and everything was return values of something else. That led me to push for (and we eventually did) ban the use of var on our project, a lot of times it doesn't hurt, but it is just abused too much in my experience.
Anonymous
April 20, 2011
Sean, it sounds more like your code was littered with poorly named variables, ie 'value'. I've worked on very large codebases where var is the norm, and can't remember ever having to wonder what the type of a particular variable is, because variables have descriptive names. Its also aided by the fact that 99.999% of the time I dont care what the type is, debugging or not. I mean...why would you?
Anonymous
April 20, 2011
Lovely . . . more analysis, discussion, and debate about using the "var" keyword. My solution is simple: K.I.S.S. I don't see how the KISS principle applies. Is it simpler to state the type redundantly, or simpler to let the compiler infer the type? It seems to me to be a judgment call which is simpler. -- Eric It's preferable to state something explicitly that shouldn't be than to not state something explicitly that should be. So . . . . . state types explicitly unless a syntactic necessity (such as when using anonymous types) and get back to solving the real business problem. I suspect that you apply your principle inconsistently. In my mortgage example, would you insert casts indicating that the subexpressions were of type decimal? You say that it is "preferable" to state something explicitly, but the types of all those subexpressions are inferred silently, not stated. When, in your opinion, does your principle apply? For example, do you always state the type when calling a generic method, or do you sometimes let the compiler infer the type? How do you decide? Do you always insert redundant casts on method arguments indicating their types to the reader, or do you let the compiler work those out for you? How do you decide? I think your decision-making process is considerably more complex than simply choosing more explicitness every time. I think you probably have some "metric" for determining when explicitness adds value and when it impedes clarity. What is that metric? -- Eric
Anonymous
April 21, 2011
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Anonymous
April 21, 2011
I'll second Alex Stockton's notion of using explicitly typed locals as assertions. I like being able to use var where the type of the expression is easily inferred (by humans and the compiler) from the RHS of the expression, as is the case with var theList = new List<int>(); and I love var when I have do something like var map = new Dictionary<int, List<string>>(); However, if the type of the local depends on the return value of a method, I prefer to explicitly type it so that if the return type of the method changes in an incompatible way, the calling code breaks as well. This prevents the class of errors which David Corbin mentioned earlier. Basically, the farther apart the type information for the LHS is from the RHS for the human reader, the more likely I am to explicitly type the variable.
Anonymous
April 21, 2011
There is also a scenario I've run into that would not have happened if the var keyword was used in foreach loops. Regardless of naming issues and the wisdom of such a design, I've actually seen this type of code in the real world. Say you have the following: ITest2 inherits from ITest, but Test2 does not inherit from Test. IEnumerable<ITest2> items = new ITest2[] { new Test2(), new Test2(), new Test2() }; foreach (ITest2 item in items) { Console.WriteLine(item.GetValue()); } Then you change the enumerable to: IEnumerable<ITest> items = new ITest[] { new Test(), new Test(), new Test() }; The code will compile, but it will fail at runtime with a cast exception.
Anonymous
April 21, 2011
@Alex Stockton, your point is clearer now, but the example is not realistic. Why would I write "var car = (Car)CreateTaxi()" unless CreateTaxi() already returned a type that required the cast? In this case, "var car" is no worse than "Car car", since the compile-time error check (on the cast) is the same. I agree that casts "should not be used for documentation purposes", but I think another example is required to illustrate this ideal where var is concerned. (I leave it to you to provide one.) FWIW, I use var nearly everywhere. As for K.I.S.S., I find the principle applies: var visually and lexically simplifies my code, which translates into faster, easier reading.
Anonymous
April 21, 2011
Considering the way Haskell programmers complain about how often they have to write types in F# programs, I'd have to say that naming types almost seems like a micro-optimization. Why worry about types except in the few cases where it matters?
Anonymous
April 21, 2011
Great, now C# has the equivalent of BASIC's Dim. Next up is GOTO: continuations!
Anonymous
April 21, 2011
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Anonymous
April 21, 2011
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Anonymous
April 21, 2011
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Anonymous
April 21, 2011
I was surprised by Stuart's comment. It even seemed to me that he must have done something wrong (sorry mate), but the quick test showed that yes, indeed my compiler behaves in the same way: interface IThingy { bool Orly(); } class Thingy : IThingy { public bool Orly() { return true; } } class TestClass { public IThingy CreateThingyFromSomething(dynamic whatever) { return new Thingy(); } } class Program { static void Main(string[] args) { dynamic val = 42; var myThingy = new TestClass().CreateThingyFromSomething(val); myThingy.Orly(); myThingy.WhyAmIDynamic(); //why indeed? this gives an error at runtime, not at compile time } } Now I would like to apologize to Eric for my laziness - maybe I didn't read the C# specification thoroughly enough. What is the reason of such behaviour? Or is it really just a bug (with a simple workaround, I admit)?
Anonymous
April 21, 2011
I think the behavior of dynamic is completely self-consistent, even if it's a little unexpected. "dynamic" means "all operations with this value will be resolved at runtime". Not "We'll make some attempt sometimes to resolve some operations at compiletime but if we can't do that it'll be done at runtime". So I think the current behavior is technically correct, but it'd be nice if there were some way to override it.
