Develop Reference

Why ! operator works while == false doesn't in this case? [duplicate]

The following works as expected:
dynamic foo = GetFoo();
if (foo != null)
{
if (foo is Foo i)
{
Console.WriteLine(i.Bar);
}
}
but if I combine the if statements like so:
if (foo != null && foo is Foo i)
{
Console.WriteLine(i.Bar);
}
then I receive a compiler warning
Use of unassigned local variable 'i'
Can anyone explain why this happens?

It would appear that this is not, in fact, a compiler error.
It was previously reported as a bug here.
However, it has been closed as not a bug. The reason is because of this part of the C# language spec (note: I am quoting here from user gafter on GitHub - this is NOT original content from myself):
If an operand of a conditional logical operator has the compile-time type dynamic, then the expression is dynamically bound (Dynamic binding). In this case the compile-time type of the expression is dynamic, and the resolution described below will take place at run-time using the run-time type of those operands that have the compile-time type dynamic.
Specifically, the && operation is not a compile-time boolean short-circuiting operation because its right-hand operand is of type dynamic.
Subtle stuff, and as DavidG says above, another reason to avoid dynamic where possible! (And I must confess, I'm still not completely convinced it's not a bug, but that's just me not understanding everything I guess...)

Ok, I was thinking for some time about this issue, and it looks that compiler behavior is pretty damn correct and it's not so hard to reproduce it even without dynamic values.
Some strange code will be required, nevertheless.
To start with we will overload && operator for our Foo type. It's not possible to overload short circuit logical operators directly, so we will overload true, false and & separately.
public static bool operator true(Foo x) => true;
public static bool operator false(Foo x) => true;
public static Foo operator &(Foo foo, Foo val) => new Foo();
Initially we had an expression foo != null && foo is Foo i in if block, now we want && from it to bound to our overload. For this reason we will overload != operator and == as well as they should be paired always.
public static Foo operator !=(Foo val, Foo val2) => new Foo();
public static Foo operator ==(Foo val, Foo val2) => new Foo();
For now foo != null evaluates to Foo and foo is Foo evaluates to bool, but our && overload has signature (Foo, Foo) — still a mismatch, will add one more overload for implicit conversion from bool.
public static implicit operator Foo(bool val) => new Foo();
Here is the code for Foo type we've got so far
class Foo
{
public static bool operator true(Foo x) => true;
public static bool operator false(Foo x) => true;
public static Foo operator &(Foo foo, Foo val) => new Foo();
public static implicit operator Foo(bool val) => new Foo();
public static Foo operator !=(Foo val, Foo val2) => new Foo();
public static Foo operator ==(Foo val, Foo val2) => new Foo();
}
And voila! We have the same error for this piece.
static void Main(string[] args)
{
Foo foo = GetFoo();
if (foo != null && foo is Foo i)
{
// Use of unassigned local variable i
// Local variable 'i' might not be initialized before accessing
Console.WriteLine(i);
}
}
static Foo GetFoo() => new Foo();
And indeed, if we for example use foo is string i instead of foo is Foo i, i won't be initialized at runtime, but we will be inside if block.
Initial issue is rather equivalent, because of dynamic values involved. foo != null is dynamic, as far as foo is dynamic, so it means && should be bound at runtime, and we have no guarantees that i will be initialized.
Looks like Matthew quoted the same thing from github issue, but I personally wasn't able to grasp it from the beginning.

Generic Equality Comparer [duplicate]

