All excellent answers! however, for someone looking for an answer, these appear to be somewhat incomplete.
As a standard String can only of Size X, 2Gb to 4Gb depending on your configuration, these answers do not really fulfil the OP's question. One method is to work with a List of Strings:
List<string> Words = new List<string>();
using (StreamReader sr = new StreamReader(@"C:\Temp\file.txt"))
{
string line = string.Empty;
while ((line = sr.ReadLine()) != null)
{
Words.Add(line);
}
}
Some may want to Tokenise and split the line when processing. The String List now can contain very large volumes of Text.