How do I add a key to an existing dictionary? It doesn&#39;t have an `.add()` method.

How can I add new keys to a dictionary?

How can I find the index of an item in a list without looping through it?

Currently this doesn&#39;t look very nice - searching through the list for the same item twice, just to get the index:

    var oProp = something;

    int theThingIActuallyAmInterestedIn = myList.IndexOf(myList.Single(i =&gt; i.Prop == oProp));





How can I get the index of an item in a list in a single step?



Recently began branching out from my safe place (R) into Python and and am a bit confused by the cell localization/selection in `Pandas`. I&#39;ve read the documentation but I&#39;m struggling to understand the practical implications of the various localization/selection options. 

 Is there a reason why I should ever use `.loc` or `.iloc` over `at`, and `iat` or vice versa?  **In what situations should I use which method?**


-----

&gt; &lt;sub&gt;Note: future readers be aware that this question is old and was written before pandas v0.20 when there used to exist a function called `.ix`. This method was later split into two - `loc` and `iloc` - to make the explicit distinction between positional and label based indexing. Please beware that `ix` was discontinued due to inconsistent behavior and being hard to grok, and no longer exists in current versions of pandas (&gt;= 1.0).&lt;/sub&gt;

pandas loc vs. iloc vs. at vs. iat?

I would like to lookup an enum from its string value (or possibly any other value). I&#39;ve tried the following code but it doesn&#39;t allow static in initialisers. Is there a simple way?

    public enum Verbosity {
        
        BRIEF, NORMAL, FULL;
        
        private static Map&lt;String, Verbosity&gt; stringMap = new HashMap&lt;String, Verbosity&gt;();
        
        private Verbosity() {
            stringMap.put(this.toString(), this);
        }
    
        public static Verbosity getVerbosity(String key) {
            return stringMap.get(key);
        }
    };

How can I lookup a Java enum from its String value?

The MSDN explains Lookup like this:

&gt; A [`Lookup&lt;TKey, TElement&gt;`][1]
&gt; resembles a [`Dictionary&lt;TKey,
&gt; TValue&gt;`][2]. The difference is that a
&gt; **Dictionary&amp;lt;TKey, TValue&amp;gt;** maps keys to single values, whereas a
&gt; **Lookup&amp;lt;TKey, TElement&amp;gt;** maps keys to collections of values.

I don&#39;t find that explanation particularly helpful. What is Lookup used for?


  [1]: http://msdn.microsoft.com/en-us/library/bb460184%28v=vs.90%29.aspx
  [2]: http://msdn.microsoft.com/en-us/library/xfhwa508.aspx

What is the point of Lookup&lt;TKey, TElement&gt;?

I&#39;d like to do the following:

 * Lookup a `Vec` for a certain key, and store it for later use.
 * If it doesn&#39;t exist, create an empty `Vec` for the key, but still keep it in the variable.

How to do this efficiently? Naturally I thought I could use `match`:

    use std::collections::HashMap;

    // This code doesn&#39;t compile.
    let mut map = HashMap::new();
    let key = &quot;foo&quot;;
    let values: &amp;Vec&lt;isize&gt; = match map.get(key) {
        Some(v) =&gt; v,
        None =&gt; {
            let default: Vec&lt;isize&gt; = Vec::new();
            map.insert(key, default);
            &amp;default
        }
    };

When I tried it, it gave me errors like:

&lt;!-- language: none --&gt;

    error[E0502]: cannot borrow `map` as mutable because it is also borrowed as immutable
      --&gt; src/main.rs:11:13
       |
    7  |     let values: &amp;Vec&lt;isize&gt; = match map.get(key) {
       |                                     --- immutable borrow occurs here
    ...
    11 |             map.insert(key, default);
       |             ^^^ mutable borrow occurs here
    ...
    15 | }
       | - immutable borrow ends here

 
I ended up with doing something like this, but I don&#39;t like the fact that it performs the lookup twice (`map.contains_key` and `map.get`):

    // This code does compile.
    let mut map = HashMap::new();
    let key = &quot;foo&quot;;
    if !map.contains_key(key) {
        let default: Vec&lt;isize&gt; = Vec::new();
        map.insert(key, default);
    }
    let values: &amp;Vec&lt;isize&gt; = match map.get(key) {
        Some(v) =&gt; v,
        None =&gt; {
            panic!(&quot;impossiburu!&quot;);
        }
    };

Is there a safe way to do this with just one `match`?

