Q:

Why when making sum of int variable + int variable i'm getting two numbers as result?

This is my code:
this.Invoke(new MethodInvoker(delegate {
    label13.Text = e.failedUrls + doneWebPages.ToString();
}));

The result is: 01,02,03,04.....09,111
When e.failedUrls is for example 1 now and doneWebPages is 22 so instead showing me in label13 the sum 23 i see 122.
What is wrong ?

A:

You're adding an int to a string1. The rules of the language will cause the compiler to emit instructions to convert2 the int to a string, and thus you are seeing string concatenation. Thus 
if e.failedUrls is 1, as you say, and doneWebPages is 22, also as you say, then
e.failedUrls + doneWebPages.ToString()

will result in
1.ToString() + "22"

which will result in
"1" + "22"

which is "122", as you're seeing. If you want integer arithmetic, what you want is:
label13.Text = (e.failedUrls + doneWebPages).ToString();

That is, first add the numbers, and then call ToString on the result.
1: I'm assuming that e.failedUrls is an int because you seem to think it's an int. Obviously, doneWebPages.ToString() is a string, but I'm assuming that doneWebPages is an  int since you also seem to think it's an int.
2: There is an operator + defined on String with parameters +(object x, string y). The semantics of + are
x + y -> x != null ? (x.ToString() ?? "") + y : "" + y;

