I have a small utility extension method that performs some null checks on some LINQ extension methods in IEnumerable<T>. The code looks like this
public static class MyIEnumerableExtensions
{
// Generic wrapper to allow graceful handling of null values
public static IEnumerable<T> NullableExtension<T>(this IEnumerable<T> first, IEnumerable<T> second, Func<IEnumerable<T>, IEnumerable<T>, IEnumerable<T>> f)
{
if (first == null && second == null) return Enumerable.Empty<T>();
if (first == null) return second;
if (second == null) return first;
return f(first, second);
}
// Wrap the Intersect extension method in our Nullable wrapper
public static IEnumerable<T> NullableIntersect<T>(this IEnumerable<T> first, IEnumerable<T> second)
{
// It'd be nice to write this as
//
// return first.NullableExtension<T>(second, IEnumerable<T>.Intersect );
//
return first.NullableExtension<T>(second, (a,b) => a.Intersect(b));
}
}
So, is there a way to pass in the IEnumerable<T>.Intersect extension method to NullableExtension directly rather than wrapping it in a lambda?
Edit
Because it is actually concise to pass in the Enumerable extension method, I removed the NullableIntersect (and other) methods and just call the nullable wrapper directly.
Also, as Anthony points out, the semantics of what an Empty enumerable should do is different depending on the extension method, i.e. Union versus Intersect. As such, I rename the NullableExtension method to IgnoreIfNull which better reflects the generic behavior.
public static class MyIEnumerableExtensions
{
// Generic wrappers to allow graceful handling of null values
public static IEnumerable<T> IgnoreIfNull<T>(this IEnumerable<T> first, IEnumerable<T> second, Func<IEnumerable<T>, IEnumerable<T>, IEnumerable<T>> f)
{
if (first == null && second == null) return Enumerable.Empty<T>();
if (first == null) return second;
if (second == null) return first;
return f(first, second);
}
public static IEnumerable<T> EmptyIfNull<T>(this IEnumerable<T> first, IEnumerable<T> second, Func<IEnumerable<T>, IEnumerable<T>, IEnumerable<T>> f)
{
return f(first ?? Enumerable.Empty<T>(), second ?? Enumerable.Empty<T>());
}
}
// Usage example. Returns { 1, 4 } because arguments to Union and Intersect are ignored
var items = new List<int> { 1, 4 };
var output1 = items.IgnoreIfNull(null, Enumerable.Union).IgnoreIfNull(null, Enumerable.Intersect);
// Usage example. Returns { } because arguments to Union and Intersect are set to empty
var output2 = items.EmptyIfNull(null, Enumerable.Union).EmptyIfNull(null, Enumerable.Intersect);
Intersect is defined within static class Enumerable. You can pass it into your method like below
return first.NullableExtension<T>(second, Enumerable.Intersect);
Note: You might be concerned about the behavior of your logic in the case of a null sequence. For example, in the case of
List<int> first = null;
var second = new List<int> { 1, 4 };
var output = first.NullableIntersect(second).ToList();
You have defined it such that output contains {1, 4} (the elements of second). I might expect first to instead be treated as an empty sequence, and an intersection with second would result in an empty sequence. Ultimately, that's for you to decide the behavior you desire.
Related
I found following extension method to filter list. I am very new to this hence I wanted to check if someone can help me. This method compare exact values but I want to use contains instead of exact compare. Any thoughts
public static IEnumerable<T> FilterByProperty<T>(this IEnumerable<T> source,string property,object value)
{
var propertyInfo = typeof(T).GetProperty(property);
return source.Where(p => propertyInfo.GetValue(p, null) == value);
}
If you want to change the equals == into Contains. try this:
public static IEnumerable<T> FilterByProperty<T>(this IEnumerable<T> source,string property,object value)
{
var propertyInfo = typeof(T).GetProperty(property);
return source.Where(p => propertyInfo
.GetValue(p, null)
.ToString() //be aware that this might be null
.Contains(value));
}
I need to pass in a "where" lambda expression that'll be used in a LINQ query inside a method. The problem is, I don't know what the where value will be compared against until I get into the method.
