C# 3.0 Cookbook, 3rd Edition

2.9. Decoding a Base64-Encoded Binary

Problem

You have a String that contains information such as a bitmap encoded as base64. You need to decode this data (which may have been embedded in an email message) from a String into a byte[] so that you can access the original binary.

Solution

Using the static method Convert.FromBase64String on the Convert class, an encoded String may be decoded to its equivalent byte[]:

	using System;

	static class CharStrExtMethods
	{
	     public static byte[] Base64DecodeString(this string inputStr)
	        {
	            byte[] decodedByteArray =
	              Convert.FromBase64String(inputStr);

	          return (decodedByteArray);
	        }
	}

Discussion

The static FromBase64String method on the Convert class makes decoding an encoded base64 string a simple matter. This method returns a byte[] that contains the decoded elements of the String.

If you receive a file via email, such as an image file (.bmp), that has been converted to a string, you can convert it back into its original bitmap file using something like the following:

	byte[] imageBytes = bmpAsString.Base64DecodeString();
	using (FileStream fstrm = new FileStream(@"C:\winnt_copy.bmp",
	                  FileMode.CreateNew, FileAccess.Write))
	{
	    using (BinaryWriter writer = new BinaryWriter(fstrm))
	    {
	        writer.Write(imageBytes);
	    }
	}

In this code, the bmpAsString variable was obtained from the code in the Discussion section of Recipe 2.10. The imageBytes byte[] is the bmpAsString String converted back to a byte[], which can then be written back to disk.

To encode a byte[] to a String ...