To complement the (perfectly valid) solution 1, let me explain you what is going on.
Since the strings are delimited by double quotes, if you want a double quote in the string you have two ways to do that:
1- By escaping the double quotes you want to put in the string (like in solution 1); escape character is backslash (
\
).
2- By doubling each double-quote you want in the string, but then you have to use a string literal. Thus, adapted to your question:
txtabc.Text = @"""""";
Hope this helps.