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How to use C# LINQ Union to get the Union of Custom list1 with list2

I am using the Enumerable.Union<TSource> method to get the union of the Custom List1 with the Custom List2. But somehow it does not work as it should in my case. I am getting all the items also the duplicate once.
I followed the MSDN Link to get the work done, but still I am not able to achieve the same.
Following is the Code of the custom class:-
public class CustomFormat : IEqualityComparer<CustomFormat>
{
private string mask;
public string Mask
{
get { return mask; }
set { mask = value; }
}
private int type;//0 for Default 1 for userdefined
public int Type
{
get { return type; }
set { type = value; }
}
public CustomFormat(string c_maskin, int c_type)
{
mask = c_maskin;
type = c_type;
}
public bool Equals(CustomFormat x, CustomFormat y)
{
if (ReferenceEquals(x, y)) return true;
//Check whether the products' properties are equal.
return x != null && y != null && x.Mask.Equals(y.Mask) && x.Type.Equals(y.Type);
}
public int GetHashCode(CustomFormat obj)
{
//Get hash code for the Name field if it is not null.
int hashProductName = obj.Mask == null ? 0 : obj.Mask.GetHashCode();
//Get hash code for the Code field.
int hashProductCode = obj.Type.GetHashCode();
//Calculate the hash code for the product.
return hashProductName ^ hashProductCode;
}
}
This I am calling as follows:-
List<CustomFormat> l1 = new List<CustomFormat>();
l1.Add(new CustomFormat("#",1));
l1.Add(new CustomFormat("##",1));
l1.Add(new CustomFormat("###",1));
l1.Add(new CustomFormat("####",1));
List<CustomFormat> l2 = new List<CustomFormat>();
l2.Add(new CustomFormat("#",1));
l2.Add(new CustomFormat("##",1));
l2.Add(new CustomFormat("###",1));
l2.Add(new CustomFormat("####",1));
l2.Add(new CustomFormat("## ###.0",1));
l1 = l1.Union(l2).ToList();
foreach(var l3 in l1)
{
Console.WriteLine(l3.Mask + " " + l3.Type);
}
Please suggest the appropriate way to achieve the same!

The oddity here is that your class implement IEqualityComparer<CustomClass> instead of IEquatable<CustomClass>. You could pass in another instance of CustomClass which would be used as the comparer, but it would be more idiomatic to just make CustomClass implement IEquatable<CustomClass>, and also override Equals(object).
The difference between IEquatable<T> and IEqualityComparer<T> is that IEquatable<T> says "I know how to compare myself with another instance of T" whereas IEqualityComparer<T> says "I know how to compare two instances of T". The latter is normally provided separately - just as it can be provided to Union via another parameter. It's very rare for a type to implement IEqualityComparer<T> for its own type - whereas IEquatable<T> should pretty much only be used to compare values of the same type.
Here's an implementation using automatically implemented properties for simplicity and more idiomatic parameter names. I'd probably change the hash code implementation myself and use expression-bodied members, but that's a different matter.
public class CustomFormat : IEquatable<CustomFormat>
{
public string Mask { get; set; }
public int Type { get; set; }
public CustomFormat(string mask, int type)
{
Mask = mask;
Type = type;
}
public bool Equals(CustomFormat other)
{
if (ReferenceEquals(this, other))
{
return true;
}
return other != null && other.Mask == Mask && other.Type == Type;
}
public override bool Equals(object obj)
{
return Equals(obj as CustomFormat);
}
public override int GetHashCode()
{
// Get hash code for the Name field if it is not null.
int hashProductName = Mask == null ? 0 : Mask.GetHashCode();
//Get hash code for the Code field.
int hashProductCode = Type.GetHashCode();
//Calculate the hash code for the product.
return hashProductName ^ hashProductCode;
}
}
Now it doesn't help that (as noted in comments) the documentation for Enumerable.Union is wrong. It currently states:
The default equality comparer, Default, is used to compare values of the types that implement the IEqualityComparer<T> generic interface.
It should say something like:
The default equality comparer, Default, is used to compare values when a specific IEqualityComparer<T> is not provided. If T implements IEquatable<T>, the default comparer will use that implementation. Otherwise, it will use the implementation of Equals(object).

