How to Decode Base64 Data In C# & .NET

In a previous post, “How to Encode Base64 Data In C# & .NET”, we looked at how to correctly encode arbitrary data in Base64 for representation and transmission as text

In this post, we will look at the opposite - how to decode Base64 data

Here, like before, we have three options:

The FromBase64String in the Convert class.
The DecodeFromUtf8 or DecodeFromChars in the Base64Url class
The DecodeFromUtf8 method in the Base64 class.

Let us use the encoded string form the example in our previous post:

eyJTZWFyY2hUZXh0IjoiY29ucmFkICIsIlNvcnRDb2x1bW4iOiJOYW1lIiwiUGFnZU51bWJlciI6MCwiUGFnZVNpemUiOjUwLCJEYXRhYmFzZSI6IkFuaW1hbHMiLCJBY3RpdmVPbmx5Ijp0cnVlfQ==

The code is as follows:

const string encodedData = "eyJTZWFyY2hUZXh0IjoiY29ucmFkICIsIlNvcnRDb2x1bW4iOiJOYW1lIiwiUGFnZU51bWJlciI6MCwiUGFnZVNpemUiOjUwLCJEYXRhYmFzZSI6IkFuaW1hbHMiLCJBY3RpdmVPbmx5Ijp0cnVlfQ==";

var convert = Convert.FromBase64String(encodedData);
var base64 = Base64.DecodeFromUtf8(Encoding.Default.GetBytes(encodedData));
var base64Url = Base64Url.DecodeFromUtf8(Encoding.Default.GetBytes(encodedData));

The result of the conversion is a byte array, byte[].

We can test for equality using the LINQ SequenceEquals method as follows:

Console.WriteLine(convert.SequenceEqual(base64) && base64.SequenceEqual(base64Url));

This will print the following:

decodeCheck

If you want the result back as a string, you can retrieve it as follows:

var original = Encoding.Default.GetString(base64Url);
Console.WriteLine(original);

This returns the following:

originalResult

TLDR

There are 3 main ways to decode Base64 data.

The code is in my GitHub.

Happy hacking!