According to the documentation of the == operator in MSDN,
For predefined value types, the
equality operator (==) returns true if
the values of its operands are equal,
false otherwise. For reference types
other than string, == returns true if
its two operands refer to the same
object. For the string type, ==
compares the values of the strings.
User-defined value types can overload
the == operator (see operator). So can
user-defined reference types, although
by default == behaves as described
above for both predefined and
user-defined reference types.
So why does this code snippet fail to compile?
bool Compare<T>(T x, T y) { return x == y; }
I get the error Operator '==' cannot be applied to operands of type 'T' and 'T'. I wonder why, since as far as I understand the == operator is predefined for all types?
Edit: Thanks, everybody. I didn't notice at first that the statement was about reference types only. I also thought that bit-by-bit comparison is provided for all value types, which I now know is not correct.
But, in case I'm using a reference type, would the == operator use the predefined reference comparison, or would it use the overloaded version of the operator if a type defined one?
Edit 2: Through trial and error, we learned that the == operator will use the predefined reference comparison when using an unrestricted generic type. Actually, the compiler will use the best method it can find for the restricted type argument, but will look no further. For example, the code below will always print true, even when Test.test<B>(new B(), new B()) is called:
class A { public static bool operator==(A x, A y) { return true; } }
class B : A { public static bool operator==(B x, B y) { return false; } }
class Test { void test<T>(T a, T b) where T : A { Console.WriteLine(a == b); } }

As others have said, it will only work when T is constrained to be a reference type. Without any constraints, you can compare with null, but only null - and that comparison will always be false for non-nullable value types.
Instead of calling Equals, it's better to use an IComparer<T> - and if you have no more information, EqualityComparer<T>.Default is a good choice:
public bool Compare<T>(T x, T y)
{
return EqualityComparer<T>.Default.Equals(x, y);
}
Aside from anything else, this avoids boxing/casting.

"...by default == behaves as described above for both predefined and user-defined reference types."
Type T is not necessarily a reference type, so the compiler can't make that assumption.
However, this will compile because it is more explicit:
bool Compare<T>(T x, T y) where T : class
{
return x == y;
}
Follow up to additional question, "But, in case I'm using a reference type, would the the == operator use the predefined reference comparison, or would it use the overloaded version of the operator if a type defined one?"
I would have thought that == on the Generics would use the overloaded version, but the following test demonstrates otherwise. Interesting... I'd love to know why! If someone knows please share.
namespace TestProject
{
class Program
{
static void Main(string[] args)
{
Test a = new Test();
Test b = new Test();
Console.WriteLine("Inline:");
bool x = a == b;
Console.WriteLine("Generic:");
Compare<Test>(a, b);
}
static bool Compare<T>(T x, T y) where T : class
{
return x == y;
}
}
class Test
{
public static bool operator ==(Test a, Test b)
{
Console.WriteLine("Overloaded == called");
return a.Equals(b);
}
public static bool operator !=(Test a, Test b)
{
Console.WriteLine("Overloaded != called");
return a.Equals(b);
}
}
}
Output
Inline:
Overloaded == called
Generic:
Press any key to continue . . .
Follow Up 2
I do want to point out that changing my compare method to
static bool Compare<T>(T x, T y) where T : Test
{
return x == y;
}
causes the overloaded == operator to be called. I guess without specifying the type (as a where), the compiler can't infer that it should use the overloaded operator... though I'd think that it would have enough information to make that decision even without specifying the type.

In general, EqualityComparer<T>.Default.Equals should do the job with anything that implements IEquatable<T>, or that has a sensible Equals implementation.
If, however, == and Equals are implemented differently for some reason, then my work on generic operators should be useful; it supports the operator versions of (among others):
Equal(T value1, T value2)
NotEqual(T value1, T value2)
GreaterThan(T value1, T value2)
LessThan(T value1, T value2)
GreaterThanOrEqual(T value1, T value2)
LessThanOrEqual(T value1, T value2)

So many answers, and not a single one explains the WHY? (which Giovanni explicitly asked)...
.NET generics do not act like C++ templates. In C++ templates, overload resolution occurs after the actual template parameters are known.
In .NET generics (including C#), overload resolution occurs without knowing the actual generic parameters. The only information the compiler can use to choose the function to call comes from type constraints on the generic parameters.

The compile can't know T couldn't be a struct (value type). So you have to tell it it can only be of reference type i think:
bool Compare<T>(T x, T y) where T : class { return x == y; }
It's because if T could be a value type, there could be cases where x == y would be ill formed - in cases when a type doesn't have an operator == defined. The same will happen for this which is more obvious:
void CallFoo<T>(T x) { x.foo(); }
That fails too, because you could pass a type T that wouldn't have a function foo. C# forces you to make sure all possible types always have a function foo. That's done by the where clause.