You need to pass an instance of an IEqualityComparer to the Union method. The method has an overload to pass in your comparer.
The easiest and ugliest solution is
var comparer = new CustomFormat(null,0);
l1 = l1.Union(l2, comparer).ToList();
You have made some mistakes in your implementation. You should not implement the IEqualityComparer method on your type (CustomFormat), but on a separate class, like CustomFormatComparer.
On your type (CustomFormat) you should implemented IEquatable.

Enumerable.Distinct - What methods does your custom class have to implement for it to work?

I've implemented every function that MSDN says is necessary, plus some additional comparison interfaces - nothing seems to work. Following is code (optimized for LinqPad).
The resulting output is all 4 items, not 2 like I expect.
Please don't post work arounds as answers - I want to know how Distinct works
void Main()
{
List<NameClass> results = new List<NameClass>();
results.Add(new NameClass("hello"));
results.Add(new NameClass("hello"));
results.Add(new NameClass("55"));
results.Add(new NameClass("55"));
results.Distinct().Dump();
}
// Define other methods and classes here
public class NameClass : Object
, IEquatable<NameClass>
, IComparer<NameClass>
, IComparable<NameClass>
, IEqualityComparer<NameClass>
, IEqualityComparer
, IComparable
{
public NameClass(string name)
{
Name = name;
}
public string Name { get; private set; }
public int Compare(NameClass x, NameClass y)
{
return String.Compare(x.Name, y.Name);
}
public int CompareTo(NameClass other)
{
return String.Compare(Name, other.Name);
}
public bool Equals(NameClass x, NameClass y)
{
return (0 == Compare(x, y));
}
public bool Equals(NameClass other)
{
return (0 == CompareTo(other));
}
public int GetHashCode(NameClass obj)
{
return obj.Name.GetHashCode();
}
public new int GetHashCode()
{
return Name.GetHashCode();
}
public new bool Equals(object a)
{
var x = a as NameClass;
if (null == x) { return false; }
return Equals(x);
}
public new bool Equals(object a, object b)
{
if (null == a && null == b) { return true; }
if (null == a && null != b) { return false; }
if (null != a && null == b) { return false; }
var x = a as NameClass;
var y = b as NameClass;
if (null == x && null == y) { return true; }
if (null == x && null != y) { return false; }
if (null != x && null == y) { return false; }
return x.Equals(y);
}
public int GetHashCode(object obj)
{
if (null == obj) { return 0; }
var x = obj as NameClass;
if (null != x) { return x.GetHashCode(); }
return obj.GetHashCode();
}
public int CompareTo(object obj)
{
if (obj == null) return 1;
NameClass x = obj as NameClass;
if (x == null)
{
throw new ArgumentException("Object is not a NameClass");
}
return CompareTo(x);
}
}

How Distinct works:
There is at least no implementation of Object.GetHashCode() which is used for initial comparison of objects: basic version of Distinct compares (actually puts in dictionary) by Object.GetHashCode first, than if hash code matches by Object.Equals.
To be precise Enumerable.Distinct(this IEnumerable source) uses EqualityComparer<NameClass>.Default to finally check for equality (note that if hash codes don't match it will not reach that portion of the comparison which is why your sample does not work).
The default equality comparer, Default, is used to compare values of the types that implement the IEquatable generic interface.
EqualityComparer.Default in turn actually allows to use class without IEquatable<T> at all falling back directly to Object.Equals:
The Default property checks whether type T implements the System.IEquatable interface and, if so, returns an EqualityComparer that uses that implementation. Otherwise, it returns an EqualityComparer that uses the overrides of Object.Equals and Object.GetHashCode provided by T.
So for basic Distinct to work you just need correct version of Equals/GetHashCode. IEquatable is optional, but must match behavior of GetHashCode in the class.
How to fix:
Your sample have public new int GetHashCode() method, which likely should be public override int GetHashCode() (Same for Equals).
Note that public new int... does not mean "override", but instead "create new version of the method that hides old one". It does not impact callers that call method via pointer to parent object.
Personally I think new should rarely be used in defining methods. Some suggestions when it is useful are covered in Usecases for method hiding using new.

