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Avoiding duplicates in a HashSet of custom types in C#

I have the following custom class deriving from Tuple:
public class CustomTuple : Tuple<List<string>, DateTime?>
{
public CustomTuple(IEnumerable<string> strings, DateTime? time)
: base(strings.OrderBy(x => x).ToList(), time)
{
}
}
and a HashSet<CustomTuple>. The problem is that when I add items to the set, they are not recognised as duplicates. i.e. this outputs 2, but it should output 1:
void Main()
{
HashSet<CustomTuple> set = new HashSet<CustomTuple>();
var a = new CustomTuple(new List<string>(), new DateTime?());
var b = new CustomTuple(new List<string>(), new DateTime?());
set.Add(a);
set.Add(b);
Console.Write(set.Count); // Outputs 2
}
How can I override the Equals and GetHashCode methods to cause this code to output a set count of 1?

You should override GetHashCode and Equals virtual methods defined in System.Object class.
Please remember that:
If two objects are logically "equal" then they MUST have the same hash code!
If two objects have the same hashcode, then it is not mandatory to have your objects equal.
Also, i've noticed an architectural problem in your code:
List is a mutable type but overriding Equals and GetHashCode usually makes your class logically to behave like a value type. So having "Item1" a mutable type and behaving like a value type is very dangerous. I suggest replacing your List with a ReadOnlyCollection . Then you would have to make a method that checks whether two ReadOnlyCollections are Equal.
For the GetHashCode () method, just compose a string from all string items found in Item1 then append a string that represents the Hash code for datetime then finally call on the concatenated result the "GetHashCode ()" overrided on string method. So normally you would have:
override int GetHashCode () {
return (GetHashCodeForList (Item1) + (Item2 ?? DateTime.MinValue).GetHashCode ()).GetHashCode ();
}
And the GetHashCodeForList method would be something like this:
private string GetHashCodeForList (IEnumerable <string> lst) {
if (lst == null) return string.Empty;
StringBuilder sb = new StringBuilder ();
foreach (var item in lst) {
sb.Append (item);
}
return sb.ToString ();
}
Final note: You could cache the GetHashCode result since it is relative expensive to get and your entire class would became immutable (if you replace List with a readonly collection).

A HashSet<T> will first call GetHashCode, so you need to work on that first. For an implementation, see this answer: https://stackoverflow.com/a/263416/1250301
So a simple, naive, implementation might look like this:
public override int GetHashCode()
{
unchecked
{
int hash = 17;
hash = hash * 23 + this.Item2.GetHashCode();
foreach (var s in this.Item1)
{
hash = hash * 23 + s.GetHashCode();
}
return hash;
}
}
However, if your lists are long, then this might not be efficient enough. So you'll have to decide where to compromise depending on how tolerant you are of collisions.
If the result of GetHashCode for two items are the same, then, and only then, will it call Equals. An implementation of Equals is going to need to compare the items in the list. Something like this:
public override bool Equals(object o1)
{
var o = o1 as CustomTuple;
if (o == null)
{
return false;
}
if (Item2 != o.Item2)
{
return false;
}
if (Item1.Count() != o.Item1.Count())
{
return false;
}
for (int i=0; i < Item1.Count(); i++)
{
if (Item1[i] != o.Item1[i])
{
return false;
}
}
return true;
}
Note that we check the date (Item2) first, because that's cheap. If the date isn't the same, we don't bother with anything else. Next we check the Count on both collections (Item1). If they don't match, there's no point iterating the collections. Then we loop through both collections and compare each item. Once we find one that doesn't match, we return false because there is no point continuing to look.
As pointed out in George's answer, you also have the problem that your list is mutable, which will cause problems with your HashSet, for example:
var a = new CustomTuple(new List<string>() {"foo"} , new DateTime?());
var b = new CustomTuple(new List<string>(), new DateTime?());
set.Add(a);
set.Add(b);
// Hashset now has two entries
((List<string>)a.Item1).Add("foo");
// Hashset still has two entries, but they are now identical.
To solve that, you need to force your IEnumerable<string> to be readonly. You could do something like:
public class CustomTuple : Tuple<IReadOnlyList<string>, DateTime?>
{
public CustomTuple(IEnumerable<string> strings, DateTime? time)
: base(strings.OrderBy(x => x).ToList().AsReadOnly(), time)
{
}
public override bool Equals(object o1)
{
// as above
}
public override int GetHashCode()
{
// as above
}
}

