C# Write Textfile into exe Folder

hi,
 
i have a sort of history in my programm wich, documents what the programm
is currently doing.
 
Now i want to create a textfile on first run, into the exe folder to write
this documentation every time into this file.
 
I am using a class for writing the documentation into a rich textbox.
the method of the class wich perform this documentation is looking like this:
 

public void Aktual(int number)
{
switch(number)
{
case 1:
string Text1 = "Hi im text1, this is currently doing";
break;
 
//And further
}
}

 
so my problems are:
 
- Creating a textfile into exe folder
SOLVED: (- Write in general into this existing file, to dont overwrite)
SOLVED: (- Open this existing textfile to view it)
 

Thank you
 
ps.: Let me know if you need more explanation
 
greets Niko