how to check if object already exists in a list

Question

I have a list    List lt MyObject gt  myList   and I am adding items to a list and I want to check if that object is already in the list     so before I do this    myList Add nextObject     I want to see if nextObject is already in the list    The object  MyObject  has a number of properties but comparison is based on matching on two properties   What is the best way to do a check before I add a new  MyObject  to this list of  MyObject s   The only solution I thought up was to change from a list to a dictionary and then make the key a concatenated string of the properties  this seems a little unelegant    Any other cleaner solutions using list or LINQ or something else

User · Answer

Another point to mention is that you should ensure that your equality function is as you expect. You should override the equals method to set up what properties of your object have to match for two instances to be considered equal.

Then you can just do mylist.contains(item)

User · Answer

Here is a quick console app to depict the concept of how to solve your issue   using System  using System Collections Generic  using System Linq  using System Text   namespace ConsoleApplication3       public class myobj               private string a   string Empty          private string b   string Empty           public myobj string a  string b                        this a   a              this b   b                     public string A                       get                               return a                                   public string B                       get                               return b                                      class Program               static void Main string   args                        List lt myobj gt  list   new List lt myobj gt                 myobj   objects     new myobj  a    b    new myobj  c    d    new myobj  a    b                    for  int i   0  i  lt  objects Length  i                                  if   list Exists  delegate myobj x    return  string Equals x A  objects i  A   amp  amp  string Equals x B  objects i  B     true   false                                             list Add objects i                                                        Enjoy

User · Answer

If it s maintainable to use those 2 properties  you could   bool alreadyExists   myList Any x  gt  x Foo   ooo   amp  amp  x Bar     bat

User · Answer

Edit  I had first said     What s inelegant about the dictionary solution  It seems perfectly elegant to me  esp since you only need to set the comparator in creation of the dictionary     Of course though  it is inelegant to use something as a key when it s also the value   Therefore I would use a HashSet  If later operations required indexing  I d create a list from it when the Adding was done  otherwise  just use the hashset

User · Answer

It depends on the needs of the specific situation  For example  the dictionary approach would be quite good assuming   The list is relatively stable  not a lot of inserts deletions  which dictionaries are not optimized for  The list is quite large  otherwise the overhead of the dictionary is pointless    If the above are not true for your situation  just use the method Any    Item wonderIfItsPresent       bool containsItem   myList Any item   gt  item UniqueProperty    wonderIfItsPresent UniqueProperty    This will enumerate through the list until it finds a match  or until it reaches the end

User · Answer

If you use EF core add    UseSerialColumn      Example  modelBuilder Entity lt JobItem gt  entity   gt                        entity ToTable  jobs                 entity Property e   gt  e Id                   HasColumnName  id                    UseSerialColumn

User · Answer

Are you sure you need a list in this case  If you are populating the list with many items  performance will suffer with myList Contains or myList Any  the run-time will be quadratic  You might want to consider using a better data structure  For example    public class MyClass               public string Property1   get  set            public string Property2   get  set                public class MyClassComparer   EqualityComparer lt MyClass gt                public override bool Equals MyClass x  MyClass y                        if x    null    y    null                 return x    y               return x Property1    y Property1  amp  amp  x Property2    y Property2                     public override int GetHashCode MyClass obj                        return obj    null   0    obj Property1 GetHashCode     obj Property2 GetHashCode                       You could use a HashSet in the following manner     var set   new HashSet lt MyClass gt  new MyClassComparer       foreach var myClass in           set Add myClass     Of course  if this definition of equality for MyClass is  universal   you needn t write an IEqualityComparer implementation  you could just override GetHashCode and Equals in the class itself

User · Answer

Simple but it works  MyList Remove nextObject  MyList Add nextObject    or   if   MyList Contains nextObject       MyList Add nextObject

User · Answer

Simply use Contains method  Note that it works based on the equality function Equals   bool alreadyExist   list Contains item

[c#] how to check if object already exists in a list

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