Why strings stored in object variables and compared with == have this strange behavior?

问题

Strange behavior when comparing two string properties with the usage of reflection.

var a = new A
{
    X = "aa",
    B = 1
};

var b = new A
{
    X = "aa",
    B = 2
};

Type type = typeof(A);

object aObjValue = type.GetProperty("X")?.GetValue(a);
object bObjValue = type.GetProperty("X")?.GetValue(b);

Console.WriteLine("aObjValue == bObjValue : " + (aObjValue == bObjValue));
Console.WriteLine("aObjValue.Equals(bObjValue) : " + aObjValue.Equals(bObjValue));

a.X = Console.ReadLine();

aObjValue = type.GetProperty("X")?.GetValue(a);

Console.WriteLine("aObjValue == bObjValue : " + (aObjValue == bObjValue));
Console.WriteLine("aObjValue.Equals(bObjValue) : " + aObjValue.Equals(bObjValue));

a.X = "aa";

aObjValue = type.GetProperty("X")?.GetValue(a);

Console.WriteLine("aObjValue == bObjValue : " + (aObjValue == bObjValue));
Console.WriteLine("aObjValue.Equals(bObjValue) : " + aObjValue.Equals(bObjValue));

Console.ReadKey();

//aObjValue == bObjValue : True
//aObjValue.Equals(bObjValue) : True
//aa
//aObjValue == bObjValue : False
//aObjValue.Equals(bObjValue) : True
//aObjValue == bObjValue : True
//aObjValue.Equals(bObjValue) : True

When using Console.ReadLine() and manually assigning a.X to "aa" I'm getting false but when assigning it in code again I get true. This is unexpected behavior for me. Can someone explain me what is going on here?

回答1:

So you know that string overloads the equality operator == to use Equals. But since you store them in a Object variable they are using the version from Object which just compares references.

Again string is a special type, it uses something that is called string interning to improve performance. So if you use a string literal "aa" this will not allocate new memory if there was already a string literal "aa". This is the case here. That is why the first aObjValue == bObjValue returns true, both are the same reference.

In the second case you enter the string "aa" to the console. This will not use string interning(which is a compiler feature), so it is a brand new instance of String. That is why the second aObjValue == bObjValue returns false. If you would cast them to String you could use == and you would get the expected behavior(same as String.Equals).

来源：https://stackoverflow.com/questions/57078896/why-strings-stored-in-object-variables-and-compared-with-have-this-strange-be

标签

reflection