Why doesnt this work, and how to fix?
public interface ITheInterface
{
string a{get;set;}
string b{get;set;}
}
public class SomeObject: ITheInterface
{
string a{get;set;}
string b{get;set;}
...
}
public class SomeGroup
{
ITheInterface Result;
...
}
var results= from y in dc.Groups
where y.id==1
select new SomeGroup
{
Result= (from x in dc.Objects
select new SomeObject{... }
).SingleOrDefault(),
}
return results.ToList();
Could not convert from type System.Collections.Generic.List to Interface
I assume your problem is with the Results.ToList() call? It will fail, because ITheInterface does not support ToList(). You are calling SingleOrDefault() on the LINQ query, which is giving you a single item. It doesn't make sense to call ToList() on a single item.
If, instead, your code read like this:
IEnumerable<SomeObject> Results = from x in dc.Objects
select new SomeObject{... };
Then, Results.ToList() will give you a List<SomeObject>.
If what you are actually looking for is a List<ITheInterface> instead, you can do this:
Results.Cast<ITheInterface>().ToList()
Results is a single object; ToList() only works on Enumerables.
You need to either write return new List { Results }; (this uses a collection initializer) or get rid of the call to SingleOrDefault and declare Results as an IEnumerable<ITheInterface>.
If you only want one object, why are you returning a List?
In addition to the other answers, when you implement an interface, you must declare the member functions as public:
public class SomeObject: ITheInterface
{
public string a{get;set;}
public string b{get;set;}
...
}
You want to say
SomeGroup result = results.SingleOrDefault()
return result;
This is because the return type of your method is SomeGroup (I'm inferring)
Related
I would like to return an object, which stores the return values from other class methods as properties on the return object. The problem is that I do not know which is the best way to do this in C#. Currently I am using a sort of JavaScript-ish approach. Because I do not know the return type, I use the dynamic keyword.
class Test {
public static dynamic MyExportingMethod() {
return new {
myString = MyStringMethod(),
myInt = MyIntMethod()
};
}
public static string MyStringMethod() {
return "Hello";
}
public static int MyIntMethod() {
return 55;
}
}
And then being able to access them like so,
var myReturnObjWithProps = Test.MyExportingMethod();
myReturnObjWithProps.myString; // should be "Hello"
So my questions are, should I use the dynamic return type? Am I not just returning an anonymous object?
should I use the dynamic return type?
Yes - a method that returns dynamic effectively returns an object, so you have to use dynamic in order to access it's properties at runtime without reflection.
Am I not just returning an anonymous object?
You are, but the declared type of the method is effectively object so its properties cannot be referenced at compile-time.
Bottom line - you should avoid returning anonymous types from methods if at all possible. Use a defined type, or keep the creation and usage of the anonymous type in one method so you can use var and let the compiler infer the type.
Make a class for the return type. You want something strongly typed, both for performance and for clarity.
class Foo
{
string MyString { get; set}
int MyInt { get; set}
}
class Test
{
public static Foo MyExportingMethod()
{
return new Foo
{
MyString = MyStringMethod(),
MyInt = MyIntMethod()
};
}
public static string MyStringMethod()
{
return "Hello";
}
public static int MyIntMethod()
{
return 55;
}
}
You should use dynamic sparingly. This way anyone can see what your method is returning. If you return dynamic the caller has no way of knowing what to look for in the dynamic without looking at the source for your method. That way the abstraction goes away and well-structured programming is all about abstraction.
An alternative to creating a new class simply for a method return would be ValueTuple:
public static (string myString, int myInt) MyExportingMethod()
{
return (MyStringMethod(), MyIntMethod());
}
var (myString, myInt) = Test.MyExportingMethod();
myString; // == "Hello"
If have following classen
public interface ISomething { int Id { get; set; } }
public class SomethingA : ISomething {...}
public class SomethingB : ISomething {...}
In another class I have following two lists:
List<SomethingA> aValues;
List<SomethingB> bValues;
My question is if there is a possibility to do something like this:
public List<ISomething> GetList(bool select) {
return select ? aValues : bValues;
}
My goal is to use this as this:
GetList(true).Single(x => x.Id) // or
foreach (var value in GetList(false))
{
value.Id = 18;
}
// anything else
UPDATE:
I see, there are good possibilities. But is there also a way to also achieve the following?