Now to explain further and clarify some of what I said above I'll come up with a bit of a contrived example.
Imagine I have a List<Products> and I need to narrow that list down to a single record using a productId property of the Products object. Normally I would do this:
var product = productList.Where(p=>p.productId == 123).FirstOrDefault();
Now take it a step further - I need to put the above logic into a method that isn't limited to a List<Products> but is instead a List<T> so ideally, I'd be calling it like this (and I know the below won't work, it's simply here to show what I am trying to achieve):
myMethod(productList, p => p.productId == X)
With the caveat being that X isn't known until I'm inside the method.
Finally, for what it's worth, I need to point out that my collection of data is an OData DataServiceQuery.
So, to re-summarize my question: I need to know how to construct a lambda "where" expression that I can pass into a method and how to use it against a collection of objects in a LINQ query.
myMethod(productList, p => p.productId == X) - you can emulate with this trick
static void myMethod<T>(List<T> list, Func<T,bool> predicate, ref int x)
{
x = 5;
var v = list.Where(predicate);
foreach (var i in v)
Console.Write(i);
Console.ReadLine();
}
static void Main(string[] args)
{
List<int> x = new List<int> { 1, 2, 3, 4, 5 };
int z = 0;
myMethod(x, p => p == z, ref z);
}
but not sure if it solves your problem in whole
For one, if you are going to query an IEnumerable<T>, you will need to ensure that your comparison will work in the first place. In that case you can make your objects implement an interface that guarantees that they will support the comparison.
Once you do that, your method can have a generic constraint that limits the input to those interfaces. At that point, your method can take a Func, which can be passed to the LINQ Where clause:
public interface Identifier
{
int Id { get; set; }
}
public class Product : Identifier
{
public int Id { get; set; }
//Other stuff
}
public T GetMatch<T>(IEnumerable<T> collection, Func<T, int, bool> predicate) where T : Identifier
{
int comparison = 5;
return collection.Where(item => predicate(item, comparison)).FirstOrDefault();
}
Which can be invoked like:
var match = GetMatch<Identifier>(collection, (x, y) => x.Id == y);
UPDATE:
I modified the above code to take in a comparison parameter
You could try to use the PredicateBuilder class from the free LinqKit library(tutorial).
You can then construct a predicate using
PredicateBuilder predicate = PredicateBuilder.True<T>();
predicate = PredicateBuilder.And(predicate, p=> p.product_id == X);
where X is of type T.
You can use this predicate in a where clause such as .Where(predicate) and return an IQueryable or return the predicate itself which would be of type Expression<Func<T, bool>>
I'd like to split a sequence in C# to a sequence of sequences using LINQ. I've done some investigation, and the closest SO article I've found that is slightly related is this.
However, this question only asks how to partition the original sequence based upon a constant value. I would like to partition my sequence based on an operation.
Specifically, I have a list of objects which contain a decimal property.
public class ExampleClass
{
public decimal TheValue { get; set; }
}
Let's say I have a sequence of ExampleClass, and the corresponding sequence of values of TheValue is:
{0,1,2,3,1,1,4,6,7,0,1,0,2,3,5,7,6,5,4,3,2,1}
I'd like to partition the original sequence into an IEnumerable<IEnumerable<ExampleClass>> with values of TheValue resembling:
{{0,1,2,3}, {1,1,4,6,7}, {0,1}, {0,2,3,5,7}, {6,5,4,3,2,1}}
I'm just lost on how this would be implemented. SO, can you help?
I have a seriously ugly solution right now, but have a "feeling" that LINQ will increase the elegance of my code.