How to lookup from and insert into a HashMap efficiently?

I have trivial question: I couldn&#39;t find a dictionary data structure in R, so I used list instead (like `&quot;word&quot;-&gt;number`).
So, how do I get the list of keys.

Working with dictionaries/lists to get list of keys

I have a function with the same name, but with different signature in a base and derived classes. When I am trying to use the base class&#39;s function in another class that inherits from the derived, I receive an error. See the following code:

    class A
    {
        public:
        void foo(string s){};
    };
    
    class B : public A
    {
        public:
        int foo(int i){};
    };
    
    class C : public B
    {
        public:
        void bar()
        {
            string s;
            foo(s);
        }
    };

I receive the following error from the gcc compiler:

    In member function `void C::bar()&#39;: no matching function for call to `C::foo(std::string&amp;)&#39; candidates are: int B::foo(int)

If I remove `int foo(int i){};` from class `B`, or if I rename it from `foo1`, everything works fine. 

What&#39;s the problem with this?

Function with same name but different signature in derived class

I have a dataset about 105000 rows and 30 columns. I have a categorical variable that I would like to assign it to a number. In Excel, I would probably do something with `VLOOKUP` and fill.

How would I go about doing the same thing in `R`?

Essentially, what I have is a `HouseType` variable, and I need to calculate the `HouseTypeNo`. Here are some sample data:

    HouseType HouseTypeNo
    Semi            1
    Single          2
    Row             3
    Single          2
    Apartment       4
    Apartment       4
    Row             3



How to do vlookup and fill down (like in Excel) in R?

When given a static set of objects (static in the sense that once loaded it seldom if ever changes) into which repeated concurrent lookups are needed with optimal performance, which is better, a `HashMap` or an array with a binary search using some custom comparator?

Is the answer a function of object or struct type?  Hash and/or Equal function performance?  Hash uniqueness?  List size?  `Hashset` size/set size?

The size of the set that I&#39;m looking at can be anywhere from 500k to 10m - incase that information is useful.

While I&#39;m looking for a C# answer, I think the true mathematical answer lies not in the language, so I&#39;m not including that tag.  However, if there are C# specific things to be aware of, that information is desired.

Which is faster, Hash lookup or Binary search?

This is not a terribly uncommon question, but I still couldn&#39;t seem to find an answer that really explained the choice.

I have a very large list of strings (ASCII representations of [SHA-256][1] hashes, to be exact), and I need to query for the presence of a string within that list.

There will be what is likely in excess of 100 million entries in this list, and I will need to repeatably query for the presence of an entry many times.

Given the size, I doubt I can stuff it all into a `HashSet&lt;string&gt;`. What would be an appropriate retrieval system to maximize performance?

I CAN pre-sort the list, I CAN put it into a SQL table, I CAN put it into a text file, but I&#39;m not sure what really makes the most sense given my application.

Is there a clear winner in terms of performance among these, or other methods of retrieval?

  [1]: http://en.wikipedia.org/wiki/SHA-2


What&#39;s an appropriate search/retrieval method for a VERY long list of strings?

I need to retrieve the entire single object hierarchy from the database as a JSON. Actually, the proposal about any other solution to achieve this result would be highly appreciated. I decided to use MongoDB with its `$lookup` support.

So I have three collections:

**party**

    { &quot;_id&quot; : &quot;2&quot;, &quot;name&quot; : &quot;party2&quot; }
    { &quot;_id&quot; : &quot;5&quot;, &quot;name&quot; : &quot;party5&quot; }
    { &quot;_id&quot; : &quot;4&quot;, &quot;name&quot; : &quot;party4&quot; }
    { &quot;_id&quot; : &quot;1&quot;, &quot;name&quot; : &quot;party1&quot; }
    { &quot;_id&quot; : &quot;3&quot;, &quot;name&quot; : &quot;party3&quot; }    
**address**