It appears that without the class constraint:
bool Compare<T> (T x, T y) where T: class
{
return x == y;
}
One should realize that while class constrained Equals in the == operator inherits from Object.Equals, while that of a struct overrides ValueType.Equals.
Note that:
bool Compare<T> (T x, T y) where T: struct
{
return x == y;
}
also gives out the same compiler error.
As yet I do not understand why having a value type equality operator comparison is rejected by the compiler. I do know for a fact though, that this works:
bool Compare<T> (T x, T y)
{
return x.Equals(y);
}

Well in my case I wanted to unit-test the equality operator. I needed call the code under the equality operators without explicitly setting the generic type. Advises for EqualityComparer were not helpful as EqualityComparer called Equals method but not the equality operator.
Here is how I've got this working with generic types by building a LINQ. It calls the right code for == and != operators:
/// <summary>
/// Gets the result of "a == b"
/// </summary>
public bool GetEqualityOperatorResult<T>(T a, T b)
{
// declare the parameters
var paramA = Expression.Parameter(typeof(T), nameof(a));
var paramB = Expression.Parameter(typeof(T), nameof(b));
// get equality expression for the parameters
var body = Expression.Equal(paramA, paramB);
// compile it
var invokeEqualityOperator = Expression.Lambda<Func<T, T, bool>>(body, paramA, paramB).Compile();
// call it
return invokeEqualityOperator(a, b);
}
/// <summary>
/// Gets the result of "a =! b"
/// </summary>
public bool GetInequalityOperatorResult<T>(T a, T b)
{
// declare the parameters
var paramA = Expression.Parameter(typeof(T), nameof(a));
var paramB = Expression.Parameter(typeof(T), nameof(b));
// get equality expression for the parameters
var body = Expression.NotEqual(paramA, paramB);
// compile it
var invokeInequalityOperator = Expression.Lambda<Func<T, T, bool>>(body, paramA, paramB).Compile();
// call it
return invokeInequalityOperator(a, b);
}

There is an MSDN Connect entry for this here
Alex Turner's reply starts with:
Unfortunately, this behavior is by
design and there is not an easy
solution to enable use of == with type
parameters that may contain value
types.

If you want to make sure the operators of your custom type are called you can do so via reflection. Just get the type using your generic parameter and retrieve the MethodInfo for the desired operator (e.g. op_Equality, op_Inequality, op_LessThan...).
var methodInfo = typeof (T).GetMethod("op_Equality",
BindingFlags.Static | BindingFlags.Public);
Then execute the operator using the MethodInfo's Invoke method and pass in the objects as the parameters.
var result = (bool) methodInfo.Invoke(null, new object[] { object1, object2});
This will invoke your overloaded operator and not the one defined by the constraints applied on the generic parameter. Might not be practical, but could come in handy for unit testing your operators when using a generic base class that contains a couple of tests.

I wrote the following function looking at the latest msdn. It can easily compare two objects x and y:
static bool IsLessThan(T x, T y)
{
return ((IComparable)(x)).CompareTo(y) <= 0;
}

bool Compare(T x, T y) where T : class { return x == y; }
The above will work because == is taken care of in case of user-defined reference types.
In case of value types, == can be overridden. In which case, "!=" should also be defined.
I think that could be the reason, it disallows generic comparison using "==".

