C# How to: Decoding/Converting Base64 strings to Bitmap images

Article purpose

This article details how to decode or convert Base64 encoded strings back into Bitmap images by means of extending the string class.

Note: This article is an update that builds upon the article: C# How to: Encoding Bitmaps to Base64 strings

Sample source code

This article is accompanied by a sample source code Visual Studio project which is available for download here.

Images as Base64 strings

From Wikipedia:

Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation. The Base64 term originates from a specific MIME content transfer encoding.

Base64 encoding schemes are commonly used when there is a need to encode binary data that need to be stored and transferred over media that are designed to deal with textual data. This is to ensure that the data remain intact without modification during transport. Base64 is commonly used in a number of applications including email via MIME, and storing complex data in XML.

From the definition quoted above the need for Base64 encoding becomes more clear. From MSDN documentation:

The base-64 digits in ascending order from zero are the uppercase characters "A" to "Z", the lowercase characters "a" to "z", the numerals "0" to "9", and the symbols "+" and "/". The valueless character, "=", is used for trailing padding.

Base64 encoding allows developers to expose binary data without potentially encountering conflicts in regards to the transfer medium. Base64 encoded binary data serves ideally when performing data transfer operations using platforms such as html, json, rest, xml, email.

A common implementation of Base64 encoding can be found when transferring image data. This article details how to convert/decode a Base64 string back into a Bitmap image.

Base64 String to Bitmap decoding implemented as an extension method

The code snippet listed below details the Base64StringToBitmap extension method targeting the String class.

public static Bitmap Base64StringToBitmap(this string
                                           base64String)
{
    Bitmap bmpReturn = null;

  
    byte[] byteBuffer = Convert.FromBase64String(base64String);
    MemoryStream memoryStream = new MemoryStream(byteBuffer);

  
    memoryStream.Position = 0;

  
    bmpReturn = (Bitmap)Bitmap.FromStream(memoryStream);

  
    memoryStream.Close();
    memoryStream = null;
    byteBuffer = null;

  
    return bmpReturn;
}

The Base64 string parameter is first converted to a byte array by invoking the Convert.FromBase64String method. Next we create a MemoryStream against the resulting byte array, which serves as a parameter to the Bitmap class’ FromStream static method.

The implementation

The Base64StringToBitmap extension method is implemented in a console based application. The sample source code creates a Bitmap instance from the local file system, which is then converted to a Base64 string.

An article detailing how to convert Bitmaps to Base64 encoded strings can be found here:

C# How to: Encoding Bitmaps to Base64 strings – The article you are currently reading was published as a follow up article.

Next the newly created Base64 string is converted back to a Bitmap image by invoking the Base64StringToBitmap extension method. In order to test if the Base64 string was decoded successfully the Bitmap image is saved to the local file system and displayed in the default installed application associated with png images. The implementation as follows:

static void Main(string[] args)
{
    StreamReader streamReader = new StreamReader(
                                   "NavForward.png");

 
    Bitmap bmp = new Bitmap(streamReader.BaseStream);
    streamReader.Close();

  
    string base64ImageString = bmp.ToBase64String(
                                   ImageFormat.Png);

  
    Console.WriteLine(base64ImageString);

  
    Bitmap bmpFromString = 
    base64ImageString.Base64StringToBitmap();

 
    bmpFromString.Save("FromBase64String.png", 
                       ImageFormat.Png);

  
    Process.Start("FromBase64String.png");

  
    Console.ReadKey();
}

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