GetList(true).Remove(myValue);
You can't return List<ISomething> because List<T> is not covariant and classes can't be. IEnumerable<T> is covariant, you may use it as readonly sequence.
Change the method to return IEnumerable<ISomething>
public static IEnumerable<ISomething> GetList(bool select)
{
return select ? (IEnumerable<ISomething>)aValues :bValues;
}
Then do
var result = GetList(true).Single(x => x.Id == 0);
foreach (var value in GetList(false))
{
value.Id = 18;
}
As for your update: If you like to remove the item you need to lose some flexibility. I.e Use non generic IList as the return type.
public static IList GetList(bool select)
{
return select ? (IList)aValues : bValues;
}
Then do
IList list = GetList(true);
foreach (var value in list.OfType<ISomething>())//OfType or Cast can be used
{
if (value.Id == 6)//Whatever condition
{
list.Remove(value);
break;
}
}
I like the OfType extension because it returns the typed list you need
var listA = initialList.OfType<TypeA>(); //return List<TypeA>
var listB = initialList.OfType<TypeB>(); //return List<TypeB>
So in your case you start with
var aValues = List<ISomething>.OfType<SomethingA>()
and then you can iterate on whichever subcollection you need. Of course you are then working with a IEnumerable, but that can be converted implicitly back to a IEnumerable<ITest>.
If you want to filter out values, I would create explicit methods to remove them but it depends on what you need to achieve in the end (for example comparing on a Id instead of the whole object):
public IEnumerable<T> Remove<T>(this List<IDisposable> values, T valueToRemove) where T: IComparable
{
return values.OfType<T>().Where(t => valueToRemove.CompareTo(t) != 0);
}
The simplest solution may be using Linq Cast() like this:
public List<ISomething> GetList(bool select)
{
return (List<ISomething>)(#select ? aValues.Cast<ISomething>() : bValues.Cast<ISomething>());
}
I see, there are good possibilities. But is there also a way to also achieve the following?
GetList(true).Remove(myValue);
To remove from the original lists, you are likely best of with a specialized Remove method on the class in question as others have suggested, as most solutions here return a copy of the original list.
You may remove the element from a copy of the list quite easily like so, but I understand that's not what you are asking.
var result = GetList(true);
result.Remove(myValue);
You can either use the .Cast<T> method like this:
if (select)
{
return aValues.Cast<ISomething>().ToList();
}
else
{
return bValues.Cast<ISomething>().ToList();
}
or add all items to a commong Lis() like this:
var ret = new List<ISomething>();
if (select)
{
ret.AddRange(aValues);
}
else
{
ret.AddRange(bValues);
}
return ret;
Since you only want to iterate it, I would write the method like this:
public IEnumerable<ISomething> GetList(bool select) {
return select ? aValues.Cast<ISomething>() : bValues.Cast<ISomething>();
}
You can also look at this StackOverflow question.
I have the following code:
public class CategoryNavItem
{
public int ID { get; set; }
public string Name { get; set; }
public string Icon { get; set; }
public CategoryNavItem(int CatID, string CatName, string CatIcon)
{
ID = CatID;
Name = CatName;
Icon = CatIcon;
}
}
public static List<Lite.CategoryNavItem> getMenuNav(int CatID)
{
List<Lite.CategoryNavItem> NavItems = new List<Lite.CategoryNavItem>();
-- Snipped code --
return NavItems.Reverse();
}
But I get the following error:
Cannot implicitly convert type 'void' to 'System.Collections.Generic.List<Lite.CategoryNavItem>'
Any ideas why this might be?
