I have an IEnumerable list of objects in C#. I can use a for each to loop through and examine each object fine, however in this case all I want to do is examine the first object is there a way to do this without using a foreach loop?
I've tried mylist[0] but that didnt work.
Thanks
(For the sake of convenience, this answer assumes myList implements IEnumerable<string>; replace string with the appropriate type where necessary.)
If you're using .NET 3.5, use the First() extension method:
string first = myList.First();
If you're not sure whether there are any values or not, you can use the FirstOrDefault() method which will return null (or more generally, the default value of the element type) for an empty sequence.
You can still do it "the long way" without a foreach loop:
using (IEnumerator<string> iterator = myList.GetEnumerator())
{
if (!iterator.MoveNext())
{
throw new WhateverException("Empty list!");
}
string first = iterator.Current;
}
It's pretty ugly though :)
In answer to your comment, no, the returned iterator is not positioned at the first element initially; it's positioned before the first element. You need to call MoveNext() to move it to the first element, and that's how you can tell the difference between an empty sequence and one with a single element in.
EDIT: Just thinking about it, I wonder whether this is a useful extension method:
public static bool TryFirst(this IEnumerable<T> source, out T value)
{
using (IEnumerator<T> iterator = source.GetEnumerator())
{
if (!iterator.MoveNext())
{
value = default(T);
return false;
}
value = iterator.Current;
return true;
}
}
Remember, there may be no "first element" if the sequence is empty.
IEnumerable<int> z = new List<int>();
int y = z.FirstOrDefault();
If you're not on 3.5:
using (IEnumerator<Type> ie = ((IEnumerable<Type>)myList).GetEnumerator()) {
if (ie.MoveNext())
value = ie.Current;
else
// doesn't exist...
}
or
Type value = null;
foreach(Type t in myList) {
value = t;
break;
}
Related
i am attempting to create an
IEnumrable<PropertyInfo>
iv'e got a method called Disassemble which iterates recursively throw a given object and all it's child objects of it's properties .
please do not concern your self with the Inner wrapper objects of type INameValueWrapper
The problem below is when i encounter a property which is a Class i wan't to call Disassemble on it as well
and add it to the same iteration of the IEnumrable , the Dissasemble does not occur again when it is called
where i put the comment :
// The problem is here .
public static IEnumerable<T> Dissasemble<T>(this object sourceObj) where T : INameValueWrapper
{
var properties = sourceObj.GetType().GetProperties();
foreach (var prop in properties)
{
var wrapper = (T)prop.WrapPropertyInfo(sourceObj);
yield return wrapper;
if (wrapper is CollectionPropertyInfoWrapper)
{
var colWrapper = wrapper as CollectionPropertyInfoWrapper;
var collection = (IList)colWrapper.Value;
int index = 0;
foreach (var item in collection)
{
yield return (T)item.WrapItem(collection, index + 1);
index++;
}
}
else
{
var propWrapper = wrapper as PropertyInfoWrapper;
if (!propWrapper.IsPrimitive)
{
var childObject = prop.GetValue(sourceObj);
childObject.Dissasemble<T>(); // here is the problem
}
}
}
yield break;
}
1) Why does it not get called and added to the iteration ?
2) What is the work around this issue ? ,
i could call childObject.Dissasemble<T>().ToList()
and then iterate that collection calling yield return on it's items
but that seems like re doing something i already did.
thanks in advance.
You're calling the method, but then ignoring the results. You may want something like:
foreach (var item in childObject.Disassemble<T>())
{
yield return item;
}
I think you're a bit confused about what yield return does - it only yields a value in the sequence returned by the currently-executing method. It doesn't add a value to some global sequence. If you ignore the value returned by the recursive Disassemble call, the code won't even execute as iterator blocks are lazy. (They only execute code when they're asked for another value.)
Also, you don't need the yield break; at the end of your method.
