c# - syntax question_ @html.labelfor(m =_ m.username) - stack overflow.pdf

8/11/2019 c# - Syntax Question_ @Html.LabelFor(m =_ m.UserName) - Stack Overflow.pdf

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rkw

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5 Answers

Jason

126k 14 208 371

Going from ASP.NET 2.0 (VB) to MVC 3 (C#), I'm very confused about the syntax being used for the View.

( )

Where did that m come from? My only guess is that it represents the model that is being passed into the

view. I tried changing the m to c and it still works fine.

Is the part of the syntax that involves the "=>" more of a MVC, C#, or Razor element?

c# asp.net-mvc razor

asked Jul 20 '11 at 17:30

@NoProblemBabe, there is someone doing it here: stackoverflow.com/questions/2595947/asp-net-mvc-label-for

user971077 Sep 29 '11 at 12:13

Where did that come from?

It's the parameter in a lambda expression.

My only guess is that it represents the model that is being passed into the view. I tried changing the m

to c and it still works fine.

That's because the name doesn't matter. It's just a parameter name, it doesn't actually refer to any existing

variable.

Is the part o f the syntax that involves the "=>" more of a MVC, C#, or Razor element?

It's C#, but uses what the compiler translates into to extract what it

needs to build up the label.

This is a very deep an intricate subject. I suggest that you find a book that you're comfortable with (e.g.,

C# in Depth is very good on this subject) to understand more. You want to read about lambda expressions

and expression trees.

answered Jul 20 '11 at 17:34

I don't he needs a book for it. ToccoJul 20 '11 at 17:36

1 +1 on C# In Depth - Jon Skeet loves you... Matthew AbbottJul 20 '11 at 17:46

It's a syntax trick that has been present since C# 3.0 (I think; maybe 3.5).

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Syntax Question: @Html.LabelFor(m => m.UserName)

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Vilx-

38k 32 142 281

NoProblemBabe

996 12 34

Sergey Metlov

6,545 8 36 87

If you were to write this in code (and your Razor view does get translated to a C# code file before

compiling, so it really isin code), there are two possible ways the compiler can compile it depending on the

context.

If the method () expects a delegate, it's compiled to an anonymous method. Alternatively, if

the method expects an type, an expression treeis

constructed. This is what is happening in your case.

The reason for this convulted syntax is that the expression tree contains enough information, that

(combined with Reflection) the () method can extract the actual propertyto which you are

referring. If you were to pass it simply as () , there wouldn't be enough

information for the () to do this. It would just get the value from the property. But

now it knows where it came from, and can use more Reflection to have a glance at the attributes of theproperty. Attributes such as , , and others.

The (or or whatever) is actually your model. is an extension method and it simply

passes your model back to your Lambda expression, so that the whole expression tree trick can work. You

could also write it like () , but I don't think it would work (I haven't tried

though, maybe it does).


Ah, thanks for the explaining the reason behind the extended syntax. rkwJul 20 '11 at 17:49

Lambda expressions a syntax trick? They're a really important language feature! Mike GoodwinSep 24 '13 at

21:21

Yes. It's a really important syntax trick. :) OK, well, I suppose a lot of language features could be called "syntaxtricks". I use the term to mean "something that can be done without using the feature, except it gets a lot more

tedious to write". For example, a lambda expression could be replaced with the more ancient

syntax. Or, if it is used to construct a , then you could use

the explicit methods to build the tree. Of course, the code becomes tedious to the point where it's

useless. But it still can be done. Vilx-Sep 24 '13 at 21:48

@MikeGoodwin - Similarly, a is a "syntax trick", because you can replace it with

. The same about and (because you can replace them with a ), etc.

Vilx-Sep 24 '13 at 21:50

That's a Lambda Expression, link.

The deal is this: the m is a variable that receives the instance of the model in the given circunstance.

Inside the labelFor, it will call a compile-time created class, which has a method that does what you havepassed as an argument to the LabelFor.

A lambda expression can be switched by a delegate, with the exact same results, excepting a minor, really

minor performance boost, once.

The general idea is that you are passing a method to be execute somewhere in the LabelFor method.

ex: the method:

()

()(" ")

Should be used as:

( ())

edited Jul 20 '11 at 17:40 answered Jul 20 '11 at 17:33

This is Lambda Expression. From MSDN:

A lambda expression is an anonymous function that can contain expressions and statements, and can be

used to create delegates or expression tree types.

All lambda expressions use the lambda operator =>, which is read as "goes to". The left side of the lambda

operator specifies the input parameters (if any) and the right side holds the expression or statement block.

The lambda expression x => x * x is read "x goes to x times x."


The LabelFor is a Razor function that has knowledge of the Model internally. In other words, the model


3 14/08/2014 13:34


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Moises Issi

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resides inside the LabelFor function logic. When you pass the lambda expression, you are passing an

anonymous function to LabelFor so it knows what to do with its internal Model object to extract

information.

Typically you would pass a Model as parameter and LabelFor would use internal logic to extract the

information needed. But in this format, it is the opposite. The LabelFor knows the parameter internally (the

Model) but it doesn't know what to do with it. You tell it what to do by passing the Lambda function.

When you run the code, you pass the Lambda expression as parameter to LabelFor. LabelFor, takes the

lambda expression and passes it an internal copy of the Model object. The Lambda expression returns the

UserName property of the Model object, and the LabelFor uses it to build your HTML code.

edited Sep 24 '13 at 21:10 answered Sep 24 '13 at 20:59

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