    { &quot;_id&quot; : &quot;a3&quot;, &quot;street&quot; : &quot;Address3&quot;, &quot;party_id&quot; : &quot;2&quot; }
    { &quot;_id&quot; : &quot;a6&quot;, &quot;street&quot; : &quot;Address6&quot;, &quot;party_id&quot; : &quot;5&quot; }
    { &quot;_id&quot; : &quot;a1&quot;, &quot;street&quot; : &quot;Address1&quot;, &quot;party_id&quot; : &quot;1&quot; }
    { &quot;_id&quot; : &quot;a5&quot;, &quot;street&quot; : &quot;Address5&quot;, &quot;party_id&quot; : &quot;5&quot; }
    { &quot;_id&quot; : &quot;a2&quot;, &quot;street&quot; : &quot;Address2&quot;, &quot;party_id&quot; : &quot;1&quot; }
    { &quot;_id&quot; : &quot;a4&quot;, &quot;street&quot; : &quot;Address4&quot;, &quot;party_id&quot; : &quot;3&quot; }
**addressComment**

    { &quot;_id&quot; : &quot;ac2&quot;, &quot;address_id&quot; : &quot;a1&quot;, &quot;comment&quot; : &quot;Comment2&quot; }
    { &quot;_id&quot; : &quot;ac1&quot;, &quot;address_id&quot; : &quot;a1&quot;, &quot;comment&quot; : &quot;Comment1&quot; }
    { &quot;_id&quot; : &quot;ac5&quot;, &quot;address_id&quot; : &quot;a5&quot;, &quot;comment&quot; : &quot;Comment6&quot; }
    { &quot;_id&quot; : &quot;ac4&quot;, &quot;address_id&quot; : &quot;a3&quot;, &quot;comment&quot; : &quot;Comment4&quot; }
    { &quot;_id&quot; : &quot;ac3&quot;, &quot;address_id&quot; : &quot;a2&quot;, &quot;comment&quot; : &quot;Comment3&quot; }

I need to retrieve all parties with all corresponding addresses and address comments as part of the record. My aggregation:

    db.party.aggregate([{
    	$lookup: {
    		from: &quot;address&quot;,
    		localField: &quot;_id&quot;,
    		foreignField: &quot;party_id&quot;,
    		as: &quot;address&quot;
    	}
    },
    {
    	$unwind: &quot;$address&quot;
    },
    {
    	$lookup: {
    		from: &quot;addressComment&quot;,
    		localField: &quot;address._id&quot;,
    		foreignField: &quot;address_id&quot;,
    		as: &quot;address.addressComment&quot;
    	}
    }])

The result is pretty weird. Some records are ok. But Party with `_id: 4` is missing (there is no address for it). Also, there are two Party `_id: 1` in the result set (but with different addresses):

    {
    	&quot;_id&quot;: &quot;1&quot;,
    	&quot;name&quot;: &quot;party1&quot;,
    	&quot;address&quot;: {
    		&quot;_id&quot;: &quot;2&quot;,
    		&quot;street&quot;: &quot;Address2&quot;,
    		&quot;party_id&quot;: &quot;1&quot;,
    		&quot;addressComment&quot;: [{
    			&quot;_id&quot;: &quot;3&quot;,
    			&quot;address_id&quot;: &quot;2&quot;,
    			&quot;comment&quot;: &quot;Comment3&quot;
    		}]
    	}
    }{
    	&quot;_id&quot;: &quot;1&quot;,
    	&quot;name&quot;: &quot;party1&quot;,
    	&quot;address&quot;: {
    		&quot;_id&quot;: &quot;1&quot;,
    		&quot;street&quot;: &quot;Address1&quot;,
    		&quot;party_id&quot;: &quot;1&quot;,
    		&quot;addressComment&quot;: [{
    			&quot;_id&quot;: &quot;1&quot;,
    			&quot;address_id&quot;: &quot;1&quot;,
    			&quot;comment&quot;: &quot;Comment1&quot;
    		},
    		{
    			&quot;_id&quot;: &quot;2&quot;,
    			&quot;address_id&quot;: &quot;1&quot;,
    			&quot;comment&quot;: &quot;Comment2&quot;
    		}]
    	}
    }{
    	&quot;_id&quot;: &quot;3&quot;,
    	&quot;name&quot;: &quot;party3&quot;,
    	&quot;address&quot;: {
    		&quot;_id&quot;: &quot;4&quot;,
    		&quot;street&quot;: &quot;Address4&quot;,
    		&quot;party_id&quot;: &quot;3&quot;,
    		&quot;addressComment&quot;: []
    	}
    }{
    	&quot;_id&quot;: &quot;5&quot;,
    	&quot;name&quot;: &quot;party5&quot;,
    	&quot;address&quot;: {
    		&quot;_id&quot;: &quot;5&quot;,
    		&quot;street&quot;: &quot;Address5&quot;,
    		&quot;party_id&quot;: &quot;5&quot;,
    		&quot;addressComment&quot;: [{
    			&quot;_id&quot;: &quot;5&quot;,
    			&quot;address_id&quot;: &quot;5&quot;,
    			&quot;comment&quot;: &quot;Comment5&quot;
    		}]
    	}
    }{
    	&quot;_id&quot;: &quot;2&quot;,
    	&quot;name&quot;: &quot;party2&quot;,
    	&quot;address&quot;: {
    		&quot;_id&quot;: &quot;3&quot;,
    		&quot;street&quot;: &quot;Address3&quot;,
    		&quot;party_id&quot;: &quot;2&quot;,
    		&quot;addressComment&quot;: [{
    			&quot;_id&quot;: &quot;4&quot;,
    			&quot;address_id&quot;: &quot;3&quot;,
    			&quot;comment&quot;: &quot;Comment4&quot;
    		}]
    	}
    }
Please help me with this. I&#39;m pretty new to MongoDB but I feel it can do what I need from it.

