Value equality

  • Thread starter Thread starter tshad
  • Start date Start date
T

tshad

I was reading about referential equality vs value equality and don't
understand why I am always getting value equality wrong on 2 objects that I
just created. The values should be the same (the referential equalities
should be different).

If I have:

Type type1 = typeof(MyClass2);
//Create an instance of the type
object obj = Activator.CreateInstance(type1);
object obj2 = new MyClass2();
object obj3 = new MyClass2();

I would have expected all 3 of these objects to be equal value wise. But
they aren't (at least not as I can tell).

If I do this right afterword:

if (obj == obj2)
textBox1.Text += "obj Referentially equal obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not Referentially equal obj2" +
Environment.NewLine;

if (obj.Equals(obj2))
textBox1.Text += "obj has value equality to obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not have value equality to obj2" +
Environment.NewLine;

if (obj2.Equals(obj3))
textBox1.Text += "obj2 has value equality to obj3" +
Environment.NewLine;
else
textBox1.Text += "obj2 not have value equality to obj3" +
Environment.NewLine;

I get not equal for all of these.

I expect the 1st one not to be equal, but why would the second and third one
be equal. Especially the third one where they are created exactly the same
way???

The results I get are:

obj not Referentially equal obj2
obj not have value equality to obj2
obj2 not have value equality to obj3

Why is that????

Thanks,

Tom
 
I was reading about referential equality vs value equality and don't
understand why I am always getting value equality wrong on 2 objects that I
just created. The values should be the same (the referential equalities
should be different).

If I have:

Type type1 = typeof(MyClass2);
//Create an instance of the type
object obj = Activator.CreateInstance(type1);
object obj2 = new MyClass2();
object obj3 = new MyClass2();

I would have expected all 3 of these objects to be equal value wise. But
they aren't (at least not as I can tell).

If I do this right afterword:

if (obj == obj2)
textBox1.Text += "obj Referentially equal obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not Referentially equal obj2" +
Environment.NewLine;

if (obj.Equals(obj2))
textBox1.Text += "obj has value equality to obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not have value equality to obj2" +
Environment.NewLine;

if (obj2.Equals(obj3))
textBox1.Text += "obj2 has value equality to obj3" +
Environment.NewLine;
else
textBox1.Text += "obj2 not have value equality to obj3" +
Environment.NewLine;

I get not equal for all of these.

I expect the 1st one not to be equal, but why would the second and third one
be equal. Especially the third one where they are created exactly the same
way???

The results I get are:

obj not Referentially equal obj2
obj not have value equality to obj2
obj2 not have value equality to obj3

Why is that????

Thanks,

Tom
Click to expand...

Unless you have overridden Equals, then the default of implementation of
Equals is to compare references... "Value Equality" is something you have to
implement :)
 
tshad said:
I was reading about referential equality vs value equality and don't
understand why I am always getting value equality wrong on 2 objects that I
just created. The values should be the same (the referential equalities
should be different).

If I have:

Type type1 = typeof(MyClass2);
//Create an instance of the type
object obj = Activator.CreateInstance(type1);
object obj2 = new MyClass2();
object obj3 = new MyClass2();

I would have expected all 3 of these objects to be equal value wise. But
they aren't (at least not as I can tell).

If I do this right afterword:

if (obj == obj2)
textBox1.Text += "obj Referentially equal obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not Referentially equal obj2" +
Environment.NewLine;

if (obj.Equals(obj2))
textBox1.Text += "obj has value equality to obj2" +
Environment.NewLine;
else
textBox1.Text += "obj not have value equality to obj2" +
Environment.NewLine;

if (obj2.Equals(obj3))
textBox1.Text += "obj2 has value equality to obj3" +
Environment.NewLine;
else
textBox1.Text += "obj2 not have value equality to obj3" +
Environment.NewLine;

I get not equal for all of these.

I expect the 1st one not to be equal, but why would the second and third
one be equal. Especially the third one where they are created exactly the
same way???

The results I get are:

obj not Referentially equal obj2
obj not have value equality to obj2
obj2 not have value equality to obj3

Why is that????

Thanks,

Tom
Click to expand...


Here is a class exemplifying Tom's answer.

public class MyClass2
{
private string Name { get; set; }
public override bool Equals(object obj)
{
if (obj.GetType() != this.GetType()) return false;
return this.Name == ((MyClass2) obj).Name;
}
public override int GetHashCode()
{
return this.Name.GetHashCode();
}
}
 
I would make the following changes...
public class MyClass2
{
Click to expand...

public string Name { get; private set; }

public override bool Equals(object obj)
{
Click to expand...

if (obj == null)
return false;
if (obj.GetType() != this.GetType()) return false;
return this.Name == ((MyClass2) obj).Name;
}
public override int GetHashCode()
{
Click to expand...

//Be aware that you'd have to check Name for != null
//in the object's constructor
 
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