CODEWITHSUNDEEP
c#
Scott Baker
Scott Baker

Reputation: 10443

The "is" keyword: there a difference in these methods?

Not really sure about the is keyword, but I think these two are the same. Can anyone confirm this?

public bool Equals(Object obj)
{ 
   if (obj == null) return false;
   MyType t = (MyType)obj;
   if (t == null) return false;
   return true;
}

and the other is...

public bool Equals(Object obj)
{ 
   return obj is MyType;
}

Upvotes: 2

Views: 114

Answers (5)

David Basarab
David Basarab

Reputation: 73311

An is expression evaluates to true if both of the following conditions are met:

expression is not null.

expression can be cast to type.

That is, a cast expression of the form (type)(expression) will complete without throwing an exception. For more information, see 7.6.6 Cast expressions. A compile-time warning will be issued if the expression expression is type is known to always be true or always be false.

From MSDN

The methods are not the same. If obj is not of MyType it will throw an exception while is will not, it will return false.

Upvotes: 1

Hans Passant
Hans Passant

Reputation: 941635

Not the same, the 1st version bombs with an InvalidCastException if an object of an unrelated type is passed.

Equals() is supposed to test for value equality, not type equality.

Upvotes: 1

Paul Bellora
Paul Bellora

Reputation: 55223

In the first method, you risk the cast failing and throwing an exception. I think you might be mixed up with the as keyword, which returns null if the cast fails instead. Once that's fixed, the methods would be roughly equivalent:

public bool Equals(Object obj)
{ 
   return (obj as MyType) != null;
}

But the latter is more concise and clear anyway.

Upvotes: 2

svick
svick

Reputation: 244837

They are not the same. If obj is not MyType, the first version throws an exception, the second version returns false.

And this implementation of Equals() is quite strange. Are you sure you want to consider all instances of MyType as equal?

Upvotes: 2

Mehrdad Afshari
Mehrdad Afshari

Reputation: 422026

No, they are definitely not equal. The cast to (MyType) will throw InvalidCastException immediately if obj isn't an instance of MyType. The expression will not evaluate to null if the cast fails. as operator exhibits the behavior you are expecting from that cast. My answer to another question discusses this in a bit more detail.

Upvotes: 4

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