I know I can name parameters when I create a tuple implicitly like:
var me = (age: 21, favoriteFood: "Custard");
Is it possible to name parameters when a tuple is created explicitly? i.e.
var me = new ValueTuple<int, string>(21, "Custard");
No, you can't. The ValueTuple types are actually independent of the named field support in C#. The latter works more like named properties for anonymous types. That is, the compiler analyzes the code and generates aliases to the appropriate members according to your declarations and usages. It is through the assignment that the compiler learns the names of the fields. Since the basic constructor syntax doesn't provide a mechanism to name the fields, you can't use that to directly generate a tuple with named fields.
Of course, there are ways you can re-interpret the value returned from the constructor syntax, to assign names to that returned value. I'm assuming you're aware of that approach and are looking for something more direct.
As an example of what I mean by "re-interpret", you could do something like this:
static (int value, string text) ConvertToNamed((int, string) t) => t;
then this would name the fields, in a new variable:
var t1 = new ValueTuple<int, string>(21, "hello");
var t2 = ConvertToNamed(t1);
The variable t1 is stuck with Item1 and Item2. But the compiler will implicitly generate the desired names for the variable t2.
Maybe a better example is one where you don't require the additional method:
(int value, string text) t = new ValueTuple<int, string>(21, "hello");
Again, you're not really naming the fields in the constructor syntax, but they are reinterpreted by the local variable declaration.
This is probably not a serious limitation. In a scenario where there's a desire to have a persistent, easily-assigned name, it's probably better to declare a user-defined type than to use the tuple syntax anyway. You can write deconstructors for user-defined types as well, and declaring types like that means the names are first-class citizens when it comes to reflection, dynamic, etc.
You can cast to named tuple during assignment:
var me = ((int value, string text)) new ValueTuple<int, string>(21, "Custard");
Was looking for this when using ValueTuple in Linq. I did a F2-rename variable and it created the following syntax:
var query = from x in myList
select (First: x.something, Second: x.other) into g
select new Whatever(g.First, g.Second);
... C# 7.0 doesn’t enable the use of custom item names when using the
explicit System.ValueTuple<…> data type. Therefore, if you replace var
in Example 8 of Figure 1, you’ll end up with warnings that each item
name will be ignored.
Source
I'm posting this answer because the closest the docs come to explaining this is:
Data members of ValueTuple types are fields. Data members of Tuple
types are properties.
I don't believe so. Here are docs that I found on ValueTuple:
https://learn.microsoft.com/en-us/dotnet/api/system.valuetuple-7?view=netframework-4.7
I personally have not used the ValueTuble type. I have used the Tuple class before like this:
var tuple = new Tuple<int, string>(21, "Custard");
var number = tuple.Item1;
var st = tuple.Item2;
However I find using Tuples, especially when passed thru methods to be clunky. Always having to know what is in Item1 and Item2 etc. According to the ValueTuple docs, it is used in the same way.
Shouldn't you just make an entity class?
I have used the Anonymous Type which I think will suit your needs.
var anyom = new
{
age = 21,
favoriteFood = "Custard"
};
number = anyom.age;
st = anyom.favoriteFood;
Related
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Use of var keyword in C#
After discussion with colleagues regarding the use of the 'var' keyword in C# 3 I wondered what people's opinions were on the appropriate uses of type inference via var?
For example I rather lazily used var in questionable circumstances, e.g.:-
foreach(var item in someList) { // ... } // Type of 'item' not clear.
var something = someObject.SomeProperty; // Type of 'something' not clear.
var something = someMethod(); // Type of 'something' not clear.
More legitimate uses of var are as follows:-
var l = new List<string>(); // Obvious what l will be.
var s = new SomeClass(); // Obvious what s will be.
Interestingly LINQ seems to be a bit of a grey area, e.g.:-
var results = from r in dataContext.SomeTable
select r; // Not *entirely clear* what results will be here.
It's clear what results will be in that it will be a type which implements IEnumerable, however it isn't entirely obvious in the same way a var declaring a new object is.
It's even worse when it comes to LINQ to objects, e.g.:-
var results = from item in someList
where item != 3
select item;
This is no better than the equivilent foreach(var item in someList) { // ... } equivilent.
There is a real concern about type safety here - for example if we were to place the results of that query into an overloaded method that accepted IEnumerable<int> and IEnumerable<double> the caller might inadvertently pass in the wrong type.
