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C# | Convert.FromBase64String(String) Method - GeeksforGeeks

C# | Convert.FromBase64String(String) Method

Last Updated : 11 Jul, 2025

This method is used to convert the specified string, which encodes binary data as base-64 digits, to an equivalent 8-bit unsigned integer array.

Syntax:

public static byte[] FromBase64String (string s);

Here,

is the string to convert.

Return Value:

This method returns an array of 8-bit unsigned integers that is equivalent to

Exceptions

ArgumentNullException: If the s is null.
FormatException: If the length of s, ignoring white-space characters, is not zero or a multiple of 4. OR the format of s is invalid. s contains a non-base-64 character, more than two padding characters, or a non-white space character among the padding characters.

Below programs illustrate the use of

Convert.FromBase64String(String)

Method:

Example 1: csharp


 // C# program to demonstrate the
// Convert.FromBase64String(String) Method
using System;

class GFG {

    // Main Method
    public static void Main()
    {
        try {

            // defining and initializing
            // byte1 and byte2
            byte[] byte1 = {2, 4, 6, 8, 10, 
                       12, 14, 16, 18, 20};

            byte[] byte2 = {10, 20, 30, 
                                40, 50};
          
            // calling get() Method
            get(byte1, "byte1");

            Console.WriteLine("");
            get(byte2, "byte2");
        }

        catch (ArgumentNullException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }

        catch (FormatException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), 
                                  e.Message);
        }
    }

    // Defining get() method
    public static void get(byte[] bytes, string str)
    {

        Console.WriteLine("For {0}", str);

        // converting byte to base 64 string
        string s = Convert.ToBase64String(bytes);

        Console.WriteLine("The base 64 string: '{0}'", s);
        Console.WriteLine("The base 64 string: '{0}'", s);

        // converting base 64 string to byte array
        byte[] val = Convert.FromBase64String(s);
        Console.WriteLine("Converted byte value: {0}", 
                          BitConverter.ToString(val));
    }
}

Output:

For byte1
The base 64 string: 'AgQGCAoMDhASFA=='
The base 64 string: 'AgQGCAoMDhASFA=='
Converted byte value: 02-04-06-08-0A-0C-0E-10-12-14

For byte2
The base 64 string: 'ChQeKDI='
The base 64 string: 'ChQeKDI='
Converted byte value: 0A-14-1E-28-32

Example 2:

For

ArgumentNullException csharp


 // C# program to demonstrate the
// Convert.FromBase64String(String) Method
using System;

class GFG {

    // Main Method
    public static void Main()
    {
        try {

            // defining and initializing
            // byte1 and byte2
            byte[] byte1 = {2, 4, 6, 8, 10, 12,
                               14, 16, 18, 20};

            // calling get() Method
            get(byte1, "byte1");
            Console.WriteLine("");

            // converting base 64 string to byte array
            Console.WriteLine("s is null");

            byte[] val = Convert.FromBase64String(null);
            Console.WriteLine("Converted byte value: {0}",
                              BitConverter.ToString(val));
        }
        catch (ArgumentNullException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
        catch (FormatException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
    }

    // Defining get() method
    public static void get(byte[] bytes, string str)
    {

        Console.WriteLine("For {0}", str);

        // converting byte to base 64 string
        string s = Convert.ToBase64String(bytes);
        Console.WriteLine("The base 64 string: '{0}'", s);

        // converting base 64 string to byte array
        byte[] val = Convert.FromBase64String(s);
        Console.WriteLine("Converted byte value: {0}", 
                          BitConverter.ToString(val));
    }
}

Output:

For byte1
The base 64 string: 'AgQGCAoMDhASFA=='
Converted byte value: 02-04-06-08-0A-0C-0E-10-12-14

s is null
Exception Thrown: System.ArgumentNullException

Example 3:

For

FormatException csharp


 // C# program to demonstrate the
// Convert.FromBase64String(String) Method
using System;

class GFG {

    // Main Method
    public static void Main()
    {

        try {

            // defining and initializing 
            // byte1 and byte2
            byte[] byte1 = {2, 4, 6, 8, 10, 
                       12, 14, 16, 18, 20};

            // calling get() Method
            get(byte1, "byte1");
            Console.WriteLine("");

            // converting base 64 
            // string to byte array
            Console.WriteLine("s contains a non-base-64 character");

            byte[] val = Convert.FromBase64String("sun");
            Console.WriteLine("Converted byte value: {0}", 
                              BitConverter.ToString(val));
        }
        catch (ArgumentNullException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
        catch (FormatException e) {

            Console.Write("Exception Thrown: ");
            Console.Write("{0}", e.GetType(), e.Message);
        }
    }

    // Defining get() method
    public static void get(byte[] bytes, string str)
    {

        Console.WriteLine("For {0}", str);

        // converting byte to base 64 string
        string s = Convert.ToBase64String(bytes);
        Console.WriteLine("The base 64 string: '{0}'", s);

        // converting base 64 string to byte array
        byte[] val = Convert.FromBase64String(s);
        Console.WriteLine("Converted byte value: {0}",
                           BitConverter.ToString(val));
    }
}

Output:

For byte1
The base 64 string: 'AgQGCAoMDhASFA=='
Converted byte value: 02-04-06-08-0A-0C-0E-10-12-14

s contains a non-base-64 character
Exception Thrown: System.FormatException

Reference:

https://learn.microsoft.com/en-us/dotnet/api/system.convert.frombase64string?view=netframework-4.7.2

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