MongoDB nested lookup with 3 levels

If we have to store the available positions at a company (i.e. Manager, Team Lead, ... etc). What are the best practices for storing it? I have two opinions with comments... &quot;sure, welcoming yours&quot;&lt;br/&gt;
&lt;b&gt;
&lt;ol&gt;
    &lt;li&gt;Storing it as DB table with columns ID and Name, and deal with it using queries and joins.&lt;/li&gt;
    &lt;li&gt;Storing it as Enum and forget about the DB table.&lt;/li&gt;
&lt;/ol&gt;
&lt;/b&gt;
In my opinion, I will choose the first solution if I have changing items. So that I won&#39;t hard code these options as Enum.&lt;br/&gt;
I may choose the Enum solution, if I have no doubt that data won&#39;t change (for example, Gender: Male, Female). &lt;br/&gt;&lt;br/&gt;
&lt;b&gt;NOTE:&lt;/b&gt; I code in English, and the UI Culture may be Arabic. If I will work with the Enum Solution, I will hard code the culture-based strings in the presentation layer, is it okay from the best practices perspective!!!! &lt;br/&gt;&lt;br/&gt;
I would like to know your opinions and if my thoughts correspond to what is most recommended &quot;Best Practices&quot;?? 

Lookup Tables Best Practices: DB Tables... or Enumerations

Does anyone know any simple way to retrieve the country from a given IP Address, preferably in ISO_3166-1 format?

How to get the Country according to a certain IP?

In mongo, after doing an `aggregation` with `$lookup`, I would like the request to return only some fields and not the whole document.

I have the following query :

    db.somecollection.aggregate([{
        $lookup: {
            from: &quot;campaigns&quot;,
            localField: &quot;campId&quot;,
            foreignField: &quot;_id&quot;,
            as: &quot;campaign&quot;
        }
    }, {
        $unwind: &quot;$campaign&quot;
    }, {
        $lookup: {
            from: &quot;entities&quot;,
            localField: &quot;campaign.clientid&quot;,
            foreignField: &quot;_id&quot;,
            as: &quot;campaign.client&quot;
        }
    }]);

This request will return me this :

    {
	&quot;_id&quot; : ObjectId(&quot;56cc7cd1cc2cf62803ebfdc7&quot;),
	&quot;campId&quot; : ObjectId(&quot;56c740e4479f46e402efda84&quot;),
	&quot;articleId&quot; : ObjectId(&quot;56c742c06094640103ba3843&quot;),
	&quot;campaign&quot; : {
		&quot;_id&quot; : ObjectId(&quot;56c740e4479f46e402efda84&quot;),
		&quot;clientid&quot; : ObjectId(&quot;56c740b8479f46e402efda83&quot;),
		&quot;client&quot; : [
			{
				&quot;_id&quot; : ObjectId(&quot;56c740b8479f46e402efda83&quot;),
				&quot;username&quot; : &quot;someusername&quot;,
				&quot;shhh&quot; : &quot;somehashedpassword&quot;,
				&quot;email&quot; : &quot;mail@mail.com&quot;,
			}
		]
	}

The request works well, but I would like to filter the fields in `campaign.client` to only get for example `_id` and `username`. Is there a way to do this in a MongoDB `aggregate` request?