Okay, I think we can do this...
public static IEnumerable<IEnumerable<TElement>>
PartitionMontonically<TElement, TKey>
(this IEnumerable<TElement> source,
Func<TElement, TKey> selector)
{
// TODO: Argument validation and custom comparisons
Comparer<TKey> keyComparer = Comparer<TKey>.Default;
using (var iterator = source.GetEnumerator())
{
if (!iterator.MoveNext())
{
yield break;
}
TKey currentKey = selector(iterator.Current);
List<TElement> currentList = new List<TElement> { iterator.Current };
int sign = 0;
while (iterator.MoveNext())
{
TElement element = iterator.Current;
TKey key = selector(element);
int nextSign = Math.Sign(keyComparer.Compare(currentKey, key));
// Haven't decided a direction yet
if (sign == 0)
{
sign = nextSign;
currentList.Add(element);
}
// Same direction or no change
else if (sign == nextSign || nextSign == 0)
{
currentList.Add(element);
}
else // Change in direction: yield current list and start a new one
{
yield return currentList;
currentList = new List<TElement> { element };
sign = 0;
}
currentKey = key;
}
yield return currentList;
}
}
Completely untested, but I think it might work...
alternatively with linq operators and some abuse of .net closures by reference.
public static IEnumerable<IEnumerable<T>> Monotonic<T>(this IEnumerable<T> enumerable)
{
var comparator = Comparer<T>.Default;
int i = 0;
T last = default(T);
return enumerable.GroupBy((value) => { i = comparator.Compare(value, last) > 0 ? i : i+1; last = value; return i; }).Select((group) => group.Select((_) => _));
}
Taken from some random utility code for partitioning IEnumerable's into a makeshift table for logging. If I recall properly, the odd ending Select is to prevent ambiguity when the input is an enumeration of strings.
Here's a custom LINQ operator which splits a sequence according to just about any criteria. Its parameters are:
xs: the input element sequence.
func: a function which accepts the "current" input element and a state object, and returns as a tuple:
a bool stating whether the input sequence should be split before the "current" element; and
a state object which will be passed to the next invocation of func.
initialState: the state object that gets passed to func on its first invocation.
Here it is, along with a helper class (required because yield return apparently cannot be nested):
public static IEnumerable<IEnumerable<T>> Split<T, TState>(
this IEnumerable<T> xs,
Func<T, TState, Tuple<bool, TState>> func,
TState initialState)
{
using (var splitter = new Splitter<T, TState>(xs, func, initialState))
{
while (splitter.HasNext)
{
yield return splitter.GetNext();
}
}
}
internal sealed class Splitter<T, TState> : IDisposable
{
public Splitter(IEnumerable<T> xs,
Func<T, TState, Tuple<bool, TState>> func,
TState initialState)
{
this.xs = xs.GetEnumerator();
this.func = func;
this.state = initialState;
this.hasNext = this.xs.MoveNext();
}
private readonly IEnumerator<T> xs;
private readonly Func<T, TState, Tuple<bool, TState>> func;
private bool hasNext;
private TState state;
public bool HasNext { get { return hasNext; } }
public IEnumerable<T> GetNext()
{
while (hasNext)
{
Tuple<bool, TState> decision = func(xs.Current, state);
state = decision.Item2;
if (decision.Item1) yield break;
yield return xs.Current;
hasNext = xs.MoveNext();
}
}
public void Dispose() { xs.Dispose(); }
}
Note: Here are some of the design decisions that went into the Split method:
It should make only a single pass over the sequence.
State is made explicit so that it's possible to keep side effects out of func.
Out of interest how does the GetHashCode of a concrete implementation of IEqualityComparer work?
The reason that I ask is that I'm using linq to union two collections, and when only the left collection has an item GetHashCode is called twice. Further to that, it's called four times if both collections have one row.