Try:
NavItems.Reverse();
return NavItems;
List<T>.Reverse() is an in-place reverse; it doesn't return a new list.
This does contrast to LINQ, where Reverse() returns the reversed sequence, but when there is a suitable non-extension method it is always selected in preference to an extension method. Plus, in the LINQ case it would have to be:
return someSequence.Reverse().ToList();
One workaround would be Return NavItems.AsEnumerable().Reverse();
.Reverse() on a list reverses the items within the list, it does not return a new reversed list.
Reverse() does not returns reversed list itself, it modifies original list. So rewrite it as following:
return NavItems.Reverse();
TO
NavItems.Reverse();
return NavItems;
Reverse() does not return a List as expected of your function.
NavItems.Reverse();
return NavItems;
If you have a list like in your example:
List<Lite.CategoryNavItem> NavItems
You can use the generic Reverse<> extensions method to return a new list without modifiying the original one. Just use the extension method like this:
List<Lite.CategoryNavItem> reversed = NavItems.Reverse<Lite.CategoryNavItem>();
Notes: You need to specify the <> generic tags to explicit use the extension method.
Don't forget the
using System.Linq;
.Reverse reverses the "in-place"..., try
NavItems.Reverse();
return NavItems;
I had a situation where none of the suggested options suited me. So, if you:
don't want to use someList.Reverse() because it returns nothing (void)
don't want to use someList.Reverse() because it modifies source list
use someList.AsEnumerable().Reverse() and get the Ambiguous invocation error
You can try Enumerable.Reverse(someList) instead.
Don't forget the:
using System.Linq;
I have two classes, CheckboxItemsList which extends a generic list, and CheckboxItems, which contains a list of objects of type CheckboxItem.
I want to use LINQ to be able to filter CheckboxItemsList based on properties of its CheckboxItems objects. The return type is always a generic list, though, but I want it to be a CheckboxItemsList.
So I guess the basic question is, can linq be made to return a list of the same type that it starts with? Since I can't cast a base class to a derived class, do I have any option other than iterating through the results of the linq query and rebuilding the derived list object row by row? Not that this is the end of the world, but I'm relatively new to linq and was wondering it there is a better way to do it.
What I want:
CheckboxItemsList newList = MyCheckboxItemsList.Where(item=>item.Changed);
(obviously doesn't work since the query will return List<CheckboxItems>, not CheckboxItemsList)
The objects, generally:
public class CheckboxItemsList: List<CheckboxItems>
{
// does not add any fields, just access methods
}
public class CheckboxItems : IEnumerable<CheckboxItem>
{
public long PrimaryKey=0;
protected CheckboxItem[] InnerList;
public bool Changed
{
get {
return (InnerList.Any(item => item.Changed));
}
}
....
}
No, this is not possible out of the box. You'll need to add code to do this.
For example, you can add a constructor like so:
public CheckboxItemsList(IEnumerable<CheckboxItems> checkboxItems) {
// something happens
}
Then you can say
CheckboxItemsList newList = new CheckboxItemsList(
MyCheckboxItemsList.Where(item => item.Changed)
);
Additionally, you could add an extension method like so
static class IEnumerableCheckboxItemsExtensions {
public static ToCheckboxItemsList(
this IEnumerable<CheckboxItems> checkboxItems
) {
return new CheckboxItemsList(checkboxItems);
}
}
and then
CheckboxItemsList newList =
MyCheckboxItemsList.Where(item => item.Changed)
.ToCheckboxItemsList();
LINQ works on IEnumerable<T> and IQueryable<T> and the result types of all LINQ operations (Where, Select) etc, will return one of those. The standard ToList function returns a concrete list of type List<T>, you may need to come up with an extension method, e.g.:
public static CheckboxItemsList ToItemList(this IEnumerable<CheckboxItem> enumerable)
{
return new CheckboxItemsList(enumerable);
}
No, there's no built-in way to do this. You have two main options:
Add a constructor to your CheckboxItemsList class that takes an IEnumerable<CheckboxItems> or similar. Pass that collection on to the base List<T> constructor that takes an IEnumerable<T>. That base constructor should then populate the list for you:
var newList =
new CheckboxItemsList(MyCheckboxItemsList.Where(item=>item.Changed));
// ...