You don't have to implement any interface, just GetHashCode and Equals methods correctly:
public class NameClass
{
public NameClass(string name)
{
Name = name;
}
public string Name { get; private set; }
public override bool Equals(object obj)
{
var other = obj as NameClass;
return other != null && other.Name == this.Name;
}
public override int GetHashCode()
{
return Name.GetHashCode();
}
}

Enumerable.Distinct<TSource> Method:
It uses the default equality comparer, Default, to compare values.
EqualityComparer.Default:
The Default property checks whether type T implements the System.IEquatable<T> interface and, if so, returns an EqualityComparer<T> that uses that implementation. Otherwise, it returns an EqualityComparer<T> that uses the overrides of Object.Equals and Object.GetHashCode provided by T.
IEquatable<T> Interface:
If you implement IEquatable<T>, you should also override the base class implementations of Object.Equals(Object) and GetHashCode so that their behavior is consistent with that of the IEquatable<T>.Equals method.
Overriding methods:
The override modifier is required to extend or modify the abstract or virtual implementation of an inherited method, property, indexer, or event.
So your code should look like this:
public class NameClass : IEquatable<NameClass>
{
public NameClass(string name)
{
Name = name;
}
public string Name { get; private set; }
// implement IEquatable<NameClass>
public bool Equals(NameClass other)
{
return (other != null) && (Name == other.Name);
}
// override Object.Equals(Object)
public override bool Equals(object obj)
{
return Equals(obj as NameClass);
}
// override Object.GetHashCode()
public override GetHashCode()
{
return Name.GetHashCode();
}
}

So, first off, Distinct will, as per it's documentation, use EqualityComparer<T>.Default to compare objects if no custom equality comparer is provided (you provided none).
EqualityComparer<T>.Default, as per its documentation, will look to see if the object implements IEquatable<T>, if it does it will use that implementation of Equals.
Regardless of whether or not the type implements IEquatable<T>, EqualityComparer<T>.Default will use the object.GetHashCode method to get the has code of the object. IEquatable<T>, unfortunately, does not force you to also override the object's GetHashCode implementation, and in your case, while you do implement IEquatable<T>, your code does not override the object's GetHashCode implementation.
As a result of this Distinct is actually using the proper Equals method for your type, but it's using the wrong GetHashCode method. Whenever you're hashing objects and that type has an Equals and GetHashCode implementation that's out of sync problems ensue. What's happening is that in whatever hash based collection it's sending the two "equal" objects to different buckets, so they never even get to the point where their Equals methods are called on each other. If you happened to get lucky and there was a hash collection and the objects were coincidentally sent to the same bucket, then, since the Equals method is what you intended it would actually work, but the odds of that happening are...very low. (In this specific case, about 2/2147483647, or
9.3e-10.
While you do provide a new GetHashCode method in NameClass, it is hiding the object implementation, not overriding it. If you change your GetHashCode implementation to use override rather than new then your code will work.

I just realized I messed up my sample code - my class derives from DependencyObject, not Object. I can't override thew GetHashCode or Equals functions because the DependencyObject class is sealed.

Mono implementation of Dictionary<T,T> using .Equals(obj o) instead of .GetHashCode()

By searching though msdn c# documentation and stack overflow, I get the clear impression that Dictionary<T,T> is supposed to use GetHashCode() for checking key-uniqueness and to do look-up.
The Dictionary generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table.
...
The speed of retrieval depends on the quality of the hashing algorithm of the type specified for TKey.
I Use mono (in Unity3D), and after getting some weird results in my work, I conducted this experiment:
public class DictionaryTest
{
public static void TestKeyUniqueness()
{
//Test a dictionary of type1
Dictionary<KeyType1, string> dictionaryType1 = new Dictionary<KeyType1, string>();
dictionaryType1[new KeyType1(1)] = "Val1";
if(dictionaryType1.ContainsKey(new KeyType1(1)))
{
Debug.Log ("Key in dicType1 was already present"); //This line does NOT print
}
//Test a dictionary of type1
Dictionary<KeyType2, string> dictionaryType2 = new Dictionary<KeyType2, string>();
dictionaryType2[new KeyType2(1)] = "Val1";
if(dictionaryType2.ContainsKey(new KeyType2(1)))
{
Debug.Log ("Key in dicType2 was already present"); // Only this line prints
}
}
}
//This type implements only GetHashCode()
public class KeyType1
{
private int var1;
public KeyType1(int v1)
{
var1 = v1;
}
public override int GetHashCode ()
{
return var1;
}
}
//This type implements both GetHashCode() and Equals(obj), where Equals uses the hashcode.
public class KeyType2
{
private int var1;
public KeyType2(int v1)
{
var1 = v1;
}
public override int GetHashCode ()
{
return var1;
}
public override bool Equals (object obj)
{
return GetHashCode() == obj.GetHashCode();
}
}
Only the when using type KeyType2 are the keys considered equal. To me this demonstrates that Dictionary uses Equals(obj) - and not GetHashCode().
Can someone reproduce this, and help me interpret the meaning is? Is it an incorrect implementation in mono? Or have I misunderstood something.

i get the clear impression that Dictionary is supposed to use
.GetHashCode() for checking key-uniqueness
What made you think that? GetHashCode doesn't return unique values.
And MSDN clearly says:
Dictionary requires an equality implementation to
determine whether keys are equal. You can specify an implementation of
the IEqualityComparer generic interface by using a constructor that
accepts a comparer parameter; if you do not specify an implementation,
the default generic equality comparer EqualityComparer.Default is
used. If type TKey implements the System.IEquatable generic
interface, the default equality comparer uses that implementation.