Personally I find the circumstances you describe far from questionable, since there is no point in repeating yourself unless you specifically want the static type of a variable to be different than the static type of the expression used to initialize the variable. For example:
IEnumerable<int> foo = new List<int>(); // It's IEnumerable on purpose
Furthermore, there are absolutely no type safety concerns with var. The point is not that the variable can be of "any" type. It is of a very specific type, but you simply do not care to spell that type out.
I'm only using it as a place holder until I'm sure which datatypes I'm using.
Sure this is a short answer but I think it's pretty close that when you should use the var keyword.
the var keyword is used as shorthand in the language, but isn't a .NET type. The compiler must know the type of the variable to use the var keyword - so it is type-safe.
I personally only use it if the type name is also used in the assignment and the name is possible too long to duplicate in the code.
var dictionary = new Dictionary<string, string>();
It is also used for anonymous types (but still, the compiler must know the signature of the anonymous type).
var fred = new { Age = 23, Name = "Fred" };
This method is used commonly in the select clause of LINQ queries.
Just an "abstraction" or "syntax sugar" to be able to write a code without specifying first the type (this is no your first cases)
In second case: LINQ queries, instead, to rapresent some unknown, dynamic, not concrete, if you wish, type.
could be:
var results = from item in someList
where item != 3
select item; //item a class instance
could be
var results = from item in someList
where item != 3
select item.ItemName; //string property of that class
could be
var results = from item in someList
where item != 3
select new {item.ItemName, item.ID}; //unknown type dynamically generated, that conains the string and integer, like result
As far as I know var remains strong typed. The compiler calculated what the proper type should be. In fact it has no real meaning.
It is only a trick to reduce the number of manipulations. For instance when you change a type in one class, this can result in a cascade of modification. But its only a way to migrate work from the programmer to the compiler.
For instance your Linq query will result in a type IEnumerable<TA>. When you change some class so the result will be IEnumerable<TB> there is no need to change this part of the code.
When I do a query that returns an anonymous type
var assets =
from Product p in Session.CreateLinq<Product>()
where bundles.Contains(p.ProductBundle)
select new {p.Asset, p.Asset.PropertyTbl};
Can I type the return to anything other than var?
You cannot* return an anonymous type because the caller would not know what type it is and wouldn't be able to use it.
If you want to return the results, you can create objects of a non-anonymous type:
IEnumerable<Foo> assets =
from Product p in Session.CreateLinq<Product>()
where bundles.Contains(p.ProductBundle)
select new Foo { Bar = p.Asset, Baz = p.Asset.PropertyTbl};
You can also use the Tuple type in .NET 4 if you don't want to create a custom class for your values.
* This is not strictly true - it is possible but you should avoid doing it. Here is a link anyway if you really want to.
You can use object or dynamic (in .NET 4.0) instead of var but don't expect to find a name to an anonymous type. In your case using var is better as it will preserve the strong typing at least until you leave the scope of the current method.
You could define a new class:
public class AssetProp
{
public virtual string Asset {get;set;}
public virtual string PropertyTbl {get;set;}
}
And then you can return it as that class:
IEnumerable<AssetProp> assets =
from Product p in Session.CreateLinq<Product>()
where bundles.Contains(p.ProductBundle)
select new AssetProp {p.Asset, p.Asset.PropertyTbl};
Not really, since the new {p.Asset, p.Asset.PropertyTbl} code creates an anonymous type. Even using object doesn't really gain you much since you can't cast it to anything useful later on, so you would have to use reflection to access the properties.
Not really. If you cast to object you wont be able to access the properties of your anonymous class.
The var keyword was specifically introduced for dealing with anonymous classes - why would you want to avoid it? If you need to return the data you should name the class.
You can if you use lambda expressions, otherwise you can do a cast but do some good exception handling.
you can also do this (it does relate much to your problem though, because you just move "var" somewhere else, but it's interesting that it recognize those types as same)
var element = new { id = 7 };
List<object> collection = new List<object>();
element = collection.Select(item => new { id = 0 }).First();
Consider the following:
// select a subset of the DataTable
var subset = DataTable.Where(...).Select(row => new
{
Id = Convert.ToInt32(row["Id"]),
Name = row["Name"].ToString(),
Email = row["Email"].ToString()
});
// or create a new object
var subset = new {
Id = 1,
Name = "something random",
Email = "name#domain.tld"
};
Is there any way to use the subset variable as a parameter to a method, without it being cast as a plain Object? Can you somehow carry the auto-generated type of the variable?