This is rough typing but you'll get the point. GetHashCode is called twice, which I'm guessing is twice for the one item in listOne?
e.g.
var listOne = new List<SearchResult>{new SearchResult{Name="Blah"}};
var listTwo = new List<SearchResult>();
listOne.Union(listTwo, SearchResultComparer);
public class SearchResultComparer : IEqualityComparer<SearchResult>
{
public bool Equals(SearchResult x, SearchResult y){....}
public int GetHashCode(SearchResult obj)
{
unchecked
{
int result = 0;
result = (result * 397) ^ (obj.Name != null ?
return result;
}
}
}
Thanks
I'm curious about your observation, I can only observe a single check of GetHashCode for each of the items in each list. But as far as an implementation of Union using your comparer, think of it like this
static IEnumerable<T> Union<T>(this IEnumerable<T> first, IEnumerable<T> second, IEqualityComparer<T> comparer)
{
// there's undoubtedly validation against null sequences
var unionSet = new HashSet<T>(comparer);
foreach (T item in first)
{
if (unionSet.Add(item))
yield return item;
}
foreach (T item in second)
{
if (unionSet.Add(item))
yield return item;
}
}
The Add method of the HashSet will return true or false if the item can be added. In the inner implementation, it will make a call to the item's GetHashCode and get the value, and then see if this value already exists inside the collection. If it does, it compares each with the matching hash code for equality. If there is no equality match (or if the hash code did not already exist), the item is successfully added and the method returns true. Otherwise, the item is not added and the method returns false.
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Until recently, I was using a Distinct in LINQ to select a distinct category (an enum) from a table. This was working fine.
I now need to have it distinct on a class containing a category and country (both enums). The Distinct isn't working now.
What am I doing wrong?
I believe this post explains your problem:
http://blog.jordanterrell.com/post/LINQ-Distinct()-does-not-work-as-expected.aspx
The content of the above link can be summed up by saying that the Distinct() method can be replaced by doing the following.
var distinctItems = items
.GroupBy(x => x.PropertyToCompare)
.Select(x => x.First());
try an IQualityComparer
public class MyObjEqualityComparer : IEqualityComparer<MyObj>
{
public bool Equals(MyObj x, MyObj y)
{
return x.Category.Equals(y.Category) &&
x.Country.Equals(y.Country);
}
public int GetHashCode(MyObj obj)
{
return obj.GetHashCode();
}
}
then use here
var comparer = new MyObjEqualityComparer();
myObjs.Where(m => m.SomeProperty == "whatever").Distinct(comparer);
You're not doing it wrong, it is just the bad implementation of .Distinct() in the .NET Framework.
One way to fix it is already shown in the other answers, but there is also a shorter solution available, which has the advantage that you can use it as an extension method easily everywhere without having to tweak the object's hash values.
Take a look at this:
**Usage:**
var myQuery=(from x in Customers select x).MyDistinct(d => d.CustomerID);
Note: This example uses a database query, but it does also work with an enumerable object list.
Declaration of MyDistinct:
public static class Extensions
{
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query,
Func<T, V> f)
{
return query.GroupBy(f).Select(x=>x.First());
}
}
Or if you want it shorter, this is the same as above, but as "one-liner":
public static IEnumerable<T> MyDistinct<T, V>(this IEnumerable<T> query, Func<T, V> f)
=> query.GroupBy(f).Select(x => x.First());
And it works for everything, objects as well as entities. If required, you can create a second overloaded extension method for IQueryable<T> by just replacing the return type and first parameter type in the example I've given above.
Test data:
You can try it out with this test data:
List<A> GetData()
=> new List<A>()
{
new A() { X="1", Y="2" }, new A() { X="1", Y="2" },
new A() { X="2", Y="3" }, new A() { X="2", Y="3" },
new A() { X="1", Y="3" }, new A() { X="1", Y="3" },
};
class A
{
public string X;
public string Y;
}
Example:
void Main()
{
// returns duplicate rows:
GetData().Distinct().Dump();
// Gets distinct rows by i.X
GetData().MyDistinct(i => i.X).Dump();
}
For explanation, take a look at other answers. I'm just providing one way to handle this issue.
