创建TextSource派生类：

public class TextBuilder : TextSource
{
    public Line CurrentLine { get; private set; }

    public TextBuilder(FastColoredTextBox t) : base(t)
    {
        NewLine();
    }

    public void Append(string text, StyleIndex style = StyleIndex.None)
    {
        for (int i = 0; i < text.Length; i++)
        {
            char c = text[i];
            if (c == '\r' && i < text.Length - 1 && text[i + 1] == '\n') continue;
            if (c == '\r' || c == '\n') { NewLine(); continue; }
            CurrentLine.Add(new Char(c, style));
        }
    }

    public void AppendLine(string text, StyleIndex style = StyleIndex.None)
    {
        Append(text, style);
        NewLine();
    }

    public void AppendLine() => NewLine();

    void NewLine()
    {
        CurrentLine = CreateLine();
        lines.Add(CurrentLine);
    }

    public override void Initialized()
    {
        // Solution similar to CustomTextSourceSample.cs (not more than necessary)
        // (an alternative is OnTextChanged(0, lines.Count - 1) which invokes TextChanged but there may be side effects)

        if (CurrentTB.WordWrap)
        {
            OnRecalcWordWrap(new TextChangedEventArgs(0, lines.Count - 1));
        }

        CurrentTB.Invalidate();
    }
}

将此虚方法添加到TextSource类：

/// <summary>
/// Called when CurrentTB has initialized this text source.
/// </summary>
public virtual void Initialized()
{
}

在FCTB类中，将此行添加到InitTextSource方法中：

while (LineInfos.Count < ts.Count)
    LineInfos.Add(new LineInfo(-1));

ts.Initialized(); // Add this line

为方便起见，请更改Char构造函数：

public Char(char c, StyleIndex s = StyleIndex.None)
{
    this.c = c;
    style = s;
}

使用如下：

var b = new TextBuilder(fctb);
b.Styles[0] = new MarkerStyle(Brushes.LightBlue);
b.Append("text", StyleIndex.Style0);
fctb.TextSource = b;

如果你不想重新编译FCTB，你可以只添加TextBuilder类，并在赋值TextSource之后调用b.OnTextChanged(0, b.Count - 1)，但是你可能还是要调整FCTB，并在文本构建器中添加内容。