I am trying to avoid having to create new classes every time I want to pass LINQ subsets to methods.
Random generic approaches are welcome.
No, passing anonymous types about isn't generally a good idea because you lose the type information*. You should create a concrete type and use that instead.
var subset = DataTable.Where(...).Select(row => new SomeType
{
Id = Convert.ToInt32(row["Id"]),
Name = row["Name"].ToString(),
Email = row["Email"].ToString()
});
Alternatively you can use the Tuple type if you are using .NET 4. This is a simple way to create "disposable" types and still get some type-safety.
*Actually there is a workaround, but I regard it is an ugly hack and would advise that you don't do it.
If I need to do this, I use resharper's "Replace Anonymous Type With Named Class" refactoring option. Then you have an appropriate named type to expose over the API, and you haven't had to do any work. This also gives you options to create it immutable (like anonymous types) or mutable, nested vs top-level, etc.
BTW, I don't recommend struct here (from the question).
Another option is to pass the behaviour into the method - i.e. an Action<int,string,string> callback - then do something like:
foreach(item in query) callback(item);
However, I don't like this as it is not obvious that there is a likely error in:
DoSomething(args, (id, email, name) => Email(To: email, Subject: name));
(the error being that it should probably be (id, name, email), if you see what I mean)
You can use a generic method:
public static void Foo<T>(T item)
{
// Do whatever
}
Then if you call
Foo(subset);
the compiler will infer T for you. Whether or not that actually helps you is another matter... it depends on what the method is meant to do. Obviously Foo can't refer to Id, Name, Email etc.
In general, if multiple methods should know about the same members, then you should use a named type. The usual case for passing them to generic methods is where the method really doesn't care about what type is involved, such as in LINQ.
I've made a feature request for C# 5 that we should be able to create types which have all the same features as anonymous types (immutability, equality, hash code generation, ToString dumping) but for simple named types. We'll see if it actually happens...
Anonymous Types don't provide much help outside of the context they where created.
If you need to pass an Anonymous Type to a method, either this method is very generic like (Example)
void PrintAllObjectProperties(object obj);
witch you would use reflection to do the work, or you are doing something wrong.
Here's what I came up with...
Extension method on Object:
public static class ObjectExtensions
{
/// <summary>
/// Cast Object to anonymous type.
/// E.G.: new Object().ToAnonymousType(new { Property = new Type() });
/// </summary>
public static T ToAnonymousType<T>(this Object o, T t)
{
return (T)o;
}
}
Usage:
public void HandleAnonymousTypeAsParameter(Object o)
{
var anonymousType = o.ToAnonymousType(new
{
Id = new Int32(),
Foo = new String(),
Bar = new String()
});
// ... You can do this in even less characters:
var anonymousType = o.ToAnonymousType(new { Id = 0, Foo = "", Bar = "" });
}
HandleAnonymousTypeAsParameter(new
{
Id = 1,
Foo = "foo",
Bar = "bar"
});
Credits goes to John Skeet and Thomas P.
I have no clue what an "anonymous type" is in C# nor how it is used. Can somone give me a good description of it and it's use?
[Note: i really know what it is and how to use it but thought i'd ask for those that don't]
An anonymous type is a type generated by the compiler due to an expression such as:
new { Property1 = x.Value1, Property2 = y.Value2, z.Value3 }
(the last one is like Value3 = z.Value3).
The name of the anonymous type is "unspeakable" - i.e. you can't specify it in normal C# - but it's a perfectly normal type as far as the CLR is concerned. As you can't write the name, if you want to create a variable of an anonymous type (or a generic type using an anonymous type as the type argument), you need to use an implicitly typed local variable with the var keyword:
var person = new { Name = "Bill", Address = "..." };
C# anonymous types are immutable (i.e. the properties are read-only) - the generated type has a single constructor which takes values for all the properties as parameters. The property types are inferred from the values.
Anonymous types override GetHashCode, Equals and ToString in reasonably obvious ways - the default equality comparer for each property type is used for hashing and equality.
They are typically used in LINQ in the same way that you'd use "SELECT Value1 As Property1, Value2 As Property2, Value3" in SQL.
