C# 在flowdocument中创建RTF运行

有人能帮我理解RTF吗？我从某个字符串填充了一个

inputStream

inputStream = new MemoryStream(Encoding.ASCII.GetBytes((myObject.someStr)));

然后该流传递到flowdocument

FlowDocument fldoc = new FlowDocument();
TextRange tr = new TextRange(
    fldoc.ContentStart, fldoc.ContentEnd);
tr.Load(inputStream, DataFormats.Rtf);

之后，我创建自定义文本框来处理

myObject

的属性

Block curBlock = fldoc.Blocks.FirstBlock;
Inline curInline = (currentBlock as Paragraph).Inlines.FirstInline;
Run curRun = (currentInline as Span).Inlines.FirstInline as Run;
return new CustomTextBox(tr, currentRun.ContentStart, myObject)

失去焦点时，我的自定义文本框会

TextPointer ptr = CurrentRun.ContentStart;
ptr.DeleteTextInRun(ptr.GetTextRunLength(LogicalDirection.Forward));
ptr.InsertTextInRun(mainTextbox.Text);
using (MemoryStream ms = new MemoryStream())
{
    tr.Save(ms, DataFormats.Rtf);
    myObject.someStr = ASCIIEncoding.Default.GetString(ms.ToArray());
}

现在的问题是：如果

myObject.someStr==”

，那么

curBlock==null

，我无法实例化CustomTextBox。我正在努力

Paragraph p = new Paragraph();
Span s = new Span();
Run r = new Run(tr.Text);
s.Inlines.Add(r);
p.Inlines.Add(s);
fldoc.Blocks.Add(new Paragraph());
return new CustomTextBox(tr, r.ContentStart, myObject)

，但tr无论如何都是空的。我怎么办

fldoc = new FlowDocument(p);

帮助