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static Stream StringStream(string s) {
    var stream = new MemoryStream();
    var writer = new StreamWriter(stream);
    writer.Write(s);
    writer.Flush();
    stream.Position = 0;
    return stream;
}
static void setRtf(RichTextBox tb,string rtf) {
    tb.Document.Blocks.Clear();
    tb.Selection.Load(StringStream(rtf), DataFormats.Rtf);
}
void example() {
    var rtf = @"{\rtf1\ansi\ansicpg1252\deff0\deflang1033{\fonttbl{\f0\fnil\fcharset0 Arial;}}{\colortbl;\red0\green0\blue0;\red255\green0\blue0; }\viewkind4\uc1\pard\qc\cf1\fs16 test \b bold \cf2\b0\i italic\cf0\i0\fs17\par }";
    setRtf(richTextBox,rtf);
}
...