请检查答案到这个问题。

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已编辑添加

如上所述，上述答案适用于RTF到PlainText的转换，根据 RTF规范1.6 你使用 \\\☚ 显示ą， \\\⠞ ę。

语法是 \u Nd ，其中 N 是字符的十进制Unicode值，而 d 是ASCII近似值。

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修改为澄清

对于你说的话，你在RTF中有一些占位符，对吗？

你需要做的是在替换占位符时添加经过一点研究，我想你可能会使用这样的东西：

 公共函数GetRtfString（ByVal text As String）As String 

 Dim sb As New Text.StringBuilder（）
为每个c As Char在text 
 Dim code = Convert.ToInt32（c）
 If（Char.IsLetter（c）AndAlso code& H80）Then 
 sb.Append（c）
 Else 
 sb.AppendFormat（CultureInfo.InvariantCulture，\ {0} {1}，代码，RemoveDiacritics（c））
 End If 
 Next 
返回sb.ToString（） 

结束函数

公共函数RemoveDiacritics（ByVal text As String）As String 

 Dim formD = text.Normalize（System.Text。 NormalizationForm.FormD）
 Dim sb As New Text.StringBuilder（）

对于每个c As Char在formD 
 If（CharUnicodeInfo.GetUnicodeCategory（c）< UnicodeCategory.NonSpacingMark）然后
 sb.Append（c）
 End If 
 Next 

返回sb.ToString（）。Normalize（System.Text.NormalizationForm.FormC ）

结束功能
 

How to programatically insert string with special characters into RTF? I have rtf template I load to string and then replace all $MY_VARIABLE$ with data. Data contains special chars like 'ąęść' and the problem is that in result file these characters are replaced with '?'. It's something wrong with encoding but what?

My code looks like:

StreamReader reader = new StreamReader("template.rtf");
StringBuilder form = new StringBuilder(reader.ReadToEnd());
// here I replace variables in rtf with data
Encoding srcEncoding = new UTF8Encoding();
Encoding dstEncoding = new ASCIIEncoding();
byte[] utf = srcEncoding.GetBytes(form.ToString());
byte[] asci = Encoding.Convert(Encoding.UTF8, Encoding.ASCII, utf);
return dstEncoding.GetString(asci);

Please, check the answer to this question.

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Edited to Add

As you say that the above answer applies to the conversion of RTF to PlainText, according to RTF Specification 1.6 you use \u261a to display ą, \u281e for ę...

The syntax is \uNd where N is the decimal Unicode value for the character, and d is the ASCII approximation.

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Edited to Clarify

For what you say, you have some placeholders in the RTF, right?

What you need to do is to have a function that, when replacing the placeholders, add the proper RTF encoded characters.

After a little bit of research, I think you may use something like this:

Public Function GetRtfString(ByVal text As String) As String

  Dim sb As New Text.StringBuilder()
  For Each c As Char In text
    Dim code = Convert.ToInt32(c)
    If (Char.IsLetter(c) AndAlso code < &H80) Then
      sb.Append(c)
    Else
      sb.AppendFormat(CultureInfo.InvariantCulture, "\u{0}{1}", code, RemoveDiacritics(c))
    End If
  Next
  Return sb.ToString()

End Function

Public Function RemoveDiacritics(ByVal text As String) As String

  Dim formD = text.Normalize(System.Text.NormalizationForm.FormD)
  Dim sb As New Text.StringBuilder()

  For Each c As Char In formD
    If (CharUnicodeInfo.GetUnicodeCategory(c) <> UnicodeCategory.NonSpacingMark) Then
      sb.Append(c)
    End If
  Next

  Return sb.ToString().Normalize(System.Text.NormalizationForm.FormC)

End Function