Note that this if the recursion goes deep, this use of iterator blocks can be inefficient. That may well not be a problem for you, but it's something to think about. See posts by Wes Dyer and Eric Lippert about this.
Instead of
childObject.Dissasemble<T>(); // here is the problem
try
foreach (var a in childObject.Dissasemble<T>())
{
yield return a;
}
Surely there is an easy way to verify a collection of values has no duplicates [using the default Comparison of the collection's Type] in C#/.NET ? Doesn't have to be directly built in but should be short and efficient.
I've looked a lot but I keep hitting examples of using collection.Count() == collection.Distinct().Count() which for me is inefficient. I'm not interested in the result and want to bail out as soon as I detect a duplicate, should that be the case.
(I'd love to delete this question and/or its answer if someone can point out the duplicates)
Okay, if you just want to get out as soon as the duplicate is found, it's simple:
// TODO: add an overload taking an IEqualityComparer<T>
public bool AllUnique<T>(this IEnumerable<T> source)
{
if (source == null)
{
throw new ArgumentNullException("source");
}
var distinctItems = new HashSet<T>();
foreach (var item in source)
{
if (!distinctItems.Add(item))
{
return false;
}
}
return true;
}
... or use All, as you've already shown. I'd argue that this is slightly simpler to understand in this case... or if you do want to use All, I'd at least separate the creation of the set from the method group conversion, for clarity:
public static bool IsUnique<T>(this IEnumerable<T> source)
{
// TODO: validation
var distinctItems = new HashSet<T>();
// Add will return false if the element already exists. If
// every element is actually added, then they must all be unique.
return source.All(distinctItems.Add);
}
Doing it inline, you can replace:
collection.Count() == collection.Distinct().Count()
with
collection.All( new HashSet<T>().Add );
(where T is the type of your collection's elements)
Or you can extract the above to a helper extension method[1] so you can say:
collection.IsUnique()
[1]
static class EnumerableUniquenessExtensions
{
public static bool IsUnique<T>(this IEnumerable<T> that)
{
return that.All( new HashSet<T>().Add );
}
}
(and as Jon has pointed out in his answer, one really should separate and comment the two lines as such 'cuteness' is generally Not A Good Idea)
I have a nested while loop inside a foreach loop where I would like to advance the enumerator indefinitately while a certain condition is met. To do this I try casting the enumerator to IEnumerator< T > (which it must be if it is in a foreach loop) then calling MoveNext() on the casted object but it gives me an error saying I cannot convert it.
Cannot convert type 'System.DateTime' to System.Collections.Generic.IEnumerator via a reference conversion, boxing conversion, unboxing conversion, wrapping conversion, or null type conversion.
foreach (DateTime time in times)
{
while (condition)
{
// perform action
// move to next item
(time as IEnumerator<DateTime>).MoveNext(); // will not let me do this
}
// code to execute after while condition is met
}
What is the best way to manually increment the IEnumerator inside of the foreach loop?
EDIT:
Edited to show there is code after the while loop that I would like executed once the condition is met which is why I wanted to manually increment inside the while then break out of it as opposed to continue which would put me back at the top. If this isn't possible I believe the best thing is to redesign how I am doing it.
Many of the other answers recommend using continue, which may very well help you do what you need to do. However, in the interests of showing manually moving the enumerator, first you must have the enumerator, and that means writing your loop as a while.
using (var enumerator = times.GetEnumerator())
{
DateTime time;
while (enumerator.MoveNext())
{
time = enumerator.Current;
// pre-condition code
while (condition)
{
if (enumerator.MoveNext())
{
time = enumerator.Current;
// condition code
}
else
{
condition = false;
}
}
// post-condition code
}
}
From your comments:
How can the foreach loop advance it if it doesn't implement the IEnumerator interface?
