How does the C# conditional operator ? evaluate when used in pairs

Question

I'm trying to figure out what this code does in C# I'm not that familiar with the ? operator and this usage is confusing me. I know that if I do something like this.

Result = ans > 0 ? string.IsNullOrWhiteSpace(line[0]) : "";

It boils down to this:

if(ans > 0)
    Result = string.IsNullOrWhiteSpace(line[0]);
else 
    Result = "";

However I don't understand what happens when the line is like this instead:

Result = ans > 0 
       ? string.IsNullOrWhiteSpace(line[0]) 
       ? ""
       : line[0].Trim().ToUpper() 
       : "";

When it is written this way does it just pair the ? and with the first : it comes to? That doesn't really make sense because Result can only have one value. Hopefully this makes sense I tried to condense the code down to just the problem I am having so that it is easy to understand. Let me know if I'm not clear enough.

Answer 1

There is no ambiguity in parsing the expression of the form a ? b ? c : d : e: the only way to parse it is

a ? (b ? c : d) : e

A more interesting parse would be when a conditional expression is used as the last operand, not the middle. Microsoft documentation provides the answer to this:

The conditional operator is right-associative. The expression a ? b : c ? d : e is evaluated as a ? b : (c ? d : e), not as (a ? b : c) ? d : e.

However, it is a good idea to at least parenthesize expressions like that, because some readers of your code would need to consult a language reference in order to understand a potentially simple piece of logic.

Answer 2

From your post,

Result = ans > 0 
   ? string.IsNullOrWhiteSpace(line[0]) 
   ? ""
   : line[0].Trim().ToUpper() 
   : "";

is equivalent to

if( ans > 0 )
{
    if( string.IsNullOrWhiteSpace(line[0]) )
    {
         Result = "";
    }
    else
    {
         Result = line[0].Trim().ToUpper();
    }
}
else
{
     Result = "";
}