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In c# How to convert back unicoded characters to UTF-8?

I Have this text Grou00dfbeerenstrau00dfe and I need to convert it to Großbeerenstraße
also Eichstu00e4tt to Eichstätt
But I don't completely understand and solve this because of these reasons:
ONLY some characters (special characters) are converted, not the whole text
Unicoded texts usually have Escape characters like \u00df instead of u00df
Could you please help me to convert correctly back to its original states?
Basically, how can I convert when there is no escape character?
NOTE: If you must know, I'm sending some special charactered strings into some system. I cannot touch this system but when I request back the same string from that system, it converts Großbeerenstraße to Grou00dfbeerenstrau00dfe and so on.

Based on David's idea of looking for u and checking if the following 4 characters are valid hex numbers, it would look something like this:
public string FixGermanUnicode(string input) {
var output = new StringBuilder();
for (var i = 0; i < input.Length; i++) {
if (i < input.Length - 4 && input[i] == 'u' && input[i + 1] == '0'
&& int.TryParse(input.Substring(i + 1, 4), NumberStyles.HexNumber, null, out var code)) {
try {
output.Append(char.ConvertFromUtf32(code));
i += 4;
} catch (ArgumentOutOfRangeException) {
//not a valid unicode character
output.Append(input[i]);
}
} else {
output.Append(input[i]);
}
}
return output.ToString();
}
Console.WriteLine(FixGermanUnicode("Grou00dfbeerenstrau00dfe"));
Really, it checks for u0 to prevent cases where the next 4 characters are valid unicode, but should not have been replaced. That will work for German at least, since all the special characters in German have unicode codes starting with 0.
This will also catch scenarios where the follow 4 digits are valid hex numbers, but the resulting hex number is not a valid unicode character.

While I completely agree with #Gabriel Luci's answer, I would like to point out a more concise implementation of the same idea (it needs the ' System.Text.RegularExpression' namespace):
readonly static string unicodePattern = #"u0[0-9a-fA-F]{3}";
public static string FixGermanUnicode(string input)
{
return Regex.Replace(input, unicodePattern, match =>
{
var digits = match.Value.Substring(1);
try
{
return char.ConvertFromUtf32(int.Parse(digits, System.Globalization.NumberStyles.AllowHexSpecifier)).ToString();
}
catch (ArgumentOutOfRangeException)
{
//not a valid unicode character
return match.Value;
}
});
}

Convert any string to ASCII, Remove Backslash

This question may reveal my ignorance regarding character encoding, so if it does, I would greatly appreciate information to correct that.
I am relaying strings from new applications to an old application. The old application only accepts ASCII characters (http://www.asciitable.com/). The old application also does not support certain characters such as backslashes. The new applications support more or less anything.
Let's say I have the string:
"Whatever - 1_夜_💦💦💦"
I need to convert that to something with only ASCII characters. For example, maybe something like:
"Whatever - 1_\u001cY_=???=???=???"
Then I want to replace the remaining illegal characters with substitution strings.
Ideally, any character that is encoded to ASCII should be able to be de-coded. That is, any unique input string will have a unique output string (no arbitrary inputs "abc" and "xyz" which are different produce the same result). An algorithm could convert the output string back to the input string.
This is what I've tried:
static string ConvertToAscii(string str)
{
var return_string = "";
foreach (var c in str)
{
if ((int)c < 128)
{
return_string += c;
}
else
{
var charBytes = BitConverter.GetBytes(c);
var ascii = Encoding.ASCII.GetString(charBytes);
return_string += ascii;
}
}
return return_string;
}
When I use this with the string I mentioned above, I get:
"Whatever - 1_\u001cY_=???=???=???"
That seems great - however, the "\u001cY" is apparently a single character, rather than a collection of ASCII characters. So my target database rejects it, and I am not able to figure out how to remove the "\" while leaving the remaining characters.
How can I convert any string into a collection of ASCII characters?

The easiest approach is Base64 all bytes since you don't seem to care how strings are represented:
Convert.ToBase64String( Encoding.Unicode.GetBytes("Whatever - 1_夜_💦💦💦"))
will produce result that is guaranteed to be ASCII (even printable ASCII) - for your string result would be "VwBoAGEAdABlAHYAZQByACAALQAgADEAXwAcWV8APdim3D3Yptw92Kbc".