public class CheckboxItemsList : List<CheckboxItems>
{
public CheckboxItemsList(IEnumerable<CheckboxItems> collection)
: base(collection)
{
}
}
Create an extension method that takes an IEnumerable<CheckboxItems> or similar and returns a populated CheckboxItemsList:
var newList = MyCheckboxItemsList.Where(item=>item.Changed)
.ToCheckboxItemsList();
// ...
public static class EnumerableExtensions
{
public static CheckboxItemsList ToCheckboxItemsList(
this IEnumerable<CheckboxItems> source)
{
var list = new CheckboxItemsList();
foreach (T item in source)
{
list.Add(item);
}
return list;
}
}
(Of course, for completeness you could implement both of these options. The extension method would then just pass its IEnumerable<CheckboxItems> argument on to the constructor rather than manually looping and adding each item.)
You can also use "Conversion Operator", as below:
public class CheckboxItemsList: List<CheckboxItems>
{
public static implicit operator CheckboxItems(IEnumerable<CheckboxItems> items)
{
var list = new CheckboxItemsList();
foreach (var item in items)
{
list.Add(item);
}
return list;
}
}
Now, the below code would work.
CheckboxItemsList newList = MyCheckboxItemsList.Where(item=>item.Changed);
From MSDN:
A conversion operator declaration that includes the implicit keyword introduces a user-defined implicit conversion. Implicit conversions can occur in a variety of situations, including function member invocations, cast expressions, and assignments. This is described further in Section 6.1.
A conversion operator declaration that includes the explicit keyword introduces a user-defined explicit conversion. Explicit conversions can occur in cast expressions, and are described further in Section 6.2.
Here is what I came up with, building on the various suggestions of others. A generic extension method:
public static T ToList<T>(this IEnumerable baseList) where T : IList,new()
{
T newList = new T();
foreach (object obj in baseList)
{
newList.Add(obj);
}
return (newList);
}
So now I can do what I want:
CheckboxItemsList newList = MyCheckboxItemsList.Where(item=>item.Changed)
.ToList<CheckboxItemsList>();
Another pretty obvious solution occurred to me, which is also useful for situations where the derived list class has field properties that I need to maintain in the new list.
Just create a new instance of my derived list class, and use AddRange to populate it.
// When created with a CheckboxItemsList parameter, it creates a new empty
// list but copies fields
CheckboxItemsList newList = new CheckboxItemsList(OriginalList);
newList.AddRange(OriginalList.Where(item => item.Changed));
This is my function:
private IEnumerable<string> SeachItem(int[] ItemIds)
{
using (var reader = File.OpenText(Application.StartupPath + #"\temp\A_A.tmp"))
{
var myLine = from line in ReadLines(reader)
where line.Length > 1
let id = int.Parse(line.Split('\t')[1])
where ItemIds.Contains(id)
let m = Regex.Match(line, #"^\d+\t(\d+)\t.+?\t(item\\[^\t]+\.ddj)")
where m.Success == true
select new { Text = line, ItemId = id, Path = m.Groups[2].Value };
return myLine;
}
}
I get a compile error,because "myLine" is not a IEnumerable[string] and I don't know how to write IEnumerable[Anonymous]
"Cannot implicitly convert type 'System.Collections.Generic.IEnumerable[AnonymousType#1]' to 'System.Collections.Generic.IEnumerable[string]'"
You cannot declare IEnumerable<AnonymousType> because the type has no (known) name at build time. So if you want to use this type in a function declaration, make it a normal type. Or just modify your query to return a IENumerable<String> and stick with that type.
Or return IEnumerable<KeyValuePair<Int32, String>> using the following select statement.
select new KeyValuePair<Int32, String>(id, m.Groups[2].Value)
I am not necessarily recommending this...