Doing this:
public override bool Equals (object obj)
{
return GetHashCode() == obj.GetHashCode();
}
is wrong in the general case because you might end up with KeyType2 instances that are equal to StringBuilder, SomeOtherClass, AnythingYouCanImagine and what not instances.
You should totally do it like so:
public override bool Equals (object obj)
{
if (obj is KeyType2) {
return (obj as KeyType2).var1 == this.var1;
} else
return false;
}
When you are trying to override Equals and inherently GetHashCode you must ensure the following points (given the class MyObject) in this order (you were doing it the other way around):
1) When are 2 instances of MyObject equal ? Say you have:
public class MyObject {
public string Name { get; set; }
public string Address { get; set; }
public int Age { get; set; }
public DateTime TimeWhenIBroughtThisInstanceFromTheDatabase { get; set; }
}
And you have 1 record in some database that you need to be mapped to an instance of this class.
And you make the convention that the time you read the record from the database will be stored
in the TimeWhenIBroughtThisInstanceFromTheDatabase:
MyObject obj1 = DbHelper.ReadFromDatabase( ...some params...);
// you do that at 14:05 and thusly the TimeWhenIBroughtThisInstanceFromTheDatabase
// will be assigned accordingly
// later.. at 14:07 you read the same record into a different instance of MyClass
MyObject obj2 = DbHelper.ReadFromDatabase( ...some params...);
// (the same)
// At 14:09 you ask yourself if the 2 instances are the same
bool theyAre = obj1.Equals(obj2)
Do you want the result to be true ? I would say you do.
Therefore the overriding of Equals should like so:
public class MyObject {
...
public override bool Equals(object obj) {
if (obj is MyObject) {
var that = obj as MyObject;
return (this.Name == that.Name) &&
(this.Address == that.Address) &&
(this.Age == that.Age);
// without the syntactically possible but logically challenged:
// && (this.TimeWhenIBroughtThisInstanceFromTheDatabase ==
// that.TimeWhenIBroughtThisInstanceFromTheDatabase)
} else
return false;
}
...
}
2) ENSURE THAT whenever 2 instances are equal (as indicated by the Equals method you implement)
their GetHashCode results will be identitcal.
int hash1 = obj1.GetHashCode();
int hash2 = obj2.GetHashCode();
bool theseMustBeAlso = hash1 == hash2;
The easiest way to do that is (in the sample scenario):
public class MyObject {
...
public override int GetHashCode() {
int result;
result = ((this.Name != null) ? this.Name.GetHashCode() : 0) ^
((this.Address != null) ? this.Address.GetHashCode() : 0) ^
this.Age.GetHashCode();
// without the syntactically possible but logically challenged:
// ^ this.TimeWhenIBroughtThisInstanceFromTheDatabase.GetHashCode()
}
...
}
Note that:
- Strings can be null and that .GetHashCode() might fail with NullReferenceException.
- I used ^ (XOR). You can use whatever you want as long as the golden rule (number 2) is respected.
- x ^ 0 == x (for whatever x)

c# List<T>.Contains() Method Returns False

In the code block below I would expect dictCars to contain:
{ Chevy:Camaro, Dodge:Charger }
But, dictCars comes back empty. Because this line returns false each time it's called:
if(myCars.Contains(new Car(Convert.ToInt64(strCar.Split(':')[1]),strCar.Split(':')[2])))
Code block:
public class Car
{
public long CarID { get; set; }
public string CarName { get; set; }
public Car(long CarID, string CarName)
{
this.CarID = CarID;
this.CarName = CarName;
}
}
List<Car> myCars = new List<Car>();
myCars.Add(new Car(0,"Pinto"));
myCars.Add(new Car(2,"Camaro"));
myCars.Add(new Car(3,"Charger"));
Dictionary<string, string> dictCars = new Dictionary<string, string>();
string strCars = "Ford:1:Mustang,Chevy:2:Camaro,Dodge:3:Charger";
String[] arrCars = strCars.Split(',');
foreach (string strCar in arrCars)
{
if(myCars.Contains(new Car(Convert.ToInt64(strCar.Split(':')[1]),strCar.Split(':')[2])))
{
if (!dictCars.ContainsKey(strCar.Split(':')[0]))
{
dictCars.Add(strCar.Split(':')[0], strCar.Split(':')[2]);
}
}
}
return dictCars;
Question: What am I doing wrong with my List.Contains implementation?
Thanks in advance!

You need to tell Contains what makes two Cars equal. By default it will use ReferenceEquals which will only call two objects equal if they are the same instance.
Either override Equals and GetHashCode in your Car class or define an IEqualityComparer<Car> class and pass that to Contains.
If two Cars that have the same CarID are "equal" then the implementation is pretty straightforward:
public override bool Equals(object o)
{
if(o.GetType() != typeof(Car))
return false;
return (this.CarID == ((Car)o).CarID);
}
public override int GetHashCode()
{
return CarID.GetHashCode();
}

Your Car class is a reference type. By default reference types are compared to each other by reference, meaning they are considered the same if they reference the same instance in memory. In your case you want them to be considered equal if they contain the same values.
To change the equality behavior, you need to override Equals and GetHashCode.
If two cars are equal only when ID and Name are equal, the following is one possible implementation of the equality members:
protected bool Equals(Car other)
{
return CarID == other.CarID && string.Equals(CarName, other.CarName);
}
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj))
return false;
if (ReferenceEquals(this, obj))
return true;
var other = obj as Car;
return other != null && Equals(other);
}
public override int GetHashCode()
{
unchecked
{
return (CarID.GetHashCode() * 397) ^
(CarName != null ? CarName.GetHashCode() : 0);
}
}
This implementation has been created automatically by ReSharper.
It takes into account null values and the possibility of sub-classes of Car. Additionally, it provides a useful implementation of GetHashCode.