You might like this:
public class LambdaComparer<T>:IEqualityComparer<T>{
private readonly Func<T,T,bool> _comparer;
private readonly Func<T,int> _hash;
public LambdaComparer(Func<T,T,bool> comparer):
this(comparer,o=>0) {}
public LambdaComparer(Func<T,T,bool> comparer,Func<T,int> hash){
if(comparer==null) throw new ArgumentNullException("comparer");
if(hash==null) throw new ArgumentNullException("hash");
_comparer=comparer;
_hash=hash;
}
public bool Equals(T x,T y){
return _comparer(x,y);
}
public int GetHashCode(T obj){
return _hash(obj);
}
}
Usage:
public void Foo{
public string Fizz{get;set;}
public BarEnum Bar{get;set;}
}
public enum BarEnum {One,Two,Three}
var lst=new List<Foo>();
lst.Distinct(new LambdaComparer<Foo>(
(x1,x2)=>x1.Fizz==x2.Fizz&&
x1.Bar==x2.Bar));
You can even wrap it around to avoid writing noisy new LambdaComparer<T>(...) thing:
public static class EnumerableExtensions{
public static IEnumerable<T> SmartDistinct<T>
(this IEnumerable<T> lst, Func<T, T, bool> pred){
return lst.Distinct(new LambdaComparer<T>(pred));
}
}
Usage:
lst.SmartDistinct((x1,x2)=>x1.Fizz==x2.Fizz&&x1.Bar==x2.Bar);
NB: works reliably only for Linq2Objects
I know this is an old question, but I am not satisfied with any of the answers. I took time to figure this out for myself and I wanted to share my findings.
First it is important to read and understand these two things:
IEqualityComparer
EqualityComparer
Long story short in order to make the .Distinct() extension understand how to determine equality of your object - you must define a "EqualityComparer" for your object T. When you read the Microsoft docs it literally states:
We recommend that you derive from the EqualityComparer class
instead of implementing the IEqualityComparer interface...
That is how you determine what to use, because it had been decided for you already.
For the .Distinct() extension to work successfully you must ensure that your objects can be compared accurately. In the case of .Distinct() the GetHashCode() method is what really matters.
You can test this out for yourself by writing a GetHashCode() implementation that just returns the current Hash Code of the object being passed in and you will see the results are bad because this value changes on each run. That makes your objects too unique which is why it is important to actually write a proper implementation of this method.
Below is an exact copy of the code sample from IEqualityComparer<T>'s page with test data, small modification to the GetHashCode() method and comments to demonstrate the point.
//Did this in LinqPad
void Main()
{
var lst = new List<Box>
{
new Box(1, 1, 1),
new Box(1, 1, 1),
new Box(1, 1, 1),
new Box(1, 1, 1),
new Box(1, 1, 1)
};
//Demonstration that the hash code for each object is fairly
//random and won't help you for getting a distinct list
lst.ForEach(x => Console.WriteLine(x.GetHashCode()));
//Demonstration that if your EqualityComparer is setup correctly
//then you will get a distinct list
lst = lst
.Distinct(new BoxEqualityComparer())
.ToList();
lst.Dump();
}
public class Box
{
public Box(int h, int l, int w)
{
this.Height = h;
this.Length = l;
this.Width = w;
}
public int Height { get; set; }
public int Length { get; set; }
public int Width { get; set; }
public override String ToString()
{
return String.Format("({0}, {1}, {2})", Height, Length, Width);
}
}
public class BoxEqualityComparer
: EqualityComparer<Box>
{
public override bool Equals(Box b1, Box b2)
{
if (b2 == null && b1 == null)
return true;
else if (b1 == null || b2 == null)
return false;
else if (b1.Height == b2.Height && b1.Length == b2.Length
&& b1.Width == b2.Width)
return true;
else
return false;
}
public override int GetHashCode(Box bx)
{
#region This works
//In this example each component of the box object are being XOR'd together
int hCode = bx.Height ^ bx.Length ^ bx.Width;
//The hashcode of an integer, is that same integer
return hCode.GetHashCode();
#endregion
#region This won't work
//Comment the above lines and uncomment this line below if you want to see Distinct() not work
//return bx.GetHashCode();
#endregion
}
}