This is is what I went for, which outputs 1 as desired:
private class CustomTuple : Tuple<List<string>, DateTime?>
{
public CustomTuple(IEnumerable<string> strings, DateTime? time)
: base(strings.OrderBy(x => x).ToList(), time)
{
}
public override bool Equals(object obj)
{
if (obj == null || GetType() != obj.GetType())
{
return false;
}
var that = (CustomTuple) obj;
if (Item1 == null && that.Item1 != null || Item1 != null && that.Item1 == null) return false;
if (Item2 == null && that.Item2 != null || Item2 != null && that.Item2 == null) return false;
if (!Item2.Equals(that.Item2)) return false;
if (that.Item1.Count != Item1.Count) return false;
for (int i = 0; i < Item1.Count; i++)
{
if (!Item1[i].Equals(that.Item1[i])) return false;
}
return true;
}
public override int GetHashCode()
{
int hash = 17;
hash = hash*23 + Item2.GetHashCode();
return Item1.Aggregate(hash, (current, s) => current*23 + s.GetHashCode());
}
}

Intersection of List of List

I have a list of lists which looks like the following
public class FilteredVM
{
public int ID { get; set; }
public string Name { get; set; }
public string Number { get; set; }
}
List<List<FilteredVM>> groupedExpressionResults = new List<List<FilteredVM>>();
I would like to Intersect the lists within this list based upon the ID's, whats the best way to tackle this?

Here's an optimized extension method:
public static HashSet<T> IntersectAll<T>(this IEnumerable<IEnumerable<T>> series, IEqualityComparer<T> equalityComparer = null)
{
if (series == null)
throw new ArgumentNullException("series");
HashSet<T> set = null;
foreach (var values in series)
{
if (set == null)
set = new HashSet<T>(values, equalityComparer ?? EqualityComparer<T>.Default);
else
set.IntersectWith(values);
}
return set ?? new HashSet<T>();
}
Use this with the following comparer:
public class FilteredVMComparer : IEqualityComparer<FilteredVM>
{
public static readonly FilteredVMComparer Instance = new FilteredVMComparer();
private FilteredVMComparer()
{
}
public bool Equals(FilteredVM x, FilteredVM y)
{
return x.ID == y.ID;
}
public int GetHashCode(FilteredVM obj)
{
return obj.ID;
}
}
Like that:
series.IntersectAll(FilteredVMComparer.Instance)
You could just write
series.Aggregate((a, b) => a.Intersect(b, FilteredVMComparer.Instance))
but it 'd be wasteful because it'd have to construct multiple sets.

Intersect will work when the type are dead equals, which in your case won't apply because you haven't implemented the GetHashCode and Equals methods, which is the best and complete way.
Thus, If you only intended to take elements that contains in both lists, than the following solution will suit you right.
Assuming list1 and list2 are type List<FilteredVM> than, The most simple way, will be doing this:
var intersectByIDs = list1.Where(elem => list2.Any(elem2 => elem2.ID == elem.ID));

If you are a fan of one-liner solutions you can use this:
List<FilteredVM> result = groupedExpressionResults.Aggregate((x, y) => x.Where(xi => y.Select(yi => yi.ID).Contains(xi.ID)).ToList());
And if you just want the IDs you can just add .Select(x => x.ID), like this:
var ids = groupedExpressionResults.Aggregate((x, y) => x.Where(xi => y.Select(yi => yi.ID).Contains(xi.ID)).ToList()).Select(x => x.ID);
Working Demo

Is there a way to reduce amount of boilerplate code in Equals and GetHashCode?