MongoDB aggregation with $lookup only include (or project) some fields to return from query

A great programming resource, Bit Twiddling Hacks, proposes ([here][1]) the following method to compute log2 of a 32-bit integer:

    #define LT(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n
    static const char LogTable256[256] = 
    {
        -1, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3,
        LT(4), LT(5), LT(5), LT(6), LT(6), LT(6), LT(6),
        LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7), LT(7)
    };

    unsigned int v; // 32-bit word to find the log of
    unsigned r;     // r will be lg(v)
    register unsigned int t, tt; // temporaries
    if (tt = v &gt;&gt; 16)
    {
        r = (t = tt &gt;&gt; 8) ? 24 + LogTable256[t] : 16 + LogTable256[tt];
    }
    else 
    {
        r = (t = v &gt;&gt; 8) ? 8 + LogTable256[t] : LogTable256[v];
    }

and mentions that

&gt; The lookup table method takes only about 7 operations to find the log
&gt; of a 32-bit value. If extended for 64-bit quantities, it would take
&gt; roughly 9 operations.

but, alas, doesn&#39;t give any additional info about which way one should actually go to extend the algorithm to 64-bit integers.

Any hints about how a 64-bit algorithm of this kind would look like?


  [1]: http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogLookup

Fast computing of log2 for 64-bit integers

I have a Perl script that is counting the number of occurrences of various strings in a text file. I want to be able to check if a certain string is not yet a key in the hash. Is there a better way of doing this altogether?

Here is what I am doing:

    foreach $line (@lines){
        if(($line =~ m|my regex|) )
        {
            $string = $1;
            if ($string is not a key in %strings) # &quot;strings&quot; is an associative array
            {
                $strings{$string} = 1;
            }
            else
            {
                $n = ($strings{$string});
                $strings{$string} = $n +1;
            }
        }
    }


How can I see if a Perl hash already has a certain key?

I have two collections

1. User
   
        {
           &quot;_id&quot; : ObjectId(&quot;584aac38686860d502929b8b&quot;),
           &quot;name&quot; : &quot;John&quot;
        }

2. Role
 

        {
           &quot;_id&quot; : ObjectId(&quot;584aaca6686860d502929b8d&quot;),
           &quot;role&quot; : &quot;Admin&quot;,
           &quot;userId&quot; : &quot;584aac38686860d502929b8b&quot;  
        }


I want to join these collection based on the **userId** (in **role** collection) - **_id** ( in **user** collection).

I tried the below query:

   

    db.role.aggregate({
      &quot;$lookup&quot;: {
        &quot;from&quot;: &quot;user&quot;,
        &quot;localField&quot;: &quot;userId&quot;,
        &quot;foreignField&quot;: &quot;_id&quot;,
        &quot;as&quot;: &quot;output&quot;
      }
    })

This gives me expected results as long as i store userId as a ObjectId. When my userId is a string there are no results. 
Ps: I tried 

&gt; foreignField: &#39;_id&#39;.valueOf()

 and 

&gt; foreignField: &#39;_id&#39;.toString()

. But no luck to match/join based on a ObjectId-string fields.

Any help will be appreciated.



Mongodb Join on _id field from String to ObjectId

I am having some trouble replacing values in a dataframe. I would like to replace values based on a separate table. Below is an example of what I am trying to do.

I have a table where every row is a customer and every column is an animal they purchased. Lets call this dataframe `table`.

    &gt; table
    #       P1     P2     P3
    # 1    cat lizard parrot
    # 2 lizard parrot    cat
    # 3 parrot    cat lizard

I also have a table that I will reference called `lookUp`.