The .Equals() works for me while TKey is a generic type.
public virtual TOutputDto GetOne(TKey id)
{
var entity =
_unitOfWork.BaseRepository
.FindByCondition(x =>
!x.IsDelete &&
x.Id.Equals(id))
.SingleOrDefault();
// ...
}

I have 2 solutions and they're very simply.
Solution 1: Cast the generic typed variable to object and use == operator.
Example:
void Foo<T>(T t1, T t2)
{
object o1 = t1;
object o2 = t2;
if (o1 == o2)
{
// ...
// ..
// .
}
}
Solution 2: Use object.Equals(object, object) method.
Example:
void Foo<T>(T t1, T t2)
{
if (object.Equals(t1, t2)
{
// ...
// ..
// .
}
}

You can do this with C# 11 and .NET 7+:
static void Main()
{
Console.WriteLine(Compare(2, 2));
Console.WriteLine(Compare(2, 3));
}
static bool Compare<T>(T x, T y) where T : IEqualityOperators<T, T, bool>
{
return x == y;
}
(you may prefer to use where T : INumber<T>, which covers this scenario and a lot more, but it depends on your specific needs; not all equatable types are numbers)

Can one call `operator true()` explicitly? When is it used to convert to bool?

Out of curiosity, I explored overloading operators in C#. There is an interesting post by Eric Lippert discussing the design decisions in C++ and C# concerning operator&&. In C# its overloads are defined implicitly by overloading operator& plus overloading the boolean operators trueand false, which allows the language to preserve the sometimes essential short-circuit semantics.
Playing with the somewhat exotic operator true() and false() I found that I was not able to call them directly. Instead, they are implicitly called in certain places where a bool is required. As far as I can see, these are the language constructs which directly require a bool, namely the ternary conditionaly operator and the if clause, plus calls to operator&& resp. || when the argument type has overloaded operators & resp. |.
Edit: The book "The C# Programming Language" (7.11) as well as the annotated C# Standard in 14.11.2 -- both found via google search result -- have a code example with a direct operator call which I didn't understand must be pseudo code. I tried to replicate that.
As an aside, it is harder to provoke a call to operator false(); the ternary conditional as well as an if clause always test by calling operator true(). It seems as if the only way to call it is by calling operator||().
The motivation to call the boolean operators explicitly is that it would be nice to define only one of them directly and define the other one in terms of that, so that the definitions are always consistent. Below is a little example program with a few things I tried. Is there a syntax which I missed?
using System;
namespace TriviallyTrue
{
public class T
{
public static bool operator true(T t) { Console.WriteLine("In op. true"); return true; }
public static bool operator false(T t) { return true; }
}
class Program
{
static void Main(string[] args)
{
T t = new T();
// bool b = T.true(t); // Identifier expected; 'true' is a keyword
// ok, I see. Let's use the keyword syntax.
// bool b = T.#true(t); //'TriviallyTrue.T' does not contain a definition for 'true'
// That's so not true!
// oh. perhaps we need to use cast syntax, akin to invoking operator int()?
// bool b = (true)t; // ; expected
// hm. It's in T's namespace...
// bool b = (T.true)t; // Identifier expected;
// we know that.
// another cast try.
// bool b = (T.#true)t; // The type name 'true' does not exist in the type 'TriviallyTrue.T'
// ah, a type is expected. Well, the type is bool, right? But casting to bool
// doesn't work either, see below and in Main().
// bool b = (T.#bool)t; // The type name 'bool' does not exist in the type 'TriviallyTrue.T'
// well, it exists *some*what
if (t) // works
{
// Console.WriteLine("t was " + (bool)t); // Cannot convert type 'TriviallyTrue.T' to 'bool'
// That's so not true!
Console.WriteLine("t was " + (t ? "True" : "False" )); // works!
}
}
}
}
Sample session:
In op. true
In op. true
t was True

I cannot answer the question in the title, but I think I can cover this part
The motivation to call the boolean operators explicitly is that it would be nice to define only one of them directly and define the other one in terms of that, so that the definitions are always consistent.
Without questioning in any way what #Eric Lippert wrote in that post, C# has an easier way of doing all that when a one of the true or false is logically the inverse on the other, which is the most common practical case. Instead of overriding 4 operators (false, true, & and |), one can simply provide a single implicit conversion to bool.
For instance
public class T
{
public static implicit operator bool(T t) { return t != null; }
}
Now all these work
T a = new T(), b = null;
if (a) { }
if (!a) { }
if (b) { }
if (!b) { }
if (a && b) { }
if (b && a) { }
if (a & b) { }
if (a || b) { }
if (b || a) { }
if (a | b) { }
var c1 = a ? 1 : 0;
var c2 = b ? 1 : 0;

