Base64 in C#

How Do You Encode a String to Base64 in C#?

Use Convert.ToBase64String() combined with Encoding.UTF8.GetBytes() to encode a string to Base64. The GetBytes() call converts the string into a UTF-8 byte array, and ToBase64String() produces the Base64-encoded output. This method has been available since .NET Framework 1.0 and works across all .NET versions.

using System;
using System.Text;

string original = "Hello, World!";
byte[] bytes = Encoding.UTF8.GetBytes(original);
string encoded = Convert.ToBase64String(bytes);
Console.WriteLine(encoded); // SGVsbG8sIFdvcmxkIQ==

Always specify the character encoding explicitly using Encoding.UTF8 rather than Encoding.Default to ensure consistent behavior across platforms. The returned string uses the standard Base64 alphabet (A-Z, a-z, 0-9, +, /) with = padding. For a browser-based alternative, use the Base64 text encoder.

How Do You Decode Base64 in C#?

Use Convert.FromBase64String() to convert a Base64 string back to a byte array, then Encoding.UTF8.GetString() to reconstruct the original string. The FromBase64String() method validates the input and throws FormatException if the string contains invalid Base64 characters or has incorrect padding.

using System;
using System.Text;

string encoded = "SGVsbG8sIFdvcmxkIQ==";
byte[] bytes = Convert.FromBase64String(encoded);
string decoded = Encoding.UTF8.GetString(bytes);
Console.WriteLine(decoded); // Hello, World!

Validate Base64 input before decoding in production code. The decoder rejects whitespace characters other than spaces, tabs, and newlines. To decode Base64 strings directly in your browser, use the Base64 text decoder.

How Do You Encode a File to Base64 in C#?

Read the file into a byte array using File.ReadAllBytes(), then pass the array to Convert.ToBase64String(). This approach loads the entire file into memory and works for files that fit within available RAM.

using System;
using System.IO;

byte[] fileBytes = File.ReadAllBytes("image.png");
string encoded = Convert.ToBase64String(fileBytes);

// Create a data URI for HTML embedding
string dataUri = "data:image/png;base64," + encoded;
Console.WriteLine(dataUri);

For images, prepend the appropriate MIME type to create a data URI. The Base64 file encoder performs the same operation in your browser without uploading the file. To decode back to a file, use File.WriteAllBytes(path, Convert.FromBase64String(base64)).

What Base64 Methods Does .NET Provide?

The System.Convert class provides 5 Base64-related methods. .NET Core 2.1 added TryFromBase64String() for non-throwing validation. The Base64FormattingOptions enum controls whether the output includes line breaks for MIME compatibility.

The Base64FormattingOptions.InsertLineBreaks flag inserts CRLF every 76 characters, matching MIME format (RFC 2045). The default Base64FormattingOptions.None produces a continuous string with no line breaks. For a complete reference on the Base64 alphabet, see the Base64 character table.

How Do You Use Base64 with Streams in C#?

Use CryptoStream with ToBase64Transform or FromBase64Transform from the System.Security.Cryptography namespace to process large files without loading them entirely into memory. The transform classes process data in fixed-size blocks through a stream pipeline.

using System.IO;
using System.Security.Cryptography;

// Encode a large file to Base64 using streams
using (var input = File.OpenRead("large-file.bin"))
using (var output = File.Create("large-file.b64"))
using (var transform = new ToBase64Transform())
using (var cryptoStream = new CryptoStream(output, transform, CryptoStreamMode.Write))
{
    input.CopyTo(cryptoStream);
}

// Decode a Base64 file using streams
using (var input = File.OpenRead("large-file.b64"))
using (var output = File.Create("decoded-file.bin"))
using (var transform = new FromBase64Transform())
using (var cryptoStream = new CryptoStream(input, transform, CryptoStreamMode.Read))
{
    cryptoStream.CopyTo(output);
}

The ToBase64Transform processes 3 input bytes at a time and outputs 4 Base64 characters. The FromBase64Transform reverses this operation. This approach handles files of any size because only a small buffer resides in memory. Learn more about Base64 encoding fundamentals in the What is Base64 guide.

How Do You Create URL-Safe Base64 in C#?

Replace + with - and / with _ in the standard Base64 output, then optionally trim trailing = padding. In ASP.NET Core, use the built-in WebEncoders.Base64UrlEncode() method from Microsoft.AspNetCore.WebUtilities which handles the URL-safe alphabet automatically.

using System;
using System.Text;

byte[] data = Encoding.UTF8.GetBytes("subjects?abcd");

// Manual URL-safe encoding
string base64 = Convert.ToBase64String(data);
string urlSafe = base64.Replace('+', '-').Replace('/', '_').TrimEnd('=');
Console.WriteLine(urlSafe); // c3ViamVjdHM_YWJjZA

// ASP.NET Core: using Microsoft.AspNetCore.WebUtilities
// string urlSafe = WebEncoders.Base64UrlEncode(data);

URL-safe Base64 prevents encoding conflicts when the string appears in query parameters or URL paths. This variant follows RFC 4648 Section 5. The URL-safe Base64 tool provides browser-based encoding with the same alphabet.

How Do You Handle Base64 Errors in C#?

Convert.FromBase64String() throws System.FormatException when the input is invalid. Common causes include non-Base64 characters, incorrect string length, and malformed padding. Use Convert.TryFromBase64String() in .NET Core 2.1+ for non-throwing validation.

using System;

// Approach 1: Try-catch (all .NET versions)
try
{
    byte[] decoded = Convert.FromBase64String(input);
}
catch (FormatException ex)
{
    Console.WriteLine($"Invalid Base64: {ex.Message}");
}

// Approach 2: TryFromBase64String (.NET Core 2.1+)
Span<byte> buffer = new byte[input.Length];
if (Convert.TryFromBase64String(input, buffer, out int bytesWritten))
{
    // Valid Base64 - use buffer[..bytesWritten]
}
else
{
    Console.WriteLine("Invalid Base64 input");
}

The TryFromBase64String() method avoids the performance overhead of exception handling in validation scenarios. For quick browser-based validation, use the Base64 validator tool.

Frequently Asked Questions

Related Base64 Tools and Guides

• What is Base64? - Comprehensive guide to the Base64 encoding format
• Base64 Text Encoder - Encode text to Base64 in your browser
• Base64 File Encoder - Encode files to Base64 client-side
• URL-Safe Base64 Encoder - Encode with the URL-safe alphabet
• Base64 Validator - Validate Base64 strings instantly
• Base64 Character Table - Complete alphabet reference
• All Base64 Tools - Browse the complete toolset