How to: Write to a Text File (C# Programming Guide)

These examples show various ways to write text to a file. The first two examples use static convenience methods on the System.IO.File class to write each element of any IEnumerable<string> and a string to a text file. Example 3 shows how to add text to a file when you have to process each line individually as you write to the file. Examples 1-3 overwrite all existing content in the file, but example 4 shows you how to append text to an existing file.

These examples all write string literals to files. If you want to format text written to a file, use the Format method or C# string interpolation feature.

Example

class WriteTextFile
{
    static void Main()
    {

        // These examples assume a "C:\Users\Public\TestFolder" folder on your machine.
        // You can modify the path if necessary.
        

        // Example #1: Write an array of strings to a file.
        // Create a string array that consists of three lines.
        string[] lines = { "First line", "Second line", "Third line" };
        // WriteAllLines creates a file, writes a collection of strings to the file,
        // and then closes the file.  You do NOT need to call Flush() or Close().
        System.IO.File.WriteAllLines(@"C:\Users\Public\TestFolder\WriteLines.txt", lines);


        // Example #2: Write one string to a text file.
        string text = "A class is the most powerful data type in C#. Like a structure, " +
                       "a class defines the data and behavior of the data type. ";
        // WriteAllText creates a file, writes the specified string to the file,
        // and then closes the file.    You do NOT need to call Flush() or Close().
        System.IO.File.WriteAllText(@"C:\Users\Public\TestFolder\WriteText.txt", text);

        // Example #3: Write only some strings in an array to a file.
        // The using statement automatically flushes AND CLOSES the stream and calls 
        // IDisposable.Dispose on the stream object.
        // NOTE: do not use FileStream for text files because it writes bytes, but StreamWriter
        // encodes the output as text.
        using (System.IO.StreamWriter file = 
            new System.IO.StreamWriter(@"C:\Users\Public\TestFolder\WriteLines2.txt"))
        {
            foreach (string line in lines)
            {
                // If the line doesn't contain the word 'Second', write the line to the file.
                if (!line.Contains("Second"))
                {
                    file.WriteLine(line);
                }
            }
        }

        // Example #4: Append new text to an existing file.
        // The using statement automatically flushes AND CLOSES the stream and calls 
        // IDisposable.Dispose on the stream object.
        using (System.IO.StreamWriter file = 
            new System.IO.StreamWriter(@"C:\Users\Public\TestFolder\WriteLines2.txt", true))
        {
            file.WriteLine("Fourth line");
        }
    }
}
 //Output (to WriteLines.txt):
 //   First line
 //   Second line
 //   Third line
     
 //Output (to WriteText.txt):
 //   A class is the most powerful data type in C#. Like a structure, a class defines the data and behavior of the data type.
     
 //Output to WriteLines2.txt after Example #3:
 //   First line
 //   Third line
      
 //Output to WriteLines2.txt after Example #4:
 //   First line
 //   Third line
 //   Fourth line

Robust Programming

The following conditions may cause an exception:

  • The file exists and is read-only.

  • The path name may be too long.

  • The disk may be full.

See Also