I frequently have to override Equals and GetHashCode methods for the purpose of unit testing. After this my classes begin to look like this:
public class TestItem
{
public bool BoolValue { get; set; }
public DateTime DateTimeValue { get; set; }
public double DoubleValue { get; set; }
public long LongValue { get; set; }
public string StringValue { get; set; }
public SomeEnumType EnumValue { get; set; }
public decimal? NullableDecimal { get; set; }
public override bool Equals(object obj)
{
var other = obj as TestItem;
if (other == null)
{
return false;
}
if (object.ReferenceEquals(this, other))
{
return true;
}
return this.BoolValue == other.BoolValue
&& this.DateTimeValue == other.DateTimeValue
&& this.DoubleValue == other.DoubleValue // that's not a good way, but it's ok for demo
&& this.EnumValue == other.EnumValue
&& this.LongValue == other.LongValue
&& this.StringValue == other.StringValue
&& this.EnumValue == other.EnumValue
&& this.NullableDecimal == other.NullableDecimal;
}
public override int GetHashCode()
{
return this.BoolValue.GetHashCode()
^ this.DateTimeValue.GetHashCode()
^ this.DoubleValue.GetHashCode()
^ this.EnumValue.GetHashCode()
^ this.LongValue.GetHashCode()
^ this.NullableDecimal.GetHashCode()
^ (this.StringValue != null ? this.StringValue.GetHashCode() : 0);
}
}
While it's not hard to do it, time after time it gets boring and error prone to maintain list of same fields in Equals and GetHashCode. Is there any way to list filelds used for equality checking and hash code function only once? Equals and GetHashCode should be implemented in terms of this setup list.
In my imagination configuration and usage of such setup list may look like
public class TestItem
{
// same properties as before
private static readonly EqualityFieldsSetup Setup = new EqualityFieldsSetup<TestItem>()
.Add(o => o.BoolValue)
.Add(o => o.DateTimeValue)
// ... and so on
// or even .Add(o => o.SomeFunction())
public override bool Equals(object obj)
{
return Setup.Equals(this, obj);
}
public override int GetHashCode()
{
return Setup.GetHashCode(this);
}
}
There's a way to auto implement hashCode and equals in java, project lombok for example. I wonder is there anything serving the purpose of reducing boilerplate code readily available for C#.

I think it would be possible to implement pretty much the same thing as Lombok in C#, but I'm not feeling that ambitious at the moment.
I believe this is what you are after though (pretty much exactly as you have described it). This implementation does box all value types into objects, so it's not the most efficient implementation, but it should be good enough for your purpose of unit tests.
public class EqualityFieldsSetup<T>
where T : class
{
private List<Func<T, object>> _propertySelectors;
public EqualityFieldsSetup()
{
_propertySelectors = new List<Func<T, object>>();
}
public EqualityFieldsSetup<T> Add(Func<T, object> propertySelector)
{
_propertySelectors.Add(propertySelector);
return this;
}
public bool Equals(T objA, object other)
{
//If both are null, then they are equal
// (a condition I think you missed)
if (objA == null && other == null)
return true;
T objB = other as T;
if (objB == null)
return false;
if (object.ReferenceEquals(objA, objB))
return true;
foreach (Func<T, object> propertySelector in _propertySelectors)
{
object objAProperty = propertySelector.Invoke(objA);
object objBProperty = propertySelector.Invoke(objB);
//If both are null, then they are equal
// move on to the next property
if (objAProperty == null && objBProperty == null)
continue;
//Boxing requires the use of Equals() instead of '=='
if (objAProperty == null && objBProperty != null ||
!objAProperty.Equals(objBProperty))
return false;
}
return true;
}
public int GetHashCode(T obj)
{
int hashCode = 0;
foreach (Func<T, object> propertySelector in _propertySelectors)
{
object objProperty = propertySelector.Invoke(obj);
if (objProperty != null)
hashCode ^= objProperty.GetHashCode();
}
return hashCode;
}
}