You can't call any operator methods explicitly in C#. operator true and operator false are no exception. It's just that most operator methods have a more straightforward method of invoking them implicitly.
If it makes sense to call the operator method in other scenarios than as an overload operator, provide it as a regular method instead. It's generally more readable and can nicely avoid the whole problem of having multiple separate implementations that you wanted to solve.
public class T
{
private bool asBoolean() { ... } // or even make it public
public static bool operator true(T t) { return t.asBoolean(); }
public static bool operator false(T t) { return !t.asBoolean(); }
}
For completeness, you can implement operator false as e.g.
public static bool operator false(T t) { return t ? false : true; }
but please don't unless you absolutely need to.

Direct Comparison Behavior in Generics [duplicate]

According to the documentation of the == operator in MSDN,
For predefined value types, the
equality operator (==) returns true if
the values of its operands are equal,
false otherwise. For reference types
other than string, == returns true if
its two operands refer to the same
object. For the string type, ==
compares the values of the strings.
User-defined value types can overload
the == operator (see operator). So can
user-defined reference types, although
by default == behaves as described
above for both predefined and
user-defined reference types.
So why does this code snippet fail to compile?
bool Compare<T>(T x, T y) { return x == y; }
I get the error Operator '==' cannot be applied to operands of type 'T' and 'T'. I wonder why, since as far as I understand the == operator is predefined for all types?
Edit: Thanks, everybody. I didn't notice at first that the statement was about reference types only. I also thought that bit-by-bit comparison is provided for all value types, which I now know is not correct.
But, in case I'm using a reference type, would the == operator use the predefined reference comparison, or would it use the overloaded version of the operator if a type defined one?
Edit 2: Through trial and error, we learned that the == operator will use the predefined reference comparison when using an unrestricted generic type. Actually, the compiler will use the best method it can find for the restricted type argument, but will look no further. For example, the code below will always print true, even when Test.test<B>(new B(), new B()) is called:
class A { public static bool operator==(A x, A y) { return true; } }
class B : A { public static bool operator==(B x, B y) { return false; } }
class Test { void test<T>(T a, T b) where T : A { Console.WriteLine(a == b); } }

As others have said, it will only work when T is constrained to be a reference type. Without any constraints, you can compare with null, but only null - and that comparison will always be false for non-nullable value types.
Instead of calling Equals, it's better to use an IComparer<T> - and if you have no more information, EqualityComparer<T>.Default is a good choice:
public bool Compare<T>(T x, T y)
{
return EqualityComparer<T>.Default.Equals(x, y);
}
Aside from anything else, this avoids boxing/casting.

"...by default == behaves as described above for both predefined and user-defined reference types."
Type T is not necessarily a reference type, so the compiler can't make that assumption.
However, this will compile because it is more explicit:
bool Compare<T>(T x, T y) where T : class
{
return x == y;
}
Follow up to additional question, "But, in case I'm using a reference type, would the the == operator use the predefined reference comparison, or would it use the overloaded version of the operator if a type defined one?"
I would have thought that == on the Generics would use the overloaded version, but the following test demonstrates otherwise. Interesting... I'd love to know why! If someone knows please share.
namespace TestProject
{
class Program
{
static void Main(string[] args)
{
Test a = new Test();
Test b = new Test();
Console.WriteLine("Inline:");
bool x = a == b;
Console.WriteLine("Generic:");
Compare<Test>(a, b);
}
static bool Compare<T>(T x, T y) where T : class
{
return x == y;
}
}
class Test
{
public static bool operator ==(Test a, Test b)
{
Console.WriteLine("Overloaded == called");
return a.Equals(b);
}
public static bool operator !=(Test a, Test b)
{
Console.WriteLine("Overloaded != called");
return a.Equals(b);
}
}
}
Output
Inline:
Overloaded == called
Generic:
Press any key to continue . . .
Follow Up 2
I do want to point out that changing my compare method to
static bool Compare<T>(T x, T y) where T : Test
{
return x == y;
}
causes the overloaded == operator to be called. I guess without specifying the type (as a where), the compiler can't infer that it should use the overloaded operator... though I'd think that it would have enough information to make that decision even without specifying the type.

