Method magic

15y848Azure

Hi,

How does the WriteLine() Method know to print to the console?

Is is using deferred execution behind the scenes in that an expression tree of some sort is being used?

Regards,

8 Replies

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Vulpes15y ago
Accepted answer
Behind the scenes, it uses a TextWriter to print to the console's output stream. Low level Windows functions then render the characters to the console.

In fact Console.WriteLine() is just a convenience method for the more long-winded Console.Out.WriteLine() where the Out property is a reference to the output stream.

It was introduced in .NET 1.0 and certainly doesn't use deferred execution or expression trees which were much later additions.
Sam Hobbs15y ago
Yes.
Vulpes15y ago
Ah, I understand now what you're getting at.

However, we can still use the ReadFile() and WriteFile() functions whether the standard input and output has been redirected or not. If passed a console handle, these functions (in effect) treat the console buffer as though it were a file.
Sam Hobbs15y ago
Note that in that page it says "the console functions can only be used with console handles" and that that is important whith "standard handles that may have been redirected". Which means that anything written to the console using the WriteConsole function is not redirected and that ReadConsole will never read anything that has been redirected. Then on that page, click on "High-Level Console Input and Output Functions". It says:

"ReadConsole and WriteConsole fail if used with a standard handle that has been redirected and is no longer a console handle."
....
"The functions do not return keyboard events involving the function keys or arrow keys. Input events generated by mouse, window, focus, or menu input are discarded."
....
if "line input mode is enabled" .... "do not return to the calling application until the ENTER key is pressed"

Just think about it. Streams only move forward; the console API is used to randomly move around the screen. The two are fundamentally incompatible.
Vulpes15y ago
I'm not sure what you mean about the Win32 API functions not being compatible with the standard input and output streams as they do provide functions to read from or write to these streams (see http://msdn.microsoft.com/en-us/library/ms683458(VS.85).aspx). The standard input, output and error streams can all be redirected using the freopen() function and its varieties.

You're correct that the .NET Console class also allows for redirection by calling its SetIn(), SetOut() or SetError() functions.
Sam Hobbs15y ago
The Windows API console functions are not compatible with streams such as standard input and standard output, correct? I am not sure if there are ways to get them to work together but as far as I know it is not as easy as we would like. Streams such as standard input and standard output are buffered whereas the console API cannot be buffered.

I had assumed that the .Net Console reads and writes could be redirected; is that incorrect? I thought that the Windows API console functions could not be redirected.
Vulpes15y ago
.NET does in fact use the standard Windows console, rather than manufacturing its own character based window.

All the Console API functions (see http://msdn.microsoft.com/en-us/library/ms682087(VS.85).aspx) can be P/Invoked from C# and work fine if you need any extra functionality, such as mouse support, which is not wrapped by the System.Console class.
Sam Hobbs15y ago
What WriteLine() Method? If you mean the one that is a member of the Console class then WriteLine writes to the console because that is what the Console class is.

The C++ language defines standard input and standard output devices, which are typically a console window. The concept of a console is the result of the time period when C++ was designed, in which dumb terminals were used with computers. I don't know the C# language has a similar definition but it probably does. I assume that the C# language defines the Console class to be an object that could be the equivalent of a dumb terminal, although I am sure they don't use the term dumb terminal. A Console is likely defined to be a text-only window.