It is a kind of subversion of the type system but you could do this:
1) change your method signature to return IEnumerable (the non generic one)
2) add a cast by example helper:
public static class Extensions{
public static IEnumerable<T> CastByExample<T>(
this IEnumerable sequence,
T example) where T: class
{
foreach (Object o in sequence)
yield return o as T;
}
}
3) then call the method something like this:
var example = new { Text = "", ItemId = 0, Path = "" };
foreach (var x in SeachItem(ids).CastByExample(example))
{
// now you can access the properties of x
Console.WriteLine("{0},{1},{2}", x.Text, x.ItemId, x.Path);
}
And you are done.
The key to this is the fact that if you create an anonymous type with the same order, types and property names in two places the types will be reused. Knowing this you can use generics to avoid reflection.
Hope this helps
Alex
The method signature on SearchItem indicates that the method returns an IEnumerable<string> but the anonymous type declared in your LINQ query is not of type string. If you want to keep the same method signature, you have to change your query to only select strings. e.g.
return myLine.Select(a => a.Text);
If you insist on returning the data selected by your query, you can return an IEnumerable<object> if you replace your return statement with
return myLine.Cast<object>();
Then you can consume the objects using reflection.
But really, if your going to be consuming an anonymous type outside the method that it is declared in, you should define a class an have the method return an IEnumerable of that class. Anonymous types are convenience but they are subject to abuse.
Your function is trying to return IEnumerable<string>, when the LINQ statement you are executing is actually returning an IEnumerable<T> where T is a compile-time generated type. Anonymous types are not always anonymous, as they take on a specific, concrete type after the code is compiled.
Anonymous types, however, since they are ephemeral until compiled, can only be used within the scope they are created in. To support your needs in the example you provided, I would say the simplest solution is to create a simple entity that stores the results of your query:
public class SearchItemResult
{
public string Text { get; set; }
public int ItemId { get; set; }
public string Path { get; set; }
}
public IEnumerable<SearchItemResult> SearchItem(int[] itemIds)
{
// ...
IEnumerable<SearchItemResult> results = from ... select new SearchItemResult { ... }
}
However, if your ultimate goal is not to retrieve some kind of object, and you are only interested in, say, the Path...then you can still generate an IEnumerable<string>:
IEnumerable<string> lines = from ... select m.Groups[2].Value;
I hope that helps clarify your understanding of LINQ, enumerables, and anonymous types. :)
Return a ValueTuple instead of an anonymous class. Ex (using "named tuples")-
(Text: line, ItemId: id, Path: m.Groups[2].Value)
https://learn.microsoft.com/en-us/dotnet/csharp/tuples
Instead of-
new { Text = line, ItemId = id, Path = m.Groups[2].Value }
The ValueTuple is part of C# version 7 and was originally implemented as a separate NuGet package (System.ValueTuple). Starting with .NET 4.7 it is a built-in type. For .NET Core, versions prior to 2.0 required the NuGet package but it is built-in with version 2.0.
The question was asked a long time ago, I hope it helps someone...
"You cannot declare IEnumerable", instead, must convert it to a "custom" IEnumerable:
public class MyString
{
public string String { get; set; } = string.Empty;
}
public IEnumerable<MyString> GetMyStrings(List<string> Strings)
{
var AnonString = from S in Strings group S by S into Grouped select new { String = Grouped.Key };
IEnumerable<MyString> Result = AnonString.Select(x => new MyString() { String = x.String }).ToArray();
return Result;
}
Regards.
this link could be useful for others who end up here
https://idreesdotnet.blogspot.com/2019/08/c-how-to-create-list-of-anonymous-type.html
the first solution (of 6) is delightlfully simple
1: First create the object(s) of anonymous type and then pass it to an array and call ToList() method.
var o1 = new { Id = 1, Name = "Foo" };
var o2 = new { Id = 2, Name = "Bar" };
var list = new[] { o1, o2 }.ToList();