C# WriteLine Concatenation Questions

Question

Greetings All,
I was playing around with the C# WriteLine Concatenation after Lesson 4.1 of the C# Course. I encountered a few weird observations I cannot understand. For the full code of my practice sheet see:
https://code.sololearn.com/cA6a2157A141
First: (Example 5) in code)
Console.WriteLine("a = ", a + " ; b = " + b);
This gives as output: "a = ".
Question: Why doesn't it write beyond the 'a = ', or give an error instead?
Note: Console.WriteLine("a = ", + a + " ; b = " + b); gives the same output: why?
Second: (Example 6) in code)
int x = 3;
int y = x++;
Console.WriteLine("x = {0}", x + " ; y = " + y);
This gives as the correct output: "x = 4 ; y = 3".
Questions: Why does this work without the y = {1} placeholder?
Why doesn't it skip the y=, the same as the a & b example 5) above?
I would be grateful for any help. Thanks in advance,
Erwin!

Answer 1

Example 5:
remove the comma after "a="

Answer 2

And, that worked, indeed.... It was that simple huh?!? :D... Thanks @Rik Wittkopp

Answer 3

I understand the error better now. The first code I had written had an error in the concatenation (+ was missing):
Console.WriteLine("a = " a + " ; b = " + b);
This gave an error stating 'Comma expected', so I added a comma, which led to the incomplete output. The correct code needed a plus, not a comma:
Console.WriteLine("a = " + a + " ; b = " + b);