当GetType（）。Name匹配时，DataGridViewCell类型不匹配？

Question

When iterating over a collection of DataGridViewCell s in a DataGridViewRow , I was testing for equality of a specific cell like so:

var transactionLogId = dataGridView1.Rows[dataGridView1.SelectedCells[0].RowIndex].Cells[0].Value;
if (dgvRow.Cells[0].Value == transactionLogId) {
    // Snip
}

And to my dismay, it never entered the nested block. Though both equal the same ( 5177518 ), and both dgvRow.Cells[0].Value.GetType().Name and transactionLogId.GetType().Name are Int64 , it doesn't work unless I append a .ToString() to both sides of the equality check.

Curious as to what I'm missing here. Thanks!

Answer 1

Operators are static calls, which means that overload resolution will resolve based strictly on the operand's compile time types.

In your case, overload resolution is resolving to the following operator overload:

==(object, object)

because those are the compile time types of the operands. The == for objects simply performs a reference equality check, that is, if both objects are the same object, which they clearly are not.

The same happens in the following code:

object o1 = 1;
object o2 = 1;
var equal = o1 == o2; //false

Using Equals in this case is the correct thing to do, because it is a virtual call and it will therefore resolve to the runtime type of the callee and give you the correct result.

Then why does this work when you call ToString() on both operands? Simply because the overload resolution resolves to the ==(string, string) overload implented in the string class which performs value equality.

UPDATE

Reading comments by Machine Learning there seems to be a misunderstanding concerning the reason why == is returning false in the OP's code. I want to clarify that boxing has nothing to do with it; consider the following example:

var str1 = "a";
var str2 = 'a'.ToString(); //to avoid string interning by the compiler.
var equals = str1 == str2; //true
object o1 = str1;
object o2 = str2;
equals = o1 == o2; //false

The observed behavior is exactly the same and there is no boxing/unboxing happening here.

Answer 2

They are both boxed values so the == operator will never be true.

GetType returns the type of the corresponding unboxed instances anyway.

You need to use Equals.