In your loop, time is a DateTime. It is not the object that needs to implement an interface or pattern to work in the loop. times is a sequence of DateTime values, it is the one that must implement the enumerable pattern. This is generally fulfilled by implementing the IEnumerable<T> and IEnumerable interfaces, which simply require T GetEnumerator() and object GetEnumerator() methods. The methods return an object implementing IEnumerator<T> and IEnumerator, which define a bool MoveNext() method and a T or object Current property. But time cannot be cast to IEnumerator, because it is no such thing, and neither is the times sequence.
You cannot modify the enumerator from inside the for loop. The language does not permit this. You need to use the continue statement in order to advance to the next iteration of a loop.
However, I'm not convinced that your loop even needs a continue. Read on.
In the context of your code you would need to convert the while to an if in order to make the continue refer to the foreach block.
foreach (DateTime time in times)
{
if (condition)
{
// perform action
continue;
}
// code to execute if condition is not met
}
But written like this it is clear that the following equivalent variant is simpler still
foreach (DateTime time in times)
{
if (condition)
{
// perform action
}
else
{
// code to execute if condition is not met
}
}
This is equivalent to your pseudo-code because the part marked code to execute after while condition is met is executed for each item for which condition is false.
My assumption in all of this is that condition is evaluated for each item in the list.
Perhaps you can use continue?
You would use the continue statement:
continue;
This is just a guess, but it sounds like what you're trying to do is take a list of datetimes and move past all of them which meet a certain criteria, then perform an action on the rest of the list. If that's what you're trying to do, you probably want something like SkipWhile() from System.Linq. For example, the following code takes a series of datetimes and skips past all of them which are before the cutoff date; then it prints out the remaining datetimes:
var times = new List<DateTime>()
{
DateTime.Now.AddDays(1), DateTime.Now.AddDays(2), DateTime.Now.AddDays(3), DateTime.Now.AddDays(4)
};
var cutoff = DateTime.Now.AddDays(2);
var timesAfterCutoff = times.SkipWhile(datetime => datetime.CompareTo(cutoff) < 1)
.Select(datetime => datetime);
foreach (var dateTime in timesAfterCutoff)
{
Console.WriteLine(dateTime);
}
Console.ReadLine();
Is that the sort of thing you're trying to do?
I definitely do not condone what I am about to suggest, but you can create a wrapper around the original IEnumerable to transform it into something that returns items which can be used to navigate the underlying the enumerator. The end result might look like the following.
public static void Main(string[] args)
{
IEnumerable<DateTime> times = GetTimes();
foreach (var step in times.StepWise())
{
while (condition)
{
step.MoveNext();
}
Console.WriteLine(step.Current);
}
}
Then we need to create our StepWise extension method.
public static class EnumerableExtension
{
public static IEnumerable<Step<T>> StepWise<T>(this IEnumerable<T> instance)
{
using (IEnumerator<T> enumerator = instance.GetEnumerator())
{
while (enumerator.MoveNext())
{
yield return new Step<T>(enumerator);
}
}
}
public struct Step<T>
{
private IEnumerator<T> enumerator;
public Step(IEnumerator<T> enumerator)
{
this.enumerator = enumerator;
}
public bool MoveNext()
{
return enumerator.MoveNext();
}
public T Current
{
get { return enumerator.Current; }
}
}
}
You could use a func as your iterator and keep the state that you are changing in that delegate to be evaluated each iteration.
public static IEnumerable<T> FunkyIEnumerable<T>(this Func<Tuple<bool, T>> nextOrNot)
{
while(true)
{
var result = nextOrNot();
if(result.Item1)
yield return result.Item2;
else
break;
}
yield break;
}
Func<Tuple<bool, int>> nextNumber = () =>
Tuple.Create(SomeRemoteService.CanIContinueToSendNumbers(), 1);
foreach(var justGonnaBeOne in nextNumber.FunkyIEnumerable())
Console.Writeline(justGonnaBeOne.ToString());
One alternative not yet mentioned is to have an enumerator return a wrapper object which allows access to itself in addition to the data element being enumerated. For sample:
struct ControllableEnumeratorItem<T>
{
private ControllableEnumerator parent;
public T Value {get {return parent.Value;}}
public bool MoveNext() {return parent.MoveNext();}
public ControllableEnumeratorItem(ControllableEnumerator newParent)
{parent = newParent;}
}
This approach could also be used by data structures that want to allow collections to be modified in controlled fashion during enumeration (e.g. by including "DeleteCurrentItem", "AddBeforeCurrentItem", and "AddAfterCurrentItem" methods).
