To write the contents of a string to a file:

Dim toWrite As String = "This will be written to the file."
System.IO.File.WriteAllText("filename.txt", toWrite)

WriteAllText will open the specified file, write the data, and then close the file. If the target file exists, it is overwritten. If the target file does not exist, it is created.

To write the contents of an array to a file:

Dim toWrite As String() = {"This", "Is", "A", "Test"}
System.IO.File.WriteAllLines("filename.txt", toWrite)

WriteAllLines will open the specified file, write each value of the array on a new line, and then close the file. If the target file exists, it is overwritten. If the target file does not exist, it is created.

To read the contents to a file into a string variable:

Dim fileContents As String = System.IO.File.ReadAllText("filename.txt")

ReadAllText will open the specified file, read data to the end, then close the file.

To read a file, separating it into an array element for each line:

Dim fileLines As String() = System.IO.File.ReadAllLines("filename.txt")

ReadAllLines will open the specified file, read each line of the file into a new index in an array until the end of the file, then close the file.

Using sw As New System.IO.StreamWriter("path\to\file.txt")
    sw.WriteLine("Hello world")
End Using

The use of a Using block is recommended good practice when using an object that Implements IDisposable