How to get attribute of a generic type object?

Question

I’m having difficulties p/ retrieve an object to get a specific attribute.

I want to overwrite the method Equals in my class, to compare whether the title of objects are identical. In this case, it does not matter whether the type and other attributes are different. I just want to compare the attribute Title of both are equal.

My code is like this:

namespace Kitty.Core.Blocks
{
    class Block<T>
    {
        public string Title { get; private set; }
        public T Contents { get; private set; }

        public Block(string title, T contents)
        {
            this.Title = title;
            this.Contents = contents;
        }

        public override bool Equals(object obj)
        {
            Block<?> other = obj; // ?



            return string.Equals(this.Title, other.Title);
        }
    }
}

The problem is in this section:

public override bool Equals(object obj)
{
    Block<?> other = obj; // Como recupero "obj"?


    return string.Equals(this.Title, other.Title);
}

I tried so:

Block<?> other = (Block<?>) obj;

And so:

Block other = (Block) obj;

But I’m forced to specify the type:

Using the Generic type 'Block<T>' requires 1 type Arguments.

As I am overwriting, it is no use to change the method signature and exchange object by the same class name.

So how can I get that back obj, keeping its generic type (which I don’t know what it will be) to compare only the attribute Title of both?

Answer 1

I don’t know if you have any goals that I don’t understand, but just put the normal generic and treat the object as the type you want. If it’s not, he’ll get one nulland need to treat this:

public override bool Equals(object obj) {
    Block<T> other = obj as Block<T>;
    if (other == null) return false;
    return Title == other.Title;
}

Behold working in the ideone. And in the .NET Fiddle. Also put on the Github for future reference.

You obviously have other ways of doing the same.

Note that you cannot compare a Block<string> with a Block<List<string>> and expect them to be equal. If even the object type is different, there is no way to be equal. When trying to make a cast in the received object, will fail and the result will be null, after all you can not turn a Block<List<string>> in a Block<string>, they are structurally different. Even if you had a field (Title) the other is not even of the same type. It can only give false.

Might want to throw an exception instead of returning a false. I don’t like this solution, it would be less idiomatic than in Java (which abuses exceptions).

Could use a cast normal ((tipo)) but it could generate an exception, do not think it is desirable. The cast simple should only be used when you are sure it will not fail, which is not the case. Even if you prefer it, which I consider a mistake, and you prefer this method to throw an exception, which I also think is inappropriate, the exception should be another one, so it would be better to capture the exception of cast and launch one of ArgumentException.

Some considerations:

• In C# what is being called attribute is usually called field, since attribute is the name of another language mechanism.
• In C# it is more rare to abuse the separation of namespace, which is already something quite different from the Java packages. Each point creates a new namespace and usually you only create a new one when it’s actually used and makes sense, you don’t usually use the stitch just to separate the word.
• When you create a Equals() specialized, almost always want to create a GetHashCode() specialized also.
• It would most probably be better to create also (or only) the equality operator (==). In C# the method Equals() is not as widely used as language allows for specialized operators. This can be observed in the example I did by preferring the operator over the method.