In general, EqualityComparer<T>.Default.Equals should do the job with anything that implements IEquatable<T>, or that has a sensible Equals implementation.
If, however, == and Equals are implemented differently for some reason, then my work on generic operators should be useful; it supports the operator versions of (among others):
Equal(T value1, T value2)
NotEqual(T value1, T value2)
GreaterThan(T value1, T value2)
LessThan(T value1, T value2)
GreaterThanOrEqual(T value1, T value2)
LessThanOrEqual(T value1, T value2)

So many answers, and not a single one explains the WHY? (which Giovanni explicitly asked)...
.NET generics do not act like C++ templates. In C++ templates, overload resolution occurs after the actual template parameters are known.
In .NET generics (including C#), overload resolution occurs without knowing the actual generic parameters. The only information the compiler can use to choose the function to call comes from type constraints on the generic parameters.

The compile can't know T couldn't be a struct (value type). So you have to tell it it can only be of reference type i think:
bool Compare<T>(T x, T y) where T : class { return x == y; }
It's because if T could be a value type, there could be cases where x == y would be ill formed - in cases when a type doesn't have an operator == defined. The same will happen for this which is more obvious:
void CallFoo<T>(T x) { x.foo(); }
That fails too, because you could pass a type T that wouldn't have a function foo. C# forces you to make sure all possible types always have a function foo. That's done by the where clause.

It appears that without the class constraint:
bool Compare<T> (T x, T y) where T: class
{
return x == y;
}
One should realize that while class constrained Equals in the == operator inherits from Object.Equals, while that of a struct overrides ValueType.Equals.
Note that:
bool Compare<T> (T x, T y) where T: struct
{
return x == y;
}
also gives out the same compiler error.
As yet I do not understand why having a value type equality operator comparison is rejected by the compiler. I do know for a fact though, that this works:
bool Compare<T> (T x, T y)
{
return x.Equals(y);
}

Well in my case I wanted to unit-test the equality operator. I needed call the code under the equality operators without explicitly setting the generic type. Advises for EqualityComparer were not helpful as EqualityComparer called Equals method but not the equality operator.
Here is how I've got this working with generic types by building a LINQ. It calls the right code for == and != operators:
/// <summary>
/// Gets the result of "a == b"
/// </summary>
public bool GetEqualityOperatorResult<T>(T a, T b)
{
// declare the parameters
var paramA = Expression.Parameter(typeof(T), nameof(a));
var paramB = Expression.Parameter(typeof(T), nameof(b));
// get equality expression for the parameters
var body = Expression.Equal(paramA, paramB);
// compile it
var invokeEqualityOperator = Expression.Lambda<Func<T, T, bool>>(body, paramA, paramB).Compile();
// call it
return invokeEqualityOperator(a, b);
}
/// <summary>
/// Gets the result of "a =! b"
/// </summary>
public bool GetInequalityOperatorResult<T>(T a, T b)
{
// declare the parameters
var paramA = Expression.Parameter(typeof(T), nameof(a));
var paramB = Expression.Parameter(typeof(T), nameof(b));
// get equality expression for the parameters
var body = Expression.NotEqual(paramA, paramB);
// compile it
var invokeInequalityOperator = Expression.Lambda<Func<T, T, bool>>(body, paramA, paramB).Compile();
// call it
return invokeInequalityOperator(a, b);
}

There is an MSDN Connect entry for this here
Alex Turner's reply starts with:
Unfortunately, this behavior is by
design and there is not an easy
solution to enable use of == with type
parameters that may contain value
types.