Suppose I have a given object of type IEnumerable<string> which is the return value of method SomeMethod(), and which contains no repeated elements. I would like to be able to "zip" the following lines in a single LINQ query:
IEnumerable<string> someList = SomeMethod();
if (someList.Contains(givenString))
{
return (someList.Where(givenString));
}
else
{
return (someList);
}
Edit: I mistakenly used Single instead of First. Corrected now.
I know I can "zip" this by using the ternary operator, but that's just not the point. I would just list to be able to achieve this with a single line. Is that possible?
This will return items with given string or all items if given is not present in the list:
someList.Where(i => i == givenString || !someList.Contains(givenString))
The nature of your desired output requires that you either make two requests for the data, like you are now, or buffer the non-matches to return if no matches are found. The later would be especially useful in cases where actually getting the data is a relatively expensive call (eg: database query or WCF service). The buffering method would look like this:
static IEnumerable<T> AllIfNone<T>(this IEnumerable<T> source,
Func<T, bool> predicate)
{
//argument checking ignored for sample purposes
var buffer = new List<T>();
bool foundFirst = false;
foreach (var item in source)
{
if (predicate(item))
{
foundFirst = true;
yield return item;
}
else if (!foundFirst)
{
buffer.Add(item);
}
}
if (!foundFirst)
{
foreach (var item in buffer)
{
yield return item;
}
}
}
The laziness of this method is either that of Where or ToList depending on if the collection contains a match or not. If it does, you should get execution similar to Where. If not, you will get roughly the execution of calling ToList (with the overhead of all the failed filter checks) and iterating the result.
What is wrong with the ternary operator?
someList.Any(s => s == givenString) ? someList.Where(s => s == givenString) : someList;
It would be better to do the Where followed by the Any but I can't think of how to one-line that.
var reducedEnumerable = someList.Where(s => s == givenString);
return reducedEnumerable.Any() ? reducedEnumerable : someList;
It is not possible to change the return type on the method, which is what you're asking. The first condition returns a string and the second condition returns a collection of strings.
Just return the IEnumerable<string> collection, and call Single on the return value like this:
string test = ReturnCollectionOfStrings().Single(x => x == "test");
How can I get next element of element to which I have reference in some collection e.g. List<T> ?
I am not talking about any loops, etc. I want get just one next (or previous) element
A collection does not have the a concept of "current element". Iterators/Enumerators do. And they have a MoveNext() method. And then you can access the current element of that enumerator with Current.
You can call GetEnumerator() on almost all collections to get an enumerator.
One problem with enumerators is you usually can't copy them nor move them backwards. (Unlike C++ where most collections have more powerful iterators)
It's not the most performant, but it should work. You can even make it an extension method.
public static T GetNext<T>(IList<T> collection, T value )
{
int nextIndex = collection.IndexOf(value) + 1;
if (nextIndex < collection.Count)
{
return collection[nextIndex];
}
else
{
return value; //Or throw an exception
}
}
And you use it like this:
var list = new List<string> {"A", "B", "C", "D"};
string current = "B";
string next = GetNext(list, current);
Console.WriteLine(next); //Prints C
To get "next" you need a concept of "current". C# doesn't use iterators like C++ does, you can either get an enumerator:
var enumerator = x.GetEnumerator();
while (enumerator.MoveNext())
{
Console.WriteLine(enumerator.Current);
}
But it would be easier to just use an int indexer into the List<T>