Every anonymous type initializer expression which uses the same property names and types in the same order will refer to the same type, so you can write:
var x = new { Name = "Fred", Age = 10 };
x = new { Name = "Bill", Age = 15 };
It's also worth knowing that VB anonymous types are slightly different: by default, they're mutable. You can make each individual property immutable using the "Key" keyword. Personally I prefer the C# way, but I can see mutability being useful in some situations.
See these links:
http://www.developer.com/net/csharp/article.php/3589916/The-New-Anonymous-Types-Feature-in-C-30.htm
http://www.west-wind.com/weblog/posts/189329.aspx
http://michaelsync.net/2008/03/06/c-30-tutorials-understanding-about-anonymous-types
http://msdn.microsoft.com/en-us/library/bb397696.aspx
An anonymous type is a type that has no name. You can use it anywhere you don't need the name of the type. For instance:
var query = from x in set where x.Property1 = value select new {x.Property1, x.Property2};
foreach (var q in query) {
// do something with q.Property1, q.Property2
}
I'm using an API that has a method that requires this type of argument:
System.Collections.ObjectModel.Collection<GenericTickType> genericTickList
How do I instantiate an object for that argument? Here's what I've tried but it keeps saying that the method call has some invalid arguments.
List<TickType> ticks_to_get = new List<TickType> { TickType.Price };
I've tried instantiating a Collection directly instead of a List and that doesn't seem to work.
"I've tried instantiating a Collection directly instead of a List and that doesn't seem to work."
What error do you get? You can definitely create an instance of Collection<T> directly, it is not an abstract class and it has several public constructors, including one that's parameter-less. You can do this, for example:
var values = new System.Collections.ObjectModel.Collection<int> { 1,2,3,4 };
I noticed your sample code has a GenericTickType and a TickType. Is this a mistake or do you actually have two classes? You said it's an enum (which one?), so one cannot possibly derive from the other. If they are two enum types, Collection<GenericTickType> and Collection<TickType> are two different classes and one is not assignable to the other.
Now, if TickType is castable to GenericTickType (and they probably are if they are both enums, and assuming they share the same numeric values), you still cannot cast Collection<TickType> to Collection<GenericTickType>. There's no contra/co-variance in C# for most classes yet (coming in C# 4). But you could cast each TickType by doing something like this:
List<GenericTickType> list = new List<GenericTickType> { (GenericTickType)TickType.Price };
list.Add((GenericTickType)TickType.Price); // add more...
Collection<GenericTickType>genericTicks = new Collection<GenericTickType>(list);
If you already have a List<TickType> and have access to C# 3.0 and LINQ, you can do this:
List<TickType> ticks = new List<TickType> { TickType.Price };
list.Add(TickType.Price); // add more...
List<GenericTickType> castedList = ticks.Cast<GenericTickType>().ToList();
Collection<GenericTickType>genericTicks = new Collection<GenericTickType>(castedList);
This uses the LINQ Cast<T>() and ToList<T>() extension methods to cast each TickType in the original list to GenericTickType and creating a new List<GenericTickType> which is used to instantiate the Collecion<GenericTickType>. (I avoided using var so you could see the types in each step).
You can't pass a List<> as a Collection<>
Maybe you have problems with covariance/contravariance? You have to do the cast on your own:
List<TickType> ticks_to_get = new Collection<TickType> { TickType.Price };
genericTickList = (Collection<GenericTickType>) ticks_to_get;
Look at Dave Bauman's answer ... unless TickType.Price doesn't return an object of type TickType it will not work
EDIT: Since GenericTickType is an enum - which API are you using? Is it of your company - can you change it? It seems to be strange that you are asked to pass a collection of enum values. See, if you can change the enum to a flagged enum ... and then pass the required values by combining them with the or-operator.
What is the type of GenericTickType? Is it an enum or class? I am assuming enum. If that is the case, modify your code as such:
Collection<GenericTickType> ticks_to_get = new Collection<GenericTickType>() { GenericTickType.Price };
The above works on 3.5 framework.
You can't use {} to initialize a Collection. You can do something like this, however:
List<TickType> ticks_to_get =
new List<TickType>( new TickType[] { TickType.Price });
or
List<TickType> ticks_to_get = new List<TickType>();
ticks_to_get.Add(TickType.Price);