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Why Does Byte 150 show up as a dash in Notepad but Not when I read it programatically?

I've got a file that looks OK in Notepad (and Notepad++) but when I try to read it with a C# program, the dash shows up as a replacement character (�) instead. After some trial and error, I can reproduce the error as follows:
File.WriteAllBytes("C:\\Misc\\CharTest\\wtf.txt", new byte[] { 150 });
var readFile = File.ReadAllText("C:\\Misc\\CharTest\\wtf.txt");
Console.WriteLine(readFile);
Now, if you go and look in the wtf.txt file using Notepad, you'll see a dash... but I don't get it. I know that's not a "real" Unicode value so that's probably the root of the issue, but I don't get why it looks fine in Notepad and not when I read in the file. And how do I get the file to read it as a dash?
As an aside, a VB6 program I'm trying to rewrite in C# also reads it as a dash.

The File.ReadAllText(string) overload defaults to UTF8 encoding, in which a standalone byte with value 150 is invalid.
Specify the actual encoding of the file, for example:
var encoding = Encoding.GetEncoding(1252);
string content = File.ReadAllText(fileName, encoding);
I used the Windows-1252 encoding, which has a dash at codepoint 150.
Edit: Notepad displays the file correctly because for non-Unicode files the Windows-1252 codepage is the default for western regional settings. So likely you can use also Encoding.Default to get the correct result but keep in mind that Encoding.Default can return different code pages with different regional settings.

You are writing bytes in a textfile. And the you are reading those bytes and interpret them as chars.
Now, when you write bytes, you don't care about encoding, while you have to, in order to read those very same bytes as char.
Notepad++ seems to interpret the byte as Unicode char and therefore prints the _.
Now File.ReadAllText reads the bytes in the specified encoding, which you did not specify and there will be set to one of these and seems to be UTF-8, where 150 is not a valid entry.

What encoding be used to create MS-DOS txt file using C#(UTF8Encoding vs Encoding)

I am trying to create a flat file for a legacy system and they mandates that the data to be presented in TextEncoding of MS DOS .txt file (Text Document - MS-DOS Format CP_OEM). I am a bit confused between files generated by using UTF8Encoding class in C# (.net4.0 framework) and I think it produce a file in default txt file (Encoding: CP_ACP).
I think Encoding names CP_ACP , Winodows and ANSI refers to same thing and Windows default is ANSI and it will omit any unicode character information.
If I use UTF8Encoding class in C# library to create a text file(as below), is it going to be in the MS DOS txt file format?
byte[] title = new UTF8Encoding(true).GetBytes("New Text File");
As per the answer supplied it is evident that UTF8 is NOT equivalent to MSDOS txt format and should use Encoding.GetEncoding(850) method to get the encoding library.
I read the following posts to check on my information but nothing conclusive yet.
https://blogs.msdn.microsoft.com/oldnewthing/20120220-00?p=8273
https://blog.mh-nexus.de/2015/01/character-encoding-confusion
https://blogs.msdn.microsoft.com/oldnewthing/20090115-00?p=19483
Finally the conclusion is to go with Encoding.GetEncoding(850) when creating a byte array to be converted back to the actual file(note: i am using byte array as i can leverage existing middle wares).

You can use the File.ReadXY(String, Encoding) and File.WriteXY(String, String[], Encoding) methods, where XY is either AllLines, Lines or AllText working with string[], IEnumerable<string> and string respectively.
MS-DOS uses different code pages. Probably the code page 850 "Western European / Latin-1" or code page 437 "OEM-US / OEM / PC-8 / DOS Latin US" (as #HansPassant suggests) will be okay. If you are not sure, which code page you need, create example files containing letters like ä, ö, ü, é, è, ê, ç, à or greek letters with the legacy system and see whether they work. If you don't use such letters or other special characters, then the code page is not very critical.
File.WriteAllText(path, "Hello World", Encoding.GetEncoding(850));
The character codes from 0 to 127 (7-bit) are the same for all MS-DOS code pages, for ANSI and UTF-8. UTF files are sometimes introduced with a BOM (byte order mark).
MS-DOS knows only 8-bit characters. The codes 128 to 255 differ for the different national code pages.
See: File Class, Encoding Class and Wikipedia: Code Page.