I've done some research and found several components that were not quite what I wanted:
EqualityComparer (nuget) - does not seem to provide meaningful GetHashCode() by default and too heavyweight to my taste.
AnonymousComparer (nuget) - does not support GetHashCode() composition.
MemberwiseEqualityComparer - requires adding custom attribute to exclude member from comparison this way it's not possible to flexibly configure comparisons for existing types. Personally doing Emit for this task is a little bit overkill.
System.DataStructures.FuncComparer (nuget) - does not support composition.
And also a couple of related discussions:
How to quickly check if two data transfer objects have equal properties in C#?
Is there a better way to implment Equals for object with lots of fields?
So far idea of having explicitly configured list of members seemed unique. And I implemented my own library https://github.com/msugakov/YetAnotherEqualityComparer. It's better than the code suggested by TylerOhlsen in that it does not box extracted members and it uses EqualityComparer<T> to compare members.
Now the code looks like:
public class TestItem
{
private static readonly MemberEqualityComparer<TestItem> Comparer = new MemberEqualityComparer<TestItem>()
.Add(o => o.BoolValue)
.Add(o => o.DateTimeValue)
.Add(o => o.DoubleValue) // IEqualityComparer<double> can (and should) be specified here
.Add(o => o.EnumValue)
.Add(o => o.LongValue)
.Add(o => o.StringValue)
.Add(o => o.NullableDecimal);
// property list is the same
public override bool Equals(object obj)
{
return Comparer.Equals(this, obj);
}
public override int GetHashCode()
{
return Comparer.GetHashCode(this);
}
}
Also the MemberEqualityComparer implements IEqualityComparer<T> and follows its semantics: it can successfully compare default(T) which may be null for reference types and Nullables.
UPDATE: There are tools that can solve the same problem of creating member based IEqualityComparer<T> but also these can provide composite IComparer<T>!
Comparers (by Stephen Cleary) (nuget).
ComparerExtensions (nuget).

How to use the IEqualityComparer

I have some bells in my database with the same number. I want to get all of them without duplication. I created a compare class to do this work, but the execution of the function causes a big delay from the function without distinct, from 0.6 sec to 3.2 sec!
Am I doing it right or do I have to use another method?
reg.AddRange(
(from a in this.dataContext.reglements
join b in this.dataContext.Clients on a.Id_client equals b.Id
where a.date_v <= datefin && a.date_v >= datedeb
where a.Id_client == b.Id
orderby a.date_v descending
select new Class_reglement
{
nom = b.Nom,
code = b.code,
Numf = a.Numf,
})
.AsEnumerable()
.Distinct(new Compare())
.ToList());
class Compare : IEqualityComparer<Class_reglement>
{
public bool Equals(Class_reglement x, Class_reglement y)
{
if (x.Numf == y.Numf)
{
return true;
}
else { return false; }
}
public int GetHashCode(Class_reglement codeh)
{
return 0;
}
}

Your GetHashCode implementation always returns the same value. Distinct relies on a good hash function to work efficiently because it internally builds a hash table.
When implementing interfaces of classes it is important to read the documentation, to know which contract you’re supposed to implement.1
In your code, the solution is to forward GetHashCode to Class_reglement.Numf.GetHashCode and implement it appropriately there.
Apart from that, your Equals method is full of unnecessary code. It could be rewritten as follows (same semantics, ¼ of the code, more readable):
public bool Equals(Class_reglement x, Class_reglement y)
{
return x.Numf == y.Numf;
}
Lastly, the ToList call is unnecessary and time-consuming: AddRange accepts any IEnumerable so conversion to a List isn’t required. AsEnumerable is also redundant here since processing the result in AddRange will cause this anyway.
1 Writing code without knowing what it actually does is called cargo cult programming. It’s a surprisingly widespread practice. It fundamentally doesn’t work.

Try This code:
public class GenericCompare<T> : IEqualityComparer<T> where T : class
{
private Func<T, object> _expr { get; set; }
public GenericCompare(Func<T, object> expr)
{
this._expr = expr;
}
public bool Equals(T x, T y)
{
var first = _expr.Invoke(x);
var sec = _expr.Invoke(y);
if (first != null && first.Equals(sec))
return true;
else
return false;
}
public int GetHashCode(T obj)
{
return obj.GetHashCode();
}
}
Example of its use would be
collection = collection
.Except(ExistedDataEles, new GenericCompare<DataEle>(x=>x.Id))
.ToList();