Here is similar code to what I ended up using to convert everything to Ascii:
internal static string ConvertToAscii(string str)
{
var returnStringBuilder = new StringBuilder();
foreach (var c in str)
{
if (char.IsControl(c))
{
// Control character
continue;
}
if (c < 127)
{
// ASCII Character
returnStringBuilder.Append(c);
}
else
{
returnStringBuilder.Append("U+" + ((int) c).ToString("X4"));
}
}
return returnStringBuilder.ToString();
}

Is there an easy way to trim the last three characters off a string

I have strings like this:
var a = "abcdefg";
var b = "xxxxxxxx";
The strings are always longer than five characters.
Now I need to trim off the last 3 characters. Is there some simple way that I can do this with C#?

In the trivial case you can just use
result = s.Substring(0, s.Length-3);
to remove the last three characters from the string.
Or as Jason suggested Remove is an alternative:
result = s.Remove(s.Length-3)
Unfortunately for unicode strings there can be a few problems:
A unicode codepoint can consist of multiple chars since the encoding of string is UTF-16 (See Surrogate pairs). This happens only for characters outside the basic plane, i.e. which have a code-point >2^16. This is relevant if you want to support Chinese.
A glyph (graphical symbol) can consist of multiple codepoints. For example ä can be written as a followed by a combining ¨.
Behavior with right-to-left writing might not be what you want either

You want String.Remove(Int32)
Deletes all the characters from this string beginning at a specified
position and continuing through the last position.
If you want to perform validation, along the lines of druttka's answer, I would suggest creating an extension method
public static class MyStringExtensions
{
public static string SafeRemove(this string s, int numCharactersToRemove)
{
if (numCharactersToRemove > s.Length)
{
throw new ArgumentException("numCharactersToRemove");
}
// other validation here
return s.Remove(s.Length - numCharactersToRemove);
}
}
var s = "123456";
var r = s.SafeRemove(3); //r = "123"
var t = s.SafeRemove(7); //throws ArgumentException

string a = "abcdefg";
a = a.Remove(a.Length - 3);

string newString = oldString.Substring(0, oldString.Length - 4);

If you really only need to trim off the last 3 characters, you can do this
string a = "abcdefg";
if (a.Length > 3)
{
a = a.Substring(0, a.Length-3);
}
else
{
a = String.Empty;
}

Removing control characters from a UTF-8 string

I found this question but it removes all valid utf-8 characters also (returns me a blank string, while there are valid utf-8 characters plus control characters). As I read about utf-8, there's not a specific range for control characters and each character set has its own control characters.
How can I modify above solution to only remove control characters ?

This is how I roll:
Regex.Replace(evilWeirdoText, #"[\u0000-\u001F]", string.Empty)
This strips out all the first 31 control characters. The next hex value up from \u001F is \u0020 AKA the space. Everything before space is all the line feed and null nonsense.
To believe me on the characters: http://donsnotes.com/tech/charsets/ascii.html

I think the following code will work for you:
public static string RemoveControlCharacters(string inString)
{
if (inString == null) return null;
StringBuilder newString = new StringBuilder();
char ch;
for (int i = 0; i < inString.Length; i++)
{
ch = inString[i];
if (!char.IsControl(ch))
{
newString.Append(ch);
}
}
return newString.ToString();
}

If you plan to use the string as a query string, you should consider using the Uri.EscapeUriString() or Uri.EscapeDataString() before sending it out.
Note: You might still need to pull out anything from char.IsControl() first?

C#: Multiline TextBox with TextBox.WordWrap Displaying Long Base64 String

I have a textbox to display a very long Base64 string. The TextBox.Multline = true and TextBox.WordWrap = true.
The issue is caused by the auto-word-boundary detection of the TextBox itself. The Base64 string has '+' as one of the 64 characters for Base64 encoding. Therefore, the TextBox will wrap it up at the '+' character, which is not what I want (because the use might think there is a newline character around the '+' character).
I just want my Base64 string displayed in Mulitline-mode in TextBox, but no word boundary detection, that is, if the TextBox.Width can only contain 80 characters, then each line should have exact 80 characters except the last line.

Smart wrap in too smart for your purposes. Just keep Multiline, turn off WordWrap and wrap the text yourself:
public IEnumerable<string> SimpleWrap(string line, int length)
{
var s = line;
while (s.Length > length)
{
var result = s.Substring(0, length);
s = s.Substring(length);
yield return result;
}
yield return s;
}
Update:
An estimate of the number of characters that can fit in a TextBox using a fixed-width font is:
public int GetMaxChars(TextBox tb)
{
using (var g = CreateGraphics())
{
return (int)Math.Floor(tb.Width / (g.MeasureString("0123456789", tb.Font).Width / 10));
}
}
A variable-width font is harder but can be done with MeasureCharacterRanges.