Why does if ( 'a'<= /*some char*/ <= 'z') not work?

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I have been taking an online course in C# and, while solving one of the tasks I came across this:

I needed to write a piece of code which determined if the first letter of a word is found between 'A' and 'F', so I wrote the following if statement:

string command= Console.ReadLine();    
if (('a' <= command[0] <= 'f') || ('A' <= command[0] <= 'F'))

And gave me the following error: "Operator '<=' cannot be applied to operands of type 'bool' and 'char'"

So, after a lot of head scratching I replaced it with this:

  if (('a' <= command[0] && command[0] <= 'f') || ('A' <= command[0] && command[0] <= 'F'))

And that worked just fine, but I can't figure out why the first one didn't work. Can someone explain this to me please?

If you split

'a' <= command[0] <= 'f'

into its components (as the compiler would see it), you get

Stage 1:  'a' <= command[0]
Stage 2: (result of stage 1) <= 'f'

So, the 2nd half of the original statement becomes:

 true <= 'f'

which is obviously the error you are seeing

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The compiler interprets

'a' <= command[0] <= 'f'

as if it were written like

('a' <= command[0]) <= 'f'

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In most languages you are not able to chain comparisons like that in a boolean expression.

The second works because you are breaking it down into one comparison at a time.

Valid: a < b a < b && a < c && b < c

Invalid: a < b < c

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