The letter theta "θ" in RTF text is displayed as "?" in RichTextBox

[GowthamabalajiSambathSF4978]

Describe the bug

We are facing an issue where the letter theta "θ" is displayed as "?" in RTF text when using the MS RichTextBox control. When attempting to display special characters (e.g., λ, θ), some of them are not shown correctly and instead appear as "?".

However, certain special symbols are rendered correctly, while others are not.

For example, when using the code snippet below in the button click event with the Beta letter "β", it works properly.

Code snippet:

private void button1_Click(object sender, EventArgs e)
{
    string rtf = "{\\rtf1\\ansi\\deff0" +
               "{\\fonttbl{\\f0\\fmodern Arial;}}" +
               "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
               "\\fs22\\f0\\cb2\\cf1 " + "β" +
               "\\cb0\\par" + "}";

    richTextBox1.Rtf = rtf;
}

When using the code snippet below in the button click event with the theta letter "θ", it does not work properly.

Code snippet:

private void button1_Click(object sender, EventArgs e)
{
    string rtf = "{\\rtf1\\ansi\\deff0" +
               "{\\fonttbl{\\f0\\fmodern Arial;}}" +
               "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
               "\\fs22\\f0\\cb2\\cf1 " + "θ" +
               "\\cb0\\par" + "}";

    richTextBox1.Rtf = rtf;
}

Sample Zip: WindowsFormsApp1.zip

Steps to reproduce the bug

1. Run the sample application.
2. Enter the theta "θ" character in the RichTextBox.
3. Click the "button1".

Expected behavior

The letter theta "θ" should be printed correctly.

Actual behavior

The letter theta "θ" not printed properly.

Screenshots

RichTextBox-ThetaIssue.mp4

Windows version

No response.

Additional context

No response.

[abineshPalanisamy]

Hi @GowthamabalajiSambathSF4978

The RTF header uses \ansi, which means the text is interpreted using ANSI encoding; however, ANSI supports only a limited set of characters and does not include all Unicode characters, therefore the character θ, being a Unicode character, is not available in ANSI encoding.

Workaround

Solution 1 :

private void button1_Click(object sender, EventArgs e)
{
    string rtf = "{\\rtf1\\ansi\\deff0\\uc1" +
                 "{\\fonttbl{\\f0\\fmodern Arial;}}" +
                 "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
                 "\\fs22\\f0\\cb2\\cf1 " +
                 "\\u952? " +   // θ (U+03B8)
                 "\\cb0\\par" +
                 "}";

    richTextBox1.Rtf = rtf;
}

When \u952 is used, the ANSI encoding is bypassed for that character, and the corresponding Unicode value is rendered directly. This ensures consistent rendering across different ANSI code pages.

Solution 2 :

richTextBox1.Text = "θ";

Using plain text instead of RTF avoids encoding constraints and ensures correct Unicode rendering.

[GowthamabalajiSambathSF4978]

Hi @abineshPalanisamy ,

The provided code snippet always converts the entered text into the letter theta (θ). However, this is not the expected behavior. We need the conversion to reflect the exact letter entered. For example, if we enter Beta (β) and then click the button, it should display Beta (β), not Theta (θ).

Please refer to the issue video for reference:

RichTestBox_Issue.mp4

Note: The entered text is not limited to theta; it can be any Greek letter such as Alpha, Beta, or others. The RichTextBox control should display the exact letter that is entered.

Regards,
Gowthamabalaji Sambath.

[SimonZhao888]

Hi @GowthamabalajiSambathSF4978,

I've added a method that dynamically converts based on the Unicode value of each input character.

A simplified example is shown below:

 private void button1_Click(object sender, EventArgs e)
 {
     string input = "αβγθΩ";
     string rtf = "{\\rtf1\\ansi\\deff0" +
                "{\\fonttbl{\\f0\\fmodern Arial;}}" +
                "{\\colortbl;\\red0\\green0\\blue0;\\red230\\green230\\blue230;}" +
                "\\fs22\\f0\\cb2\\cf1 " + EncodeRtfText(input) +
                "\\cb0\\par" + "}";

     richTextBox1.Rtf = rtf;
 }

 private static string EncodeRtfText(string text)
 {
     StringBuilder sb = new StringBuilder();

     foreach (char c in text)
     {
         switch (c)
         {
             case '\r':
                 break;

             case '\n':
                 sb.Append(@"\par ");
                 break;

             default:
                 if (c <= 0x7F)
                 {
                     sb.Append(c);
                 }
                 else
                 {
                     sb.Append($@"\u{(short)c}?");
                 }
                 break;
         }
     }

     return sb.ToString();
 }

Before

After

Hope this helps.
Sample Zip: WindowsFormsApp1.zip