Why will this simple code not work in C#

Quotes won't help you at all: in C# the quote character delimits single character literal values: 'a', 'B', '1'
Putting them round a line of code won't help.

So try something like this:

string path = @"D:\Test Data\MyFile.txt";
string allTheText = File.ReadAllText(path);

The '@' in front of the string disables string escaping, which is a way of getting special characters like double quotes and newlines into strings.
Alternatively, leave strignescaping on, and use it to "double up" and generate a backslash:

string path = "D:\\Test Data\\MyFile.txt";
string allTheText = File.ReadAllText(path);

You will get the same results.