In this article, we will learn about how to Base64 encode and decode in C#.

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Let’s dive in.

What is Base64 Encoding?

Base64 are symbols comprising of alphabets and numbers, with some special characters. What’s more, we can adopt these symbols to hold binary data in ASCII string format. Basically, each Base64 digit represents 6-bits of binary data stored as an 8-bits character in memory.

Encoding means manipulating data from one form to another. Based on this definition, we can define Base64 encoding as a group of schemes for converting binary data to ASCII format so that the data can be transferred in a text stream or saved as a document like XML.

Conversely, computer systems communicate through various channels with zeros and ones. When these systems are transmitting information, the data can get corrupted. Therefore, it is advisable to use Base64 to transmit sensitive and large data, like images, and email attachments, to mention a few.

We can use Base64 to transmit a large amount of binary data within systems. Also, we can use Base64 to encode data inside files like HTML, CSS, and XML. Lastly, we can store files like XML and JSON in Base64.

Implement the Base64 Encoding in C# With ToBase64String()

The ToBase64String() method creates a Base64 encoded ASCII string from an array of 8-bit unsigned integers. This method has some overloads with a varying number of parameters.

The ToBase64String() has four parameters: 

  • inArray
  • offset
  • length
  • options

Let’s check out each one and see how it can help us.


inArray is a required parameter and it’s an array of 8-bit unsigned integers. For instance, if we want to convert “Hello world!” to Base64, we need to get the bytes first. The resulting Base64 output is:

var textBytes = Encoding.UTF8.GetBytes("Hello world!");
// after: 72 101 108 108 111 32 119 111 114 108 100 33 
var base64String = Convert.ToBase64String(textBytes);
// after: SGVsbG8gd29ybGQh


offset is an Int32 optional parameter. It states the position we want our encoding to start from. Using our last example of “Hello world!”, if we want to encode only “world!”, we will set our optional parameter to 6. We must pair this parameter with the length option to make it work. The 8-bit unsigned integer array for “world!” is  119 111 114 108 100 33.

Similarly, the Base64 output will be different from the result we got when all the value was encoded:

var textBytes = Encoding.UTF8.GetBytes("Hello world!"); 
// after: 72 101 108 108 111 32 119 111 114 108 100 33 
var base64String = Convert.ToBase64String(textBytes, 6, 6);
// after: d29ybGQh


length is an optional Int32 parameter. This parameter works hand-in-hand with the offset parameter. We can use this parameter to state the number of elements or variables we want to encode. Using the last example, the value for this parameter was 6.


We can use the options parameter to insert line breaks within our Base64 output. Line breaks can improve the readability of our Base64 text and also assist us while using tools that don’t deal well with long lines. The options parameter is optional. Conversely, the encoding scheme will add a line break for every 76 characters.

NOTE: A likely reason for line breaks on 76 characters was to provide a way to include binary files in e-mails and Usenet postings which was intended for humans using monitors with 80 characters width. Let us know if you know more about this in the comments section.

This value cannot be modified because the value was defined in RFC 2045. To see this in action, we’ll use a text with more characters:

var textBytes = Encoding.UTF8.GetBytes("The great crocodile of Queensland can attain a length of 30 feet");
var base64String = Convert.ToBase64String(textBytes, Base64FormattingOptions.InsertLineBreaks);
// after: VGhlIGdyZWF0IGNyb2NvZGlsZSBvZiBRdWVlbnNsYW5kIGNhbiBhdHRhaW4gYSBsZW5ndGggb2Yg
//        MzAgZmVldA==

We can see that the value has been broken into two lines.

Decode the Base64 String in C# With FromBase64String()

This method is the opposite of the ToBase64String() method. It converts from Base64 to its equivalent 8-bit unsigned integer array. We are going to use the output from ToBase64String() conversion for “world!”, the base64String variable: 

// before: d29ybGQh
var base64EncodedBytes = Convert.FromBase64String(base64String);
// after: 119 111 114 108 100 33
var inputString = Encoding.UTF8.GetString(base64EncodedBytes);
// after: world!


In this article, we’ve learned how to Base64 encode and decode in C#. In addition, we’ve shown how to encode a string to Base64 and vice-versa.