You can add this code, by implementing IEquatable
public class Car: IEquatable<Car>
{
......
public bool Equals( Car other )
{
return this.CarID == other.CarID && this.CarName == other.CarName;
}
}
Link : http://msdn.microsoft.com/fr-fr/library/vstudio/ms131187.aspx

You are assuming that two Car instances that have the same CarID and CarName are equal.
This is incorrect. By default, each new Car(...) is different from each other car, since they are references to different objects.
There are a few ways to "fix" that:
Use a struct instead of a class for your Car.
Structs inherit ValueType's default implementation of Equals, which compares all fields and properties to determine equality.
Note that in this case, it is recommended that you make your Car struct immutable to avoid common problems with mutable structs.
Override Equals and GetHashCode.
That way, List.Contains will know that you intend Cars with the same ID and Name to be equal.
Use another method instead of List.Contains.
For example, Enumerable.Any allows you to specify a predicate that can be matched:
bool exists = myCars.Any(car => car.ID == Convert.ToInt64(strCar.Split(':')[1])
&& car.Name = strCar.Split(':')[2]);

You need to implement Equals. Most probably as:
public override bool Equals(object obj)
{
Car car = obj as Car;
if(car == null) return false;
return car.CarID == this.CarID && car.CarName == this.CarName;
}

Your car class needs to implement interface IEquatable and define an Equals method, otherwise the contains method is comparing the underlying references.

You need to implement the IEqualityComparer
More information on how to do it can be found here;
http://msdn.microsoft.com/en-us/library/bb339118.aspx
// Custom comparer for the class
class CarComparer : IEqualityComparer<Car>
{
// Products are equal if their names and product numbers are equal.
public bool Equals(Car x, Car y)
{
//Check whether the compared objects reference the same data.
if (Object.ReferenceEquals(x, y)) return true;
//Check whether any of the compared objects is null.
if (Object.ReferenceEquals(x, null) || Object.ReferenceEquals(y, null))
return false;
//Check whether the properties are equal.
return x.CarID == y.CarID && x.CarName == y.CarName;
}
// If Equals() returns true for a pair of objects
// then GetHashCode() must return the same value for these objects.
public int GetHashCode(Car car)
{
//Check whether the object is null
if (Object.ReferenceEquals(car, null)) return 0;
//Get hash code for the Name field if it is not null.
string hashCarName = car.CarName == null ? 0 : car.CarName.GetHashCode();
//Get hash code for the ID field.
int hashCarID = car.CarID.GetHashCode();
//Calculate the hash code for the product.
return hashCarName ^ hashCarID;
}
Check for equality;
CarComparer carComp = new CarComparer();
bool blnIsEqual = CarList1.Contains(CarList2, carComp);

A collection can never "contain" a newly newed object which uses the default Object.Equals comparison. (The default comparison is ReferenceEquals, which simply compares instances. This will never be true comparing an existing Car with a new Car())
To use Contains in this way, you will need to either:
Override Car.Equals (and Car.GetHashCode) to specify what it means to be equivalent, or
Implement an IEqualityComparer<Car> to compare the instances and specify that in your call to Contains.
Note the side effect that in the first option, other uses of Car.Equals(Car) will also use this comparison.
Otherwise, you can use Any and specify the comparison yourself (but IMHO this smells a little funny - a Car should know how to compare itself):
if(myCars.Any(c=> c.CarID == Convert.ToInt64(strCar.Split(':')[1]) && c.CarName == strCar.Split(':')[2]))

myCars.Contains(newCar)
myCars.Where(c => c.CarID == newCar.CarID && c.CarName==newCar.CarName).Count() > 0

