Add remarks for StandardInputEncoding

[imba-tjd]

Do reword my content if necessary.

Reason

This is a C Program that reads by UTF16:

#include <io.h>
#include <fcntl.h>
#include <stdio.h>
#include <wchar.h>

int main() {
    _setmode(_fileno(stdin), _O_U16TEXT);
    _setmode(_fileno(stdout), _O_U16TEXT);

    wchar_t input[10] = {};
    fgetws(input, 10, stdin);
    wprintf(L"%X\n", input[0]);
    fputws(input, stdout);
}

When executing it from Console, it works normally. For example type asdf:

asdf # Type
61
asdf

But when executing it from C# with Encoding.Unicode, it won't work normally:

using System;
using System.Text;
using System.Diagnostics;

class Program {
    static void Main() {
        ProcessStartInfo psi = new() {
            FileName = "tr.exe",
            UseShellExecute = false,
            RedirectStandardInput = true,
            StandardInputEncoding = Encoding.Unicode,
            // StandardInputEncoding = new UnicodeEncoding(false, false),
            RedirectStandardOutput = true,
            StandardOutputEncoding = Encoding.Unicode
        };
        var p = Process.Start(psi);
        p.StandardInput.WriteLine("asdf");
        p.WaitForExit();
        string s = p.StandardOutput.ReadToEnd();
        Console.WriteLine(s);
    }
}

The output:

FEFF
?asdf

FEFF is UTF16 BE BOM. I think it's because C is LE, the actual bytes are FF, FE, UTF16 LE, but when outputing the FE is interpreted as high part.

The correct way is to use new UnicodeEncoding(false, false).

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[opbld31]

Docs Build status updates of commit d9c97fd:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics/ProcessStartInfo.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
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Issue Details

---

Do reword my content if necessary.

Reason

This is a C Program that reads by UTF16:

#include <io.h>
#include <fcntl.h>
#include <stdio.h>
#include <wchar.h>

int main() {
    _setmode(_fileno(stdin), _O_U16TEXT);
    _setmode(_fileno(stdout), _O_U16TEXT);

    wchar_t input[10] = {};
    fgetws(input, 10, stdin);
    wprintf(L"%X\n", input[0]);
    fputws(input, stdout);
}

When executing it from Console, it works normally. For example type asdf:

asdf # Type
61
asdf

But when executing it from C# with Encoding.Unicode, it won't work normally:

using System;
using System.Text;
using System.Diagnostics;

class Program {
    static void Main() {
        ProcessStartInfo psi = new() {
            FileName = "tr.exe",
            UseShellExecute = false,
            RedirectStandardInput = true,
            StandardInputEncoding = Encoding.Unicode,
            // StandardInputEncoding = new UnicodeEncoding(false, false),
            RedirectStandardOutput = true,
            StandardOutputEncoding = Encoding.Unicode
        };
        var p = Process.Start(psi);
        p.StandardInput.WriteLine("asdf");
        p.WaitForExit();
        string s = p.StandardOutput.ReadToEnd();
        Console.WriteLine(s);
    }
}

The output:

FEFF
?asdf

The FEFF is UTF-16 LE BOM.

The correct way is to use new UnicodeEncoding(false, false).

Author:	imba-tjd
Assignees:	-
Labels:	area-System.Diagnostics.Process
Milestone:	-

[krwq]

cc: @adamsitnik @carlossanlop @jozkee

[opbld30]

Docs Build status updates of commit 25b152e:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics/ProcessStartInfo.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[opbld31]

Docs Build status updates of commit 71cc355:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics/ProcessStartInfo.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[opbld30]

Docs Build status updates of commit 71cc355:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System.Diagnostics/ProcessStartInfo.xml	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel