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    <p>There's a fundamental difference between the <em>standard input</em> of the child process, and the console that it is attached to. A command interpreter in interactive mode is expecting to be talking <em>either</em> to a console <em>or</em> to a pipe/file as its standard input, and it changes its behaviour to match which one it finds. For a pipe/file, it just makes a simple call the <a href="http://msdn.microsoft.com/en-gb/library/aa365467%28VS.85%29.aspx" rel="nofollow" title="ReadFile Function"><code>ReadFile()</code></a> function. For a console, however, it receives input through <a href="http://msdn.microsoft.com/en-gb/library/ms684961%28VS.85%29.aspx" rel="nofollow" title="ReadConsoleInput Function"><code>ReadConsoleInput()</code></a>, where non-alphanumeric keys are recognizable by their virtual key codes. </p> <p>See the <a href="http://doxygen.reactos.org./da/d2c/base_2shell_2cmd_2cmd_8h_ab56d2b86e2487d35eab0eae73ba5afcb.html#ab56d2b86e2487d35eab0eae73ba5afcb" rel="nofollow" title="ReadCommand()"><code>ReadCommand()</code></a> function in the ReactOS <code>CMD</code> for an example.</p> <p>If you write a byte with the value 9 down a pipe to the standard input of a process, that's what it will see from its call to <code>ReadFile()</code>. But it won't treat it as a <kbd>Tab</kbd> keypress, because (for starters) it isn't one; it's just byte value 9 coming down a pipe. Moreover, the command-line editing that a command interpreter does in interactive mode only occurs when the standard input is detected to be a console, not when it is a pipe or a file. The command interpreter uses <code>ReadConsoleInput()</code>, which doesn't read buffers of bytes but reads sequences of <a href="http://msdn.microsoft.com/en-gb/library/ms683499%28VS.85%29.aspx" rel="nofollow" title="INPUT_RECORD Structure"><code>INPUT_RECORDs</code></a>; and thus you have to <a href="http://msdn.microsoft.com/en-gb/library/ms687403%28VS.85%29.aspx" rel="nofollow" title="WriteConsoleInput Function"><code>WriteConsoleInput()</code></a> to that console handle an appropriate <code>INPUT_RECORD</code> simulating the <kbd>Tab</kbd> key if you want the command interpreter to see a <kbd>Tab</kbd> keypress in its input stream.</p> <p>Of course, since you've set <code>StartInfo.RedirectStandardInput</code> to <code>true</code>, under the covers .NET has connected the standard input of the command interpreter process to a pipe; and so the command interpreter is just calling <code>ReadFile()</code>, without doing any interactive command-line editing, as it would if its standard input had been a console. Even if you could find and open a handle to the command interpreter process' console, and write a <code>INPUT_RECORD</code> for a <kbd>Tab</kbd> key to it, the command interpreter is reading a pipe for its standard input, and ignoring its console.</p> <p>If you want a command interpreter to act just as it would if it were talking to a console, command line editing and all, you have to spawn the process <em>with its standard input as a console</em>, whose handle you have to (then) obtain so that you can simulate keypresses with <code>WriteConsoleInput()</code>. That's fairly difficult within .NET.</p>
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