Generate hash of object consistently

I'm trying to get a hash (md5 or sha) of an object.
I've implemented this:
http://alexmg.com/post/2009/04/16/Compute-any-hash-for-any-object-in-C.aspx
I'm using nHibernate to retrieve my POCOs from a database.
When running GetHash on this, it's different each time it's selected and hydrated from the database. I guess this is expected, as the underlying proxies will change.
Anyway,
Is there a way to get a hash of all the properties on an object, consistently each time?
I've toyed with the idea of using a StringBuilder over this.GetType().GetProperties..... and creating a hash on that, but that seems inefficient?
As a side note, this is for change-tracking these entities from one database (RDBMS) to a NoSQL store
(comparing hash values to see if objects changed between rdbms and nosql)

If you're not overriding GetHashCode you just inherit Object.GetHashCode. Object.GetHashCode basically just returns the memory address of the instance, if it's a reference object. Of course, each time an object is loaded it will likely be loaded into a different part of memory and thus result in a different hash code.
It's debatable whether that's the correct thing to do; but that's what was implemented "back in the day" so it can't change now.
If you want something consistent then you have to override GetHashCode and create a code based on the "value" of the object (i.e. the properties and/or fields). This can be as simple as a distributed merging of the hash codes of all the properties/fields. Or, it could be as complicated as you need it to be. If all you're looking for is something to differentiate two different objects, then using a unique key on the object might work for you.If you're looking for change tracking, using the unique key for the hash probably isn't going to work
I simply use all the hash codes of the fields to create a reasonably distributed hash code for the parent object. For example:
public override int GetHashCode()
{
unchecked
{
int result = (Name != null ? Name.GetHashCode() : 0);
result = (result*397) ^ (Street != null ? Street.GetHashCode() : 0);
result = (result*397) ^ Age;
return result;
}
}
The use of the prime number 397 is to generate a unique number for a value to better distribute the hash code. See http://computinglife.wordpress.com/2008/11/20/why-do-hash-functions-use-prime-numbers/ for more details on the use of primes in hash code calculations.
You could, of course, use reflection to get at all the properties to do this, but that would be slower. Alternatively you could use the CodeDOM to generate code dynamically to generate the hash based on reflecting on the properties and cache that code (i.e. generate it once and reload it next time). But, this of course, is very complex and might not be worth the effort.
An MD5 or SHA hash or CRC is generally based on a block of data. If you want that, then using the hash code of each property doesn't make sense. Possibly serializing the data to memory and calculating the hash that way would be more applicable, as Henk describes.

If this 'hash' is solely used to determine whether entities have changed then the following algorithm may help (NB it is untested and assumes that the same runtime will be used when generating hashes (otherwise the reliance on GetHashCode for 'simple' types is incorrect)):
public static byte[] Hash<T>(T entity)
{
var seen = new HashSet<object>();
var properties = GetAllSimpleProperties(entity, seen);
return properties.Select(p => BitConverter.GetBytes(p.GetHashCode()).AsEnumerable()).Aggregate((ag, next) => ag.Concat(next)).ToArray();
}
private static IEnumerable<object> GetAllSimpleProperties<T>(T entity, HashSet<object> seen)
{
foreach (var property in PropertiesOf<T>.All(entity))
{
if (property is int || property is long || property is string ...) yield return property;
else if (seen.Add(property)) // Handle cyclic references
{
foreach (var simple in GetAllSimpleProperties(property, seen)) yield return simple;
}
}
}
private static class PropertiesOf<T>
{
private static readonly List<Func<T, dynamic>> Properties = new List<Func<T, dynamic>>();
static PropertiesOf()
{
foreach (var property in typeof(T).GetProperties())
{
var getMethod = property.GetGetMethod();
var function = (Func<T, dynamic>)Delegate.CreateDelegate(typeof(Func<T, dynamic>), getMethod);
Properties.Add(function);
}
}
public static IEnumerable<dynamic> All(T entity)
{
return Properties.Select(p => p(entity)).Where(v => v != null);
}
}
This would then be useable like so:
var entity1 = LoadEntityFromRdbms();
var entity2 = LoadEntityFromNoSql();
var hash1 = Hash(entity1);
var hash2 = Hash(entity2);
Assert.IsTrue(hash1.SequenceEqual(hash2));

