c# compare two generic values

bernhardrusch_3056 0
Viewed 88885

Possible Duplicate:
Can’t operator == be applied to generic types in C#?

I've coded something like this:

public bool IsDataChanged()
{           
    T value1 = GetValue2;
    T value2 = GetValue1();

    return (valueInDB != valueFromView);
}

Right now the function doesn't compile with the error "Operator '!=' cannot be applied to operands of type 'T' and 'T'". What do I have to do to make this function work ?

generics c#
4 Answers

You cannot use operators on generic types (except for foo == null which is special cased) unless you add where T : class to indicate it is a reference type (then foo == bar is legal)

Use EqualityComparer<T>.Default to do it for you. This will not work on types which only supply an operator overload for == without also either:

  • implement IEquatable<T>
  • overrides object.Equals()

In general implementing the == operator and not also doing at least one of these would be a very bad idea anyway so this is not likely to be an issue.

public bool IsDataChanged<T>()
{           
    T value1 = GetValue2;
    T value2 = GetValue1();

    return !EqualityComparer<T>.Default.Equals(value1 , value2);
}

If you do not restrict to IEquatable<T> then the EqualityComparer default fallback may cause boxing when used with value types if they do not implement IEquatable<T> (if you control the types which are being used this may not matter). I am assuming you were using =! for performance though so restricting to the Generic type will avoid accidental boxing via the Object.Equals(object) route.

user_127480

That should work for you.

public bool test<T>(T test, T test2) where T : class
{
    return (test != test2);
}

This is simply pasted from the examples that were commented on your question.

CalvinR_559900

Your type needs to implement the IComparable or IEquatable interface.

Probably you then need to rewrite a!=b as !(a==b), or call the CompareTo() or Equals() method explicitly.

devio_213360

You can overload the .Equals() method on your objects and change your evaluation to:

return (!valueInDB.Equals(valueFromView));

Assuming that valueInDB and valueFromView are objects. Your example variables aren't named the same as those used in the compare, so I had to assume.

EDIT: Got beat by 3 seconds! A note on overloading, if you need to compare values within a type, the basic .Equals() from the Object class won't be enough, as it will only do a memory compare for complex types. You'll need to overload and provide the implementation of how you want the object to be compared.

user_304400
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