How to truncate a file in C#?

To truncate a file in C#, use the FileStream.SetLength method. This method allows you to change the size of a file by either reducing it (truncating) or expanding it to a specified length.

Syntax

Following is the syntax for the SetLength method −

public override void SetLength(long value);

Parameters

• value − A long representing the desired length of the stream in bytes.

How It Works

The behavior of SetLength depends on whether the new value is smaller or larger than the current file size −

Using SetLength to Truncate a File

Example

using System;
using System.IO;
using System.Text;

class Program {
   public static void Main() {
      string filePath = "sample.txt";
      
      // Create a sample file with content
      File.WriteAllText(filePath, "This is a long text that will be truncated to demonstrate SetLength method.");
      
      Console.WriteLine("Original file size: " + new FileInfo(filePath).Length + " bytes");
      
      // Truncate the file to 20 bytes
      using (FileStream stream = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite)) {
         stream.SetLength(20);
      }
      
      Console.WriteLine("File size after truncation: " + new FileInfo(filePath).Length + " bytes");
      Console.WriteLine("File content: " + File.ReadAllText(filePath));
   }
}

The output of the above code is −

Original file size: 78 bytes
File size after truncation: 20 bytes
File content: This is a long text

Using SetLength to Expand a File

Example

using System;
using System.IO;

class Program {
   public static void Main() {
      string filePath = "expand.txt";
      
      // Create a small file
      File.WriteAllText(filePath, "Hello");
      
      Console.WriteLine("Original file size: " + new FileInfo(filePath).Length + " bytes");
      
      // Expand the file to 20 bytes
      using (FileStream stream = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite)) {
         stream.SetLength(20);
      }
      
      Console.WriteLine("File size after expansion: " + new FileInfo(filePath).Length + " bytes");
      
      // Read and display the content (including null bytes)
      byte[] content = File.ReadAllBytes(filePath);
      Console.WriteLine("Content length: " + content.Length + " bytes");
      Console.WriteLine("First 10 bytes: [" + string.Join(", ", content[..10]) + "]");
   }
}

The output of the above code is −

Original file size: 5 bytes
File size after expansion: 20 bytes
Content length: 20 bytes
First 10 bytes: [72, 101, 108, 108, 111, 0, 0, 0, 0, 0]

Complete File Truncation Example

Example

using System;
using System.IO;

class Program {
   public static void TruncateFile(string path, long newSize) {
      try {
         using (FileStream stream = new FileStream(path, FileMode.Open, FileAccess.ReadWrite)) {
            Console.WriteLine("Original size: " + stream.Length + " bytes");
            stream.SetLength(newSize);
            Console.WriteLine("New size: " + stream.Length + " bytes");
         }
      }
      catch (Exception ex) {
         Console.WriteLine("Error: " + ex.Message);
      }
   }
   
   public static void Main() {
      string filePath = "test.txt";
      
      // Create test file
      File.WriteAllText(filePath, "This is a test file with some content for truncation demonstration.");
      
      // Truncate to 15 bytes
      TruncateFile(filePath, 15);
      
      // Display truncated content
      Console.WriteLine("Truncated content: "" + File.ReadAllText(filePath) + """);
   }
}

The output of the above code is −

Original size: 68 bytes
New size: 15 bytes
Truncated content: "This is a test "

Conclusion

The FileStream.SetLength method provides a simple way to truncate or expand files in C#. When truncating, the file is reduced to the specified size, and when expanding, the file is padded with undefined bytes (typically zeros).