GetHashCode() returns an Int32 (not an MD5).
If you create two objects with all the same property values they will not have the same Hash if you use the base or system GetHashCode().
String is an object and an exception.
string s1 = "john";
string s2 = "john";
if (s1 == s2) returns true and will return the same GetHashCode()
If you want to control equality comparison of two objects then you should override the GetHash and Equality.
If two object are the same then they must also have the same GetHash(). But two objects with the same GetHash() are not necessarily the same. A comparison will first test the GetHash() and if it gets a match there it will test the Equals. OK there are some comparisons that go straight to Equals but you should still override both and make sure two identical objects produce the same GetHash.
I use this for syncing a client with the server. You could use all the Properties or you could have any Property change change the VerID. The advantage here is a simpler quicker GetHashCode(). In my case I was resetting the VerID with any Property change already.
public override bool Equals(Object obj)
{
//Check for null and compare run-time types.
if (obj == null || !(obj is FTSdocWord)) return false;
FTSdocWord item = (FTSdocWord)obj;
return (OjbID == item.ObjID && VerID == item.VerID);
}
public override int GetHashCode()
{
return ObjID ^ VerID;
}
I ended up using ObjID alone so I could do the following
if (myClientObj == myServerObj && myClientObj.VerID <> myServerObj.VerID)
{
// need to synch
}
Object.GetHashCode Method
Two objects with the same property values. Are they equal? Do they produce the same GetHashCode()?
personDefault pd1 = new personDefault("John");
personDefault pd2 = new personDefault("John");
System.Diagnostics.Debug.WriteLine(po1.GetHashCode().ToString());
System.Diagnostics.Debug.WriteLine(po2.GetHashCode().ToString());
// different GetHashCode
if (pd1.Equals(pd2)) // returns false
{
System.Diagnostics.Debug.WriteLine("pd1 == pd2");
}
List<personDefault> personsDefault = new List<personDefault>();
personsDefault.Add(pd1);
if (personsDefault.Contains(pd2)) // returns false
{
System.Diagnostics.Debug.WriteLine("Contains(pd2)");
}
personOverRide po1 = new personOverRide("John");
personOverRide po2 = new personOverRide("John");
System.Diagnostics.Debug.WriteLine(po1.GetHashCode().ToString());
System.Diagnostics.Debug.WriteLine(po2.GetHashCode().ToString());
// same hash
if (po1.Equals(po2)) // returns true
{
System.Diagnostics.Debug.WriteLine("po1 == po2");
}
List<personOverRide> personsOverRide = new List<personOverRide>();
personsOverRide.Add(po1);
if (personsOverRide.Contains(po2)) // returns true
{
System.Diagnostics.Debug.WriteLine("Contains(p02)");
}
}
public class personDefault
{
public string Name { get; private set; }
public personDefault(string name) { Name = name; }
}
public class personOverRide: Object
{
public string Name { get; private set; }
public personOverRide(string name) { Name = name; }
public override bool Equals(Object obj)
{
//Check for null and compare run-time types.
if (obj == null || !(obj is personOverRide)) return false;
personOverRide item = (personOverRide)obj;
return (Name == item.Name);
}
public override int GetHashCode()
{
return Name.GetHashCode();
}
}