Anonymous
April 22, 2011
@Stuart-"I sort of want a way to declare that I'm using a 'dynamic' undynamically" C# already has a way to treat a 'dynamic' undynamically or to "override" the behavior: cast to object. var myThingy = new TestClass().CreateThingyFromSomething((object)val); I know it feels unnatural to cast a dynamic variable to object on a function that takes a dynamic parameter so that the method returns its actual type, but I don't think I would want C# to introduce another syntax just to handle this situation.
Anonymous
April 22, 2011
@Eggbound: "I've worked on very large codebases where var is the norm, and can't remember ever having to wonder what the type of a particular variable is, because variables have descriptive names." That is what Hungarian notation is, you move your type information into your variable names and we all know where that went. Unless somehow you are able to deduct that "customers", which is a pretty descriptive name, has to actually be ICollection, or even IList instead of just IEnumerable. Or that eventCount must be long and not just an int because it's counting events that have the potential to overflow int sized counter.
Anonymous
April 22, 2011
@Stuart - oh, yes, now I see where I was wrong. Dynamic allows double dispatch, so if the reference to Thingy would be obtained using anything more complex than a "new" operator, the resulting type would be unknown. Unknown means either object or dynamic, and since we are already dealing with dynamics, the latter option is more reasonable. So everything is correct except my intuition :)
Anonymous
April 22, 2011
iMil42: What? The static type of myThingy can be infered from CreateThingyFromSomething method. Your intuition is correct. The implicit type of myThingy sholud be IThingy.
Anonymous
April 23, 2011
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Anonymous
April 23, 2011
Any chance for "var everywhere" in C# 5.0? Fields would be nice... class Stuff { private Dictionary<string, List<decimal>> _prices = new Dictionary<string, List<decimal>>(); } with less code noise, less distracting, to simply: class Stuff { private var _prices = new Dictionary<string, List<decimal>>(); } Thanks.
Anonymous
April 24, 2011
@Mark: Eric has previous covered that, see: blogs.msdn.com/.../why-no-var-on-fields.aspx
Anonymous
April 24, 2011
@Alan: Appreciate these issues, we all tell others that "it's not so easy and here's why"...but... "Doing so would actually require a deep re-architecture of the compiler." There was a recent video on Channel 9 with Eric and Erik and "compiler as a service"...low level rewrite...any hope? -M
Anonymous
April 25, 2011
Alan, How is 'var' the cause for the bug in your case? Let's say you wrote it as DataB data = GetState(); ThreadPool.QueueUserWorkItem(_Runner, state) Your _Runner method will still fail as there is no type check performed by the compiler. Using or not using 'var' in this scenario won't make any difference. Yogi
Anonymous
April 25, 2011
I don't know if there's any relation, but var makes me think of duck typing. I'm more on the pro-var side, personally - I don't care what type GetThingy() returns as long as the Thing it returns will Frob() as requested.
Anonymous
April 26, 2011
Another disadvantage with using implicit typing: "Find usages"/"Find references" on a type will not find occurrences where implicit typing is used.
Anonymous
April 26, 2011
@Alex: I do agree that using a C# cast as a type annotation is dangerous. I was considering syntax and readability (and comparing with other languages), not semantics. @GreenMoose: Find References works fine with implicit typing.
Anonymous
April 26, 2011
@Olivier: It does? class Foo { } class Bar { Foo GetFoo() { return null; } void GetFooWithImplicitUsage() { var Foo = GetFoo(); } void GetFooWithExplicitUsage() { Foo Foo = GetFoo(); } } "Find all references" on type Foo will not find usage in GetFooWithImplicitUsage() with my vstudio2010...
Anonymous
April 27, 2011
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Anonymous
April 30, 2011
Good article, nice emphasis on the real world. Most code is written for really dull purposes. Code lives long long after it goes live. The simpler code is to understand at a glance the better. I scream while reading code, "Don't make me think!" if it's being complex for no purpose. I wonder how much code like that I wrote when I as younger... eeek.
Anonymous
June 23, 2011
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Anonymous
June 27, 2011
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Anonymous
July 29, 2011
thank for sharing your great think.....
Anonymous
August 12, 2011
Eric: > Is this sane or insane? > string fullName = (string)customer.FirstName + (string)" " + (string)customer.LastName; You're missing the point. Jeff wants code that breaks when the type of subexpressions change. Casts prevents that. Code like this: var IsValid = IsStringValid("somestring"); should not compile if IsStringValid() doesn't return a boolean value.
Anonymous
May 03, 2012
@pet - I understand your point and Eric probably does too, but let's address your concern about the casts. First define a cast function that looks something like this: T static_cast<T>(T value) { return value; } I hope you'll agree that this function allows type annotation, type checking, and limited type casting without allowing runtime failures of the kind that you're concerned about. Now let's rephrase Eric's question using this function. Is the following sane or insane: string fullName = static_cast<string>(customer.FirstName) + static_cast<string>("") + static_cast<string>(customer.LastName); If it's insane is the insanity related to the fact that C# doesn't have a static_cast-equivalent built in to the language or is it related to the fact that annotating subexpressions with types is insane all by itself?

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Uses and misuses of implicit typing

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