How do I read and write smart quotes (and other silly characters) in C#

I'm writing a program that reads all the text in a file into a string, loops over that string looking at the characters, and then appends the characters back to another string using a Stringbuilder. The issue I'm having is when it's written back out, the special characters such as “ and ” , come out looking like � characters instead. I don't need to do a conversion, I just want it written back out the way I read it in:
StringBuilder sb = new StringBuilder();
string text = File.ReadAllText(filePath);
for (int i = 0; i < text.Length; ++i) {
if (text[i] != '{') { // looking for opening curly brace
sb.Append(text[i]);
continue;
}
// Do stuff
}
File.WriteAllText(destinationFile, sb.ToString());
I tried using different Encodings (UTF-8, UTF-16, ASCII), but then it just came out even worse; I started getting question mark symbols and Chinese characters (yes, a bit of a shotgun approach, but I was just experimenting).
I did read this article: http://www.joelonsoftware.com/articles/Unicode.html
...but it didn't really explain why I was seeing what I saw, unless in C#, the reader starts cutting off bits when it hits weird characters like that. Thanks in advance for any help!

TL;DR that is definitely not UTF-8 and you are not even using UTF-8 to read the resulting file. Read as Windows1252, write as Windows1252 (If you are going to use the same viewing method to view the resulting file)
Well let's first just say that there is no way a file made by a regular user will be in UTF-8. Not all programs in windows even support it (excel, notepad..), let alone have it as default encoding (even most developer tools don't default to utf-8, which drives me insane). Since a lot of developers don't understand that such a thing as encoding even exists, then what chances do regular users have of saving their files in an utf-8 hostile environment?
This is where your problems first start. According to documentation, the overload you are using File.ReadAllText(filePath); can only detect UTF-8 or UTF-32.
Indeed, simply reading a file encoded normally in Windows-1252 that contains "a”a" results in a string "a�a", where � is the unicode replacement character (Read the wikipedia section, it describes exactly the situation you are in!) used to replace invalid bytes. When the replacement character is again encoded as UTF-8, and interpreted as Windows-1252, you will see � because the bytes for � in UTF-8 are 0xEF, 0xBF, 0xBD which are the bytes for � in Windows-1252.
So read it as Windows-1252 and you're half-way there:
Encoding windows1252 = Encoding.GetEncoding("Windows-1252");
String result = File.ReadAllText(#"C:\myfile.txt", windows1252);
Console.WriteLine(result); //Correctly prints "a”a" now
Because you saw �, the tool you are viewing the newly made file with is also using Windows-1252. So if the goal is to have the file show correct characters in that tool, you must encode the output as Windows-1252:
Encoding windows1252 = Encoding.GetEncoding("Windows-1252");
File.WriteAllText(#"C:\myFile", sb.toString(), windows1252);

Chances are the text will be UTF8.
File.ReadAllText(filePath, Encoding.UTF8)
coupled with
File.WriteAllText(destinationFile, sb.ToString(), Encoding.UTF8)
should cover off dealing with the Unicode characters. If you do one or the other you're going to get garbage output, both or nothing.

"Unable to translate Unicode character" error when saving to txt file

Additional information: Unable to
translate Unicode character \uDFFF at
index 195 to specified code page.
I made an algorithm, who's result are binary values (different lengths). I transformed it into uint, and then into chars and saved into stringbuilder, as you can see below:
uint n = Convert.ToUInt16(tmp_chars, 2);
_koded_text.Append(Convert.ToChar(n));
My problem is, that when i try to save those values into .txt i get the previously mentioned error.
StreamWriter file = new StreamWriter(filename);
file.WriteLine(_koded_text);
file.Close();
What i am saving is this: "忿췾᷿］볯褟ﶞ痢ﳻ��伞ﳴ㿯ﹽ翼蛿㐻ﰻ筹��﷿￦ﾏ랿鳿⏟麞펿"... which are some weird signs.
What i need is to convert those binary values into some kind of string of chars and save it to txt. I saw somewhere that converting to UTF8 should help, but i don't know how to. Would changing files encoding help too?

