A Linq Catalog

I thought I’d review the special methods available in Linq. In the API reference, they can be found under the Enumerable<T> class. Here they are:


static IEnumerable<T> Empty<T>()
static IEnumerable<T> Repeat<T>(T element, int count)



T[] ToArray()
List<T> ToList()
Dictionary<K,T> ToDictionary(Func<T,K> keySelector)    // puts every element in a dictionary
                                                       // using keys generated by keySelector
Dictionary<K,T> ToDictionary(Func<T,K> keySelector, IEqualityComparer<K>)
Dictionary<K,R> ToDictionary(Func<T,K> keySelector, Func<T,R> elemSelector)
                               // transforms each T to an R and puts the Rs in the dictionary
Dictionary<K,R> ToDictionary(Func<T,K> keySelector, Func<T,R> elemSelector, IEqualityComparer<K>)
Lookup<K,T> ToLookup(Func<T,K> keySelector)           // a Lookup is like a Dictionary except
                                                      // each key points to a list of values.
                                                      // In Perl terms it's a hash of arrays.
Lookup<K,T> ToLookup(Func<T,K> keySelector, IEqualityComparer<K>)
Lookup<K,R> ToLookup(Func<T,K> keySelector, Func<T,R> elemSelector)
Lookup<K,R> ToLookup(Func<T,K> keySelector, Func<T,R> elemSelector, IEqualityComparer<K>)

T Aggregate(Func<T,T,T> adder)        // like python's reduce

T ElementAt(int index)
T ElementAtOrDefault(int index)       // returns this[i] or default(T) if i >= this.Count

T First()
T First(Func<T,Boolean>)              // the first element that satisfies the predicate
T FirstOrDefault()
T FirstOrDefault(Func<T,Boolean>)
T Last()
T Last(Func<T,Boolean>)               // the last element that satisfies the predicate
T LastOrDefault()
T LastOrDefault(Func<T,Boolean>)

T Single()                        // returns the only element of the sequence,
                                  // or throws an exception
T Single(Func<T,Boolean> f)       // returns the only element for which f returns true.
                                  // Exception if there is not one and only one.
T SingleOrDefault()               // default(T) if empty. Still exception if more than one T.
T SingleOrDefault(Func<T,Boolean> f)

bool Contains(T)
bool Contains(T, IEqualityComparer<T>)
bool All(Func<T, bool> predicate)       // true if predicate is always true
bool Any(Func<T, bool> predicate)       // true if predicate is ever true
bool Any()                              // true if the sequence contains any elements
bool SequenceEqual(IEnumerable<T>)      // true if all the elements of the two sequences
                                        // are equal, using T's default equality comparer.
bool SequenceEqual(IEnumerable<T>, IEqualityComparer<T>)

int Count()
int Count(Func<T,Boolean>)    // counts how many elements satisfy the predicate.
long LongCount()
long LongCount(Func<T,Boolean>)

int Average()    
int Average(Func<T,int> transform)    // calls transform to alter each element before
                                      // including it in the average.
                                      // You could use this to get a weighted average, e.g.
int Max()
int Max(Func<T,int> transform)        // uses transform to turn each T into an int
int Min()
int Min(Func<T,int> transform)
int Sum()
int Sum(Func<T,int> transform)
// ... lots of overloads ...

IEnumerable<T> DefaultIfEmpty()          // returns [default(T)] if empty
IEnumerable<T> DefaultIfEmpty(T elem)    // returns [elem] if empty.

IEnumerable<T> AsEnumerable()    // forces the compiler to call IEnumerable<T> methods
IEnumerable<R> Cast<R>()         // casts each element to an R
IEnumerable<R> OfType<R>()       // filters out any elements not of type R

IEnumerable<T> Intersect(IEnumerable<T> compare)    // the standard set operation
IEnumerable<T> Intersect(IEnumerable<T> compare, IEqualityComparer<T>)
IEnumerable<T> Union(IEnumerable<T> compare)        // the standard set operation
IEnumerable<T> Union(IEnumerable<T> compare, IEqualityComparer<T>)
IEnumerable<T> Except(IEnumerable<T> compare)       // returns this - compare
IEnumerable<T> Except(IEnumerable<T> compare, IEqualiterComperer<T>)

IEnumerable<T> Distinct()                      // like SQL distinct keyword
IEnumerable<T> Distinct(IEqualityComperer<T>)
IEnumerable<T> Concat(IEnumerable<T>)          // like Union, but with list semantics
                                               // instead of set
IEnumerable<T> Range(int start, int count)

IEnumerable<T> Skip(int n)                       // skips n elements; returns the rest
IEnumerable<T> SkipWhile(Func<T,Boolean> f)      // skips while f is true; returns the rest
IEnumerable<T> SkipWhile(Func<T,int,Boolean> f)  // f also gets the 0-based index
IEnumerable<T> Take(int n)                       // returns Range(0,n)
IEnumerable<T> TakeWhile(Func<T,Boolean> f)      // takes while f is true; skips the rest
IEnumerable<T> TakeWhile(Func<T,int,Boolean> f)

IEnumerable<T> While(Func<T,Boolean> f)     // gets the elements where f is true.
IEnumerable<T> While(Func<T,int,Boolean> f) // f also knows the 0-based index.

IEnumerable<IGrouping<K,T>> GroupBy(Func<T,K> f)    // f generates a key for each element
IEnumerable<IGrouping<K,R>> GroupBy(Func<T,K> f, Func<K, IEnumerable<T>, R> resultSelector)
                       // resultSelector gets an enumeration of values with a given key,
                       // and it returns a single value of type R, e.g. representing
                       // their min value. Note that you could use anonymous classes
                       // to stick several values into R, e.g. a min, max, and average.
                       // There is a nice example here:
                       // http://msdn.microsoft.com/en-us/library/bb549393.aspx.
... lots more overloads ...

IOrderedEnumerable<T> OrderBy(Func<T,K> keySelector)    // orders the elements based on keys
                                                        // generated by keySelector
IOrderedEnumerable<T> OrderBy(Func<T,K> keySelector, IComparer<K> comp)
                                // orders the elements based on keys generated by keySelector
                                // and compared by comp
IOrderedEnumerable<T> OrderByDescending(Func<T,K> keySelector)
IOrderedEnumerable<T> OrderByDescending(Func<T,K> keySelector, IComparer<K> comp)
IEnumerable<T> Reverse()
IOrderedEnumerable<T> ThenBy(Func<T,K> keySelector)     // sub-sorts elements deemed equal
                                                        // by a previous sort
IOrderedEnumerable<T> ThenBy(Func<T,K> keySelector, IComparer<K> comp)
IOrderedEnumerable<T> ThenByDescending(Func<T,K> keySelector)
IOrderedEnumerable<T> ThenByDescending(Func<T,K> keySelector, IComparer<K> comp)

IEnumerable<T> Join(IEnumerable<I> inner, Func<O,K> outerKeySelector,
        Func<I,K> innerKeySelector, Func<O,I,R> resultSelector)
                // joins two sequences as an inner equijoin.
                // There is no method for outer joins,
                // but GroupJoin can get used to get that effect.
IEnumerable<O,I,K,R> GroupJoin(IEnumerable<I> inner, Func<O,K> outerKeySelector,
        Func<I,K> innerKeySelector, Func<O, IEnumerable<I>, R> resultSelector)
                // joins the two sequences (see Join) and groups the results (see GroupBy).
                // Unlike with Join, this function can be used to do a left outer join,
                // because even when there is no value in inner matching a value in outer,
                // the resultSelector gets called for each outer value,
                // just with an empty IEnuemrable<I>.

IEnumerable<R> Select(Func<T,R> f)                  // uses f to transform each T to an R
IEnumerable<R> Select(Func<T,int,R> f)              // f also knows the 0-based index of each T
IEnumerable<R> SelectMany(Func<T,IEnumerable<R> f)  // f can return many Rs. All the Rs
                                                    // are flattened into one sequence.
IEnumerable<R> SelectMany(Func<T,int,IEnumerable<R> f)
IEnumerable<R> SelectMany(Func<T,IEnumerable<U> f, Func<T,U,R> g)
                // f generates an intermediate value of type U; g uses both a T and U
                // to get an R. This is really only useful if U doesn't have access to T.
                // See the example code here:
                // http://msdn.microsoft.com/en-us/library/bb534631.aspx.
IEnumerable<R> SelectMany(Func<T,int,IEnumerable<U> f, Func<T,U,R> g)

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