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Base64 Encoder and Decoder in C# / .NET — Code Examples

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Base64 encoding converts binary or text data into a 64-character ASCII alphabet so it can be safely transmitted over text-only channels. Here is how to encode and decode Base64 in each language using only the standard library.

C# provides Convert.ToBase64String and Convert.FromBase64String in the BCL. For URL-safe Base64, replace non-URL-safe characters after encoding.

Also in:JavaScript / Node.jsPythonGoJavaPHPRubyRust
C# / .NET
using System;
using System.Text;

// Encode
byte[] bytes = Encoding.UTF8.GetBytes("Hello, world!");
string encoded = Convert.ToBase64String(bytes);
Console.WriteLine(encoded); // SGVsbG8sIHdvcmxkIQ==

// Decode
byte[] decodedBytes = Convert.FromBase64String("SGVsbG8sIHdvcmxkIQ==");
string decoded = Encoding.UTF8.GetString(decodedBytes);
Console.WriteLine(decoded); // Hello, world!

// URL-safe Base64 (replace + with - and / with _, strip =)
string urlSafe = encoded.Replace('+', '-').Replace('/', '_').TrimEnd('=');

// Decode URL-safe Base64 (restore padding before decoding)
string padded = urlSafe.Replace('-', '+').Replace('_', '/');
int rem = padded.Length % 4;
if (rem > 0) padded = padded.PadRight(padded.Length + (4 - rem), '=');
string urlDecoded = Encoding.UTF8.GetString(Convert.FromBase64String(padded));

// .NET 8+: cleaner URL-safe API
// string urlSafe = System.Buffers.Text.Base64Url.EncodeToString(bytes);
// byte[] back    = System.Buffers.Text.Base64Url.DecodeFromChars(urlSafe);
Notes & gotchas
  • Convert.FromBase64String throws FormatException on invalid input — wrap in try/catch in production code.
  • For .NET 8+, System.Buffers.Text.Base64Url provides a cleaner, allocation-friendly URL-safe API.
  • Always specify Encoding.UTF8 explicitly when converting strings to bytes to avoid platform encoding surprises.
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Base64 Encoder and Decoder in other languages

Base64 Encoder and Decoder in JavaScript / Node.js

In browsers use btoa() and atob(). In Node.js use Buffer. For Unicode safety in the browser, encode via TextEncoder first.

Base64 Encoder and Decoder in Python

Python's standard library includes the base64 module. Use b64encode and b64decode for standard Base64, and urlsafe_b64encode for URL-safe variants.

Base64 Encoder and Decoder in Go

Go's encoding/base64 package provides StdEncoding for standard Base64 and URLEncoding for the URL-safe variant.

Base64 Encoder and Decoder in Java

Java 8+ includes java.util.Base64. Use getEncoder() for standard Base64 and getUrlEncoder() for URL-safe encoding.

Base64 Encoder and Decoder in PHP

PHP has built-in base64_encode() and base64_decode() functions. For URL-safe Base64, replace the non-URL-safe characters after encoding.

Base64 Encoder and Decoder in Ruby

Ruby's standard library includes the base64 module. Use strict_encode64 and strict_decode64 for standard Base64 without line-break insertion.

Base64 Encoder and Decoder in Rust

Rust uses the base64 crate. Use general_purpose::STANDARD for standard Base64 and URL_SAFE_NO_PAD for JWT and URL contexts.