If you want to make sure the operators of your custom type are called you can do so via reflection. Just get the type using your generic parameter and retrieve the MethodInfo for the desired operator (e.g. op_Equality, op_Inequality, op_LessThan...).
var methodInfo = typeof (T).GetMethod("op_Equality",
BindingFlags.Static | BindingFlags.Public);
Then execute the operator using the MethodInfo's Invoke method and pass in the objects as the parameters.
var result = (bool) methodInfo.Invoke(null, new object[] { object1, object2});
This will invoke your overloaded operator and not the one defined by the constraints applied on the generic parameter. Might not be practical, but could come in handy for unit testing your operators when using a generic base class that contains a couple of tests.

I wrote the following function looking at the latest msdn. It can easily compare two objects x and y:
static bool IsLessThan(T x, T y)
{
return ((IComparable)(x)).CompareTo(y) <= 0;
}

bool Compare(T x, T y) where T : class { return x == y; }
The above will work because == is taken care of in case of user-defined reference types.
In case of value types, == can be overridden. In which case, "!=" should also be defined.
I think that could be the reason, it disallows generic comparison using "==".

The .Equals() works for me while TKey is a generic type.
public virtual TOutputDto GetOne(TKey id)
{
var entity =
_unitOfWork.BaseRepository
.FindByCondition(x =>
!x.IsDelete &&
x.Id.Equals(id))
.SingleOrDefault();
// ...
}

I have 2 solutions and they're very simply.
Solution 1: Cast the generic typed variable to object and use == operator.
Example:
void Foo<T>(T t1, T t2)
{
object o1 = t1;
object o2 = t2;
if (o1 == o2)
{
// ...
// ..
// .
}
}
Solution 2: Use object.Equals(object, object) method.
Example:
void Foo<T>(T t1, T t2)
{
if (object.Equals(t1, t2)
{
// ...
// ..
// .
}
}

You can do this with C# 11 and .NET 7+:
static void Main()
{
Console.WriteLine(Compare(2, 2));
Console.WriteLine(Compare(2, 3));
}
static bool Compare<T>(T x, T y) where T : IEqualityOperators<T, T, bool>
{
return x == y;
}
(you may prefer to use where T : INumber<T>, which covers this scenario and a lot more, but it depends on your specific needs; not all equatable types are numbers)

How do you override operator == when using interfaces instead of actual types?

I have some code like this:
How should I implement the operator == so that it will be called when the variables are of interface IMyClass?
public class MyClass : IMyClass
{
public static bool operator ==(MyClass a, MyClass b)
{
if (ReferenceEquals(a, b))
return true;
if ((Object)a == null || (Object)b == null)
return false;
return false;
}
public static bool operator !=(MyClass a, MyClass b)
{
return !(a == b);
}
}
class Program
{
static void Main(string[] args)
{
IMyClass m1 = new MyClass();
IMyClass m2 = new MyClass();
MyClass m3 = new MyClass();
MyClass m4 = new MyClass();
Console.WriteLine(m1 == m2); // does not go into custom == function. why not?
Console.WriteLine(m3 == m4); // DOES go into custom == function
}
}

The key is that you're not overriding an operator - you're overloading it.
There's no operator defined for
operator ==(IMyClass x, IMyClass y)
so the compiler has nothing it could call. It can't call
operator ==(MyClass x, MyClass y)
as it doesn't know that m1 and m2 will actually refer to instance of MyClass.
As far as I know there's no way of implementing an operator to be used for interfaces - after all, multiple implementations could all provide their own one, just for one point of possible ambiguity.
Personally I'm somewhat wary of trying to talk about equality over non-sealed types to start with - equality and inheritance don't mix terribly nicely. That goes doubly for interfaces, of course :) You might be best implementing an appropriate IEqualityComparer<IMyClass> and using that instead.

Look in the accepted answer for this question: == vs. Object.Equals(object) in .NET
The difference is one of identity equality vs semantic equality. Overriding == is meant for structs that are supposed to be the 'same' because they have the same values, since structs are copied by value and therefore are never references to the same object.
Equals is used for a semantic check of value equality for Reference types. By default, the two operations are the same.

Try making the operator== function virtual.