C# Console.WriteLine Weird

Question

Here is a simple script in C# which outputs two different things. The issue is not about comparing the objects - don't get misguided. This is about using Console.WriteLine to send an output.

var a = "hello";
var b = string.Copy(a);

Console.WriteLine($"a == b: {a == b}");
Console.WriteLine("a == b:" + a == b);

The first Console.WriteLine outputs a == b: True and the second one outputs False which means ignoring the part in the quotation ( "a == b:" ).

I am using VS 2015 and C# 4.5.2 - tested with 4.6 still the same result.

Any idea about this behavior highly appreciated.

Answer 1

The == operator has a lower precedence than the + operator. That means that first "a == b:" is concatenated( + ) with a and then the result is compared( == ) with b which returns False .

See: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/

Additive operators are higher than equality operators, so they have a higher precedence .

You could force it with () , in the following code a == b is evaluated first:

Console.WriteLine("a == b:" + (a == b));

Your first example is using string interpolation where the whole expression is evaluated before ToString is called and then the result is concatenated with the rest.

Answer 2

The second line is evaluating:

"a == b:" + a == b

And correctly returning False. IE

( "a == b:" + a ) == b

So the string is being built as a == b:hello' and being compared to 'hello' as the + operation is done before the ==`

You can resolve this by putting brackets around the a == b

Console.WriteLine("a == b:" + (a == b));

Answer 3

Your second line is 'string literal + a' == b, which will be false. In essence you are doing

"a == b:hello" == b

Which is always going to return false.