    &gt; lookUp
    #      pet   class
    # 1    cat  mammal
    # 2 lizard reptile
    # 3 parrot    bird

What I want to do is create a new table called `new` with a function replaces all values in `table` with the `class` column in `lookUp`. I tried this myself using an `lapply` function, but I got the following warnings.

    new &lt;- as.data.frame(lapply(table, function(x) {
      gsub(&#39;.*&#39;, lookUp[match(x, lookUp$pet) ,2], x)}), stringsAsFactors = FALSE)
    
    Warning messages:
    1: In gsub(&quot;.*&quot;, lookUp[match(x, lookUp$pet), 2], x) :
      argument &#39;replacement&#39; has length &gt; 1 and only the first element will be used
    2: In gsub(&quot;.*&quot;, lookUp[match(x, lookUp$pet), 2], x) :
      argument &#39;replacement&#39; has length &gt; 1 and only the first element will be used
    3: In gsub(&quot;.*&quot;, lookUp[match(x, lookUp$pet), 2], x) :
      argument &#39;replacement&#39; has length &gt; 1 and only the first element will be used

Any ideas on how to make this work? 

Replace values in a dataframe based on lookup table

I&#39;m just learning DDD (Eric Evans book is open in front of me) and I&#39;ve come across a problem that I can&#39;t find an answer for. What do you do in DDD when you&#39;re just trying to get a simple list of lookup records?

Ex.

EmployeeID: 123&lt;br /&gt;
EmployeeName: John Doe&lt;br /&gt;
State: Alaska (drop-down)&lt;br /&gt;
County: Wasilla (drop-down -- will be filtered based on state).

For example, let&#39;s say that you have an Employee domain object, an IEmployeeRepository interface and an EmployeeRepository class. This will be used by a UI to show a list of employees and individual details. In the UI, you want to use a drop-down for the State and County where the employee lives. The Available counties will be filtered based on which state was chosen. 
	
Unfortunately, the database tables and the UI look very different. In tblEmployees, it contains State Code=AK and County Code=02130, not the State and County Names.

The old way (before I began this DDD quest) would be pretty straightforward, just create 2 queries and use a DataReader to populate the drop-downs. Underneath the display in the drop-downs is the value, which gets automatically used in form posts.

With DDD, though, I&#39;m not sure how you&#39;re supposed to do this. I first started by creating State and County objects as well repositories and interfaces for the repositories. However, writing 4 classes + 2 interfaces and the plumbing in the hbm.xml files + Employee Business objects seems like overkill for just 2 queries for 2 drop-downs. There has to be a better way, doesn&#39;t there? I&#39;m not changing the records in the State or County tables any time soon and even if I did, it wouldn&#39;t be through this application. So I don&#39;t really want to create business objects for State and County if I don&#39;t have to.

The simplest solution that I see is to just create a helper class with methods that return dictionaries, such as GetStatesAll(), GetState() and GetCounties() and GetCounty(), but that just feels wrong from a DDD perspective. 

Please help. How can I use DDD without overengineering just a couple of simple lookups?

&lt;b&gt;Final Solution&lt;/b&gt;
I think that I finally found my answer through experience, which was to put the GetStates() method into its own Data Access class, though not a repository class. Since I was only doing read-only access, I threw it into a struct DTO. Since the database was small, I threw them altogether into a single class, like Todd below described. 

My conclusions:
&lt;ol&gt;
&lt;li&gt; Lookup tables are NEVER value objects, because lookup tables ALWAYS have an identity. If they didn&#39;t have an identity, you&#39;d have duplicates, which wouldn&#39;t make much sense.
&lt;li&gt; Read-only lookup table can have a repository, but probably don&#39;t need one. The goal of a repository is to reduce complexity by forcing access only through the aggregate. Going through the aggregate provides you with a way to ensure that business rules can be enforced, such as not adding tires if you don&#39;t have a car. 
&lt;li&gt; If you allow CRUD maintenance on the lookup table, then it makes sense for the lookup table to have its own repository.
&lt;li&gt; The fact that I ended up storing the codes as structs doesn&#39;t make them &quot;value types&quot;. Fowler says in POEAA that a struct is a value type. That&#39;s true, structs are immutable, which is why Fowler says that they are &quot;value types&quot;, however I was using them differently. I was using structs as a lightweight way to pass around DTOs that I didn&#39;t ever plan on changing after their initial creation. In truth, the structs that I used did, indeed, have identities, but since they were read-only they worked as structs.
&lt;li&gt;One pattern that I&#39;ve been using that I don&#39;t see much elsewhere is to make primary key fields be immutable. They&#39;re set by the constructor, but they&#39;re read-only (not private accessors) and cannot be changed once the object is created.
&lt;/ol&gt;

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