>>I am using it to log a process that is going on. It seems that after so many lines, it just stops outputting. I don't have a count though...
>
>I am reading Developing C# Windows Software: A Windows form Tutorial by Wrox Publishing. It is based off of .net 1.0, but I think it might still apply here. Anyway, just last night I read that the multi-line text box can only handle 65535 characters.
>
>Hope this may help

Using the following code:

MessageBox.Show(this.textBox1.Text.Length.ToString());

the messagebox tells me that I have 12000000 (that is 12 million) characters in my multiline TextBox.
I added those lines to my TextBox using the following code:

int myCounter = 0
System.Text.StringBuilder sb = new System.Text.StringBuilder();
sb.Append(this.textBox1.Text);
for (int i = 0 ; i< 1000000; i++)
{
	sb.Append(System.Environment.NewLine + this.myCounter.ToString().PadLeft(10,'0'));
	myCounter++;
}
this.textBox1.Text = sb.ToString();

(the additional 2 characters for newline accounts for 2 million characters).