You cannot transform binary data to a string directly. The Unicode characters in a string are encoded using utf16 in .NET. That encoding uses two bytes per character, providing 65536 distinct values. Unicode however has over one million codepoints. To make that work, the Unicode codepoints above \uffff (above the BMP, Basic Multilingual Plane) are encoded with a surrogate pair. The first one has a value between 0xd800 and 0xdbff, the second between 0xdc00 and 0xdfff. That provides 2 ^ (10 + 10) = 1 million additional codes.
You can perhaps see where this leads, in your case the code detects a high surrogate value (0xdfff) that isn't paired with a low surrogate. That's illegal. Lots more possible mishaps, several codepoints are unassigned, several are diacritics that get mangled when the string is normalized.
You just can't make this work. Base64 encoding is the standard way to carry binary data across a text stream. It uses 6 bits per character, 3 bytes require 4 characters. The character set is ASCII so the odds of the receiving program decoding the character back to binary incorrectly are minimal. Only a decades old IBM mainframe that uses EBCDIC could get you into trouble. Or just plain avoid encoding to text and keep it binary.

Since you're trying to encode binary data to a text stream this SO question already contains an answer to the question: "How do I encode something as base64?" From there plain ASCII/ANSI text is fine for the output encoding.

How to get correctly-encoded HTML from the clipboard?

Has anyone noticed that if you retrieve HTML from the clipboard, it gets the encoding wrong and injects weird characters?
For example, executing a command like this:
string s = (string) Clipboard.GetData(DataFormats.Html)
Results in stuff like:
<FONT size=-2>  <A href="/advanced_search?hl=en">Advanced
Search</A><BR>  Preferences<BR>  <A
href="/language_tools?hl=en">Language
Tools</A></FONT>
Not sure how MarkDown will process this, but there are weird characters in the resulting markup above.
It appears that the bug is with the .NET framework. What do you think is the best way to get correctly-encoded HTML from the clipboard?

In this case it is not so visible as it was in my case. Today I tried to copy data from clipboard but there were a few unicode characters. The data I got were as if I would read a UTF-8 encoded file in Windows-1250 encoding (local encoding in my Windows).
It seems you case is the same. If you save the html data (remember to put non-breakable space = 0xa0 after the  character, not a standard space) in Windows-1252 (or Windows-1250; both works). Then open this file as a UTF-8 file and you will see what there should be.
For my other project I made a function that fix data with corrupted encoding.
In this case simple conversion should be sufficient:
byte[] data = Encoding.Default.GetBytes(text);
text = Encoding.UTF8.GetString(data);
My original function is a little bit more complex and contains tests to ensure that data are not corrupted...
public static bool FixMisencodedUTF8(ref string text, Encoding encoding)
{
if (string.IsNullOrEmpty(text))
return false;
byte[] data = encoding.GetBytes(text);
// there should not be any character outside source encoding
string newStr = encoding.GetString(data);
if (!string.Equals(text, newStr)) // if there is any character "outside"
return false; // leave, the input is in a different encoding
if (IsValidUtf8(data) == 0) // test data to be valid UTF-8 byte sequence
return false; // if not, can not convert to UTF-8
text = Encoding.UTF8.GetString(data);
return true;
}
I know that this is not the best (or correct solution) but I did not found any other way how to fix the input...
EDIT: (July 20, 2017)
It Seems like the Microsoft already found this error and now it works correctly. I'm not sure whether the problem is in some frameworks, but I know for sure, that now the application uses a different framework as in time, when I wrote the answer. (Now it is 4.5; the previous version was 2.0)
(Now all my code fails in parsing the data. There is another problem to determine the correct behaviour for application with fix already aplied and without fix.)

You have to interpret the data as UTF-8. See MS Office hyperlinks change code page?.

DataFormats.Html specification states it's encoded in UTF-8. But there's a bug in .NET 4 Framework and lower, and it actually reads as UTF-8 as Windows-1252.
You get allot of wrong encodings, leading funny/bad characters such as
'Å','‹','Å’','Ž','Å¡','Å“','ž','Ÿ','Â','¡','¢','£','¤','Â¥','¦','§','¨','©'
Full explanation here
Debugging Chart Mapping Windows-1252 Characters to UTF-8 Bytes to Latin-1 Characters
Soln: Create a translation dictionary and search and replace.

I don't know what your original source document is, but be aware that Word and Outlook provide several versions of the clipboard in different encodings. One is usually Windows-1252 and another is UTF-8. Possibly you're grabbing the UTF-8 encoded version by default, when you're expecting the Windows-1252 (Latin-1 + Smart Quotes)? Non-ASCII characters would show up as multiple odd Latin-1 accented characters. Most "Smart Quotes" are not in the Latin-1 set and are often three bytes in UTF-8.
Can you specify which encoding you want the clipboard contents in?

Try this:
System.Windows.Forms.Clipboard.GetText(System.Windows.Forms.TextDataFormat.Html);