If you want a generic solution that creates an IEqualityComparer for your class based on a property (which acts as a key) of that class have a look at this:
public class KeyBasedEqualityComparer<T, TKey> : IEqualityComparer<T>
{
private readonly Func<T, TKey> _keyGetter;
public KeyBasedEqualityComparer(Func<T, TKey> keyGetter)
{
if (default(T) == null)
{
_keyGetter = (x) => x == null ? default : keyGetter(x);
}
else
{
_keyGetter = keyGetter;
}
}
public bool Equals(T x, T y)
{
return EqualityComparer<TKey>.Default.Equals(_keyGetter(x), _keyGetter(y));
}
public int GetHashCode(T obj)
{
TKey key = _keyGetter(obj);
return key == null ? 0 : key.GetHashCode();
}
}
public static class KeyBasedEqualityComparer<T>
{
public static KeyBasedEqualityComparer<T, TKey> Create<TKey>(Func<T, TKey> keyGetter)
{
return new KeyBasedEqualityComparer<T, TKey>(keyGetter);
}
}
For better performance with structs there isn't any boxing.
Usage is like this:
IEqualityComparer<Class_reglement> equalityComparer =
KeyBasedEqualityComparer<Class_reglement>.Create(x => x.Numf);

Just code, with implementation of GetHashCode and NULL validation:
public class Class_reglementComparer : IEqualityComparer<Class_reglement>
{
public bool Equals(Class_reglement x, Class_reglement y)
{
if (x is null || y is null))
return false;
return x.Numf == y.Numf;
}
public int GetHashCode(Class_reglement product)
{
//Check whether the object is null
if (product is null) return 0;
//Get hash code for the Numf field if it is not null.
int hashNumf = product.hashNumf == null ? 0 : product.hashNumf.GetHashCode();
return hashNumf;
}
}
Example:
list of Class_reglement distinct by Numf
List<Class_reglement> items = items.Distinct(new Class_reglementComparer());

The purpose of this answer is to improve on previous answers by:
making the lambda expression optional in the constructor so that full object equality can be checked by default, not just on one of the properties.
operating on different types of classes, even complex types including sub-objects or nested lists. And not only on simple classes comprising only primitive type properties.
Not taking into account possible list container differences.
Here, you'll find a first simple code sample that works only on simple types (the ones composed only by primitif properties), and a second one that is complete (for a wider range of classes and complex types).
Here is my 2 pennies try:
public class GenericEqualityComparer<T> : IEqualityComparer<T> where T : class
{
private Func<T, object> _expr { get; set; }
public GenericEqualityComparer() => _expr = null;
public GenericEqualityComparer(Func<T, object> expr) => _expr = expr;
public bool Equals(T x, T y)
{
var first = _expr?.Invoke(x) ?? x;
var sec = _expr?.Invoke(y) ?? y;
if (first == null && sec == null)
return true;
if (first != null && first.Equals(sec))
return true;
var typeProperties = typeof(T).GetProperties();
foreach (var prop in typeProperties)
{
var firstPropVal = prop.GetValue(first, null);
var secPropVal = prop.GetValue(sec, null);
if (firstPropVal != null && !firstPropVal.Equals(secPropVal))
return false;
}
return true;
}
public int GetHashCode(T obj) =>
_expr?.Invoke(obj).GetHashCode() ?? obj.GetHashCode();
}
I know we can still optimize it (and maybe use a recursive?)..
But that is working like a charm without this much complexity and on a wide range of classes. ;)
Edit: After a day, here is my $10 attempt:
First, in a separate static extension class, you'll need:
public static class CollectionExtensions
{
public static bool HasSameLengthThan<T>(this IEnumerable<T> list, IEnumerable<T> expected)
{
if (list.IsNullOrEmptyCollection() && expected.IsNullOrEmptyCollection())
return true;
if ((list.IsNullOrEmptyCollection() && !expected.IsNullOrEmptyCollection()) || (!list.IsNullOrEmptyCollection() && expected.IsNullOrEmptyCollection()))
return false;
return list.Count() == expected.Count();
}
/// <summary>
/// Used to find out if a collection is empty or if it contains no elements.
/// </summary>
/// <typeparam name="T">Type of the collection's items.</typeparam>
/// <param name="list">Collection of items to test.</param>
/// <returns><c>true</c> if the collection is <c>null</c> or empty (without items), <c>false</c> otherwise.</returns>
public static bool IsNullOrEmptyCollection<T>(this IEnumerable<T> list) => list == null || !list.Any();
}
Then, here is the updated class that works on a wider range of classes:
public class GenericComparer<T> : IEqualityComparer<T> where T : class
{
private Func<T, object> _expr { get; set; }
public GenericComparer() => _expr = null;
public GenericComparer(Func<T, object> expr) => _expr = expr;
public bool Equals(T x, T y)
{
var first = _expr?.Invoke(x) ?? x;
var sec = _expr?.Invoke(y) ?? y;
if (ObjEquals(first, sec))
return true;
var typeProperties = typeof(T).GetProperties();
foreach (var prop in typeProperties)
{
var firstPropVal = prop.GetValue(first, null);
var secPropVal = prop.GetValue(sec, null);
if (!ObjEquals(firstPropVal, secPropVal))
{
var propType = prop.PropertyType;
if (IsEnumerableType(propType) && firstPropVal is IEnumerable && !ArrayEquals(firstPropVal, secPropVal))
return false;
if (propType.IsClass)
{
if (!DeepEqualsFromObj(firstPropVal, secPropVal, propType))
return false;
if (!DeepObjEquals(firstPropVal, secPropVal))
return false;
}
}
}
return true;
}
public int GetHashCode(T obj) =>
_expr?.Invoke(obj).GetHashCode() ?? obj.GetHashCode();
#region Private Helpers
private bool DeepObjEquals(object x, object y) =>
new GenericComparer<object>().Equals(x, y);
private bool DeepEquals<U>(U x, U y) where U : class =>
new GenericComparer<U>().Equals(x, y);
private bool DeepEqualsFromObj(object x, object y, Type type)
{
dynamic a = Convert.ChangeType(x, type);
dynamic b = Convert.ChangeType(y, type);
return DeepEquals(a, b);
}
private bool IsEnumerableType(Type type) =>
type.GetInterface(nameof(IEnumerable)) != null;
private bool ObjEquals(object x, object y)
{
if (x == null && y == null) return true;
return x != null && x.Equals(y);
}
private bool ArrayEquals(object x, object y)
{
var firstList = new List<object>((IEnumerable<object>)x);
var secList = new List<object>((IEnumerable<object>)y);
if (!firstList.HasSameLengthThan(secList))
return false;
var elementType = firstList?.FirstOrDefault()?.GetType();
int cpt = 0;
foreach (var e in firstList)
{
if (!DeepEqualsFromObj(e, secList[cpt++], elementType))
return false;
}
return true;
}
#endregion Private Helpers
We can still optimize it but it worth give it a try ^^.

The inclusion of your comparison class (or more specifically the AsEnumerable call you needed to use to get it to work) meant that the sorting logic went from being based on the database server to being on the database client (your application). This meant that your client now needs to retrieve and then process a larger number of records, which will always be less efficient that performing the lookup on the database where the approprate indexes can be used.
You should try to develop a where clause that satisfies your requirements instead, see Using an IEqualityComparer with a LINQ to Entities Except clause for more details.

IEquatable<T> can be a much easier way to do this with modern frameworks.
You get a nice simple bool Equals(T other) function and there's no messing around with casting or creating a separate class.
public class Person : IEquatable<Person>
{
public Person(string name, string hometown)
{
this.Name = name;
this.Hometown = hometown;
}
public string Name { get; set; }
public string Hometown { get; set; }
// can't get much simpler than this!
public bool Equals(Person other)
{
return this.Name == other.Name && this.Hometown == other.Hometown;
}
public override int GetHashCode()
{
return Name.GetHashCode(); // see other links for hashcode guidance
}
}
Note you DO have to implement GetHashCode if using this in a dictionary or with something like Distinct.
PS. I don't think any custom Equals methods work with entity framework directly on the database side (I think you know this because you do AsEnumerable) but this is a much simpler method to do a simple Equals for the general case.
If things don't seem to be working (such as duplicate key errors when doing ToDictionary) put a breakpoint inside Equals to make sure it's being hit and make sure you have GetHashCode defined (with override keyword).

Should I Overload == Operator?

How does the == operator really function in C#? If it used to compare objects of class A, will it try to match all of A's properties, or will it look for pointers to the same memory location (or maybe something else)?
Let's create a hypothetical example. I'm writing an application that utilizes the Twitter API, and it has a Tweet class, which has all the properties of a single tweet: text, sender, date&time, source, etc. If I want to compare objects of class Tweet for equivalence, can I just use:
Tweet a, b;
if (a == b)
{
//do something...
}
Will that check for equivalence of all the properties of the Tweet class between a and b?
If not, would the correct approach be to overload the == operator to explicitly check for equivalence of all the fields?
UPDATE: From the first two answers, am I right in assuming:
If the == operator or Equals method is not overloaded for a class, the == operator for the object class is used.
The == operator for the object class checks for equality in memory location.
I have to overload the == operator or the Equals method to accomplish this task.
In the overload, I have to check for equivalence in properties manually, so there is no way to do it semi-automatically, say, in a loop, right?
UPDATE #2: Yuriy made a comment that it is possible to do check for equivalence in properties in the == operator with reflection. How can this be done? Could you give me some sample code? Thanks!

For reference types, the default implementations of both the == operator and the Equals() method will simply check that both objects have the same reference, and are therefore the same instance.
If you want to check the contents of two different objects are equal then you must write the code to do it yourself, one way or another. It would be possible to do with reflection (the MbUnit test framework does something along these lines) but with a heavy performance penalty and a good chance that it wouldn't do quite what you expected anyway, and you should implement == or Equals and GetHashCode by hand.

MSDN has a good example of how to do it:
public override bool Equals(object o)
{
try
{
return (bool) (this == (DBBool) o);
}
catch
{
return false;
}
}
Then you overload the == and !=:
// Equality operator. Returns dbNull if either operand is dbNull,
// otherwise returns dbTrue or dbFalse:
public static DBBool operator ==(DBBool x, DBBool y)
{
if (x.value == 0 || y.value == 0) return dbNull;
return x.value == y.value? dbTrue: dbFalse;
}
// Inequality operator. Returns dbNull if either operand is
// dbNull, otherwise returns dbTrue or dbFalse:
public static DBBool operator !=(DBBool x, DBBool y)
{
if (x.value == 0 || y.value == 0) return dbNull;
return x.value != y.value? dbTrue: dbFalse;
}
And don't forget to overload the GetHash method.
Edit:
I wrote the following quick sample for using reflection in a compare. This would have to be much more comprehensive, I might try doing a blog on it if people want me to:
public class TestEquals
{
private int _x;
public TestEquals(int x)
{
this._x = x;
}
public override bool Equals(object obj)
{
TestEquals te = (TestEquals)obj;
if (te == null) return false;
foreach (var field in typeof(TestEquals)
.GetFields(BindingFlags.NonPublic | BindingFlags.Instance))
{
if (!field.GetValue(this).Equals(field.GetValue(te)))
return false;
}
return true;
}
}

The proper approach is the overload the equals method of the Tweet class in addition to the == operator, as described here.

Will that check for equivalence of all the properties of the Tweet class between a and b?
No
If not, would the correct approach be to overload the == operator to explicitly check for equivalence of all the fields?
You can either overload the == operator, or overload the Equals function.
Edit
#Yuriy gave a good example for compating all the non public variables. Since i also wrote an example, here it is (mine compares properties)
class TwitterItem
{
private string myValue = "default value";
public string Value1
{
get { return myValue; }
set { myValue = value; }
}
public string Value2
{
get { return myValue; }
set { myValue = value; }
}
public string Value3
{
get { return myValue; }
set { myValue = value; }
}
public override bool Equals(object obj)
{
if (base.Equals(obj)) return true;
Type type = typeof(TwitterItem);
PropertyInfo[] properties = type.GetProperties();
foreach (PropertyInfo property in properties)
{
if (false == property.GetValue(this, null).Equals(property.GetValue(obj, null)))
return false;
}
return true;
}
}

You can compare the properties using reflection:
var a = new Entity() { Name = "test", ID = "1" };
var b = new Entity() { Name = "test", ID = "1" };
var c = new Entity() { Name = "test", ID = "2" };
System.Diagnostics.Debug.WriteLine(a.Equals(b));//Returns true
System.Diagnostics.Debug.WriteLine(a.Equals(c));//Returns false
public class Entity
{
public string Name { get; set; }
public string ID { get; set; }
public override bool Equals(object obj)
{
var t = obj.GetType();
foreach (var p in t.GetProperties())
{
if (t.GetProperty(p.Name).GetValue(obj, null) != t.GetProperty(p.Name).GetValue(this, null))
return false;
}
return true;
}
}