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C# difference between == and Equals()


Question

I have a condition in a silverlight application that compares 2 strings, for some reason when I use == it returns false while .Equals() returns true.

Here is the code:

if (((ListBoxItem)lstBaseMenu.SelectedItem).Content.Equals("Energy Attack"))
{
    // Execute code
}

if (((ListBoxItem)lstBaseMenu.SelectedItem).Content == "Energy Attack")
{
    // Execute code
}

Any reason as to why this is happening?

2015/12/16
1
558
12/16/2015 9:27:17 AM

Accepted Answer

When == is used on an expression of type object, it'll resolve to System.Object.ReferenceEquals.

Equals is just a virtual method and behaves as such, so the overridden version will be used (which, for string type compares the contents).

2015/12/16
437
12/16/2015 9:30:16 AM


== and .Equals are both dependent upon the behavior defined in the actual type and the actual type at the call site. Both are just methods / operators which can be overridden on any type and given any behavior the author so desires. In my experience, I find it's common for people to implement .Equals on an object but neglect to implement operator ==. This means that .Equals will actually measure the equality of the values while == will measure whether or not they are the same reference.

When I'm working with a new type whose definition is in flux or writing generic algorithms, I find the best practice is the following

  • If I want to compare references in C#, I use Object.ReferenceEquals directly (not needed in the generic case)
  • If I want to compare values I use EqualityComparer<T>.Default

In some cases when I feel the usage of == is ambiguous I will explicitly use Object.Reference equals in the code to remove the ambiguity.

Eric Lippert recently did a blog post on the subject of why there are 2 methods of equality in the CLR. It's worth the read

2015/12/29

== Operator

  1. If operands are Value Types and their values are equal, it returns true else false.
  2. If operands are Reference Types with exception of string and both refer to the same instance (same object), it returns true else false.
  3. If operands are string type and their values are equal, it returns true else false.

.Equals

  1. If operands are Reference Types, it performs Reference Equality that is if both refer to the same instance (same object), it returns true else false.
  2. If Operands are Value Types then unlike == operator it checks for their type first and if their types are same it performs == operator else it returns false.
2020/01/20

Firstly, there is a difference. For numbers

> 2 == 2.0
True

> 2.Equals(2.0)
False

And for strings

> string x = null;
> x == null
True

> x.Equals(null)
NullReferenceException

In both cases, == behaves more usefully than .Equals

2013/08/15

As far as I understand it the answer is simple:

  1. == compares object references.
  2. .Equals compares object content.
  3. String datatypes always act like content comparison.

I hope I'm correct and that it answered your question.

2019/11/20

I would add that if you cast your object to a string then it will work correctly. This is why the compiler will give you a warning saying:

Possible unintended reference comparison; to get a value comparison, cast the left hand side to type 'string'

2018/04/12

Source: https://stackoverflow.com/questions/814878
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