It's easier to help you if you say what's wrong as well, or what fails when you run it.
But from a quick glance you've confused a few things.
The following doesn't work because of a couple of issues.
if (Directory("C:\\csharp\\error report1.xls") = "")
What you are trying to do is creating a new Directory object that should point to a file and then check if there was any errors.
What you are actually doing is trying to call a function named Directory() and then assign a string to the result. This won't work since 1/ you don't have a function named Directory(string str) and you cannot assign to the result from a function (you can only assign a value to a variable).
What you should do (for this line at least) is the following
FileInfo fi = new FileInfo("C:\\csharp\\error report1.xls");
if(!fi.Exists)
{
// Create the xl file here
}
else
{
// Open file here
}
As to why the Excel code doesn't work, you have to check the documentation for the Excel library which google should be able to provide for you.