this is not a question, this is snippet to encode any file to base64 string and decode that encoded string to that file back. i attach the project to this post too.

Form1.cs :

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace encode_decode_Base64
{
	public partial class Form1 : Form
	{
		public Form1()
		{
			InitializeComponent();
		}
		private void btnEncode_Click(object sender, EventArgs e)
		{
			if (!string.IsNullOrEmpty(txtInFile.Text))
			{
				FileStream fs = new FileStream(txtInFile.Text,
														 FileMode.Open,
														 FileAccess.Read);
				byte[] filebytes = new byte[fs.Length];
				fs.Read(filebytes, 0, Convert.ToInt32(fs.Length));
				string encodedData =
					 Convert.ToBase64String(filebytes,
													Base64FormattingOptions.InsertLineBreaks);
				txtEncoded.Text = encodedData;
			}

		}

		private void btnDecode_Click(object sender, EventArgs e)
		{
			if (saveFileDialog1.ShowDialog() == DialogResult.OK)
			{
				
					byte[] filebytes = Convert.FromBase64String(txtEncoded.Text);
					FileStream fs = new FileStream(saveFileDialog1.FileName,
															 FileMode.CreateNew,
															 FileAccess.Write,
															 FileShare.None);
					fs.Write(filebytes, 0, filebytes.Length);
					fs.Close();
				
			}

		}

		private void button1_Click(object sender, EventArgs e)
		{
			if (openFileDialog1.ShowDialog() == DialogResult.OK)
			{
				txtInFile.Text = openFileDialog1.FileName;
			}
		}

		
	}
}

Form1.Designer.cs:

namespace encode_decode_Base64
{
	partial class Form1
	{
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.IContainer components = null;

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		/// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
		protected override void Dispose(bool disposing)
		{
			if (disposing && (components != null))
			{
				components.Dispose();
			}
			base.Dispose(disposing);
		}

		#region Windows Form Designer generated code

		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.btnEncode = new System.Windows.Forms.Button();
			this.btnDecode = new System.Windows.Forms.Button();
			this.txtEncoded = new System.Windows.Forms.TextBox();
			this.txtInFile = new System.Windows.Forms.TextBox();
			this.openFileDialog1 = new System.Windows.Forms.OpenFileDialog();
			this.button1 = new System.Windows.Forms.Button();
			this.lblBrowse = new System.Windows.Forms.Label();
			this.lblEncoded = new System.Windows.Forms.Label();
			this.saveFileDialog1 = new System.Windows.Forms.SaveFileDialog();
			this.SuspendLayout();
			// 
			// btnEncode
			// 
			this.btnEncode.Location = new System.Drawing.Point(288, 87);
			this.btnEncode.Name = "btnEncode";
			this.btnEncode.Size = new System.Drawing.Size(75, 23);
			this.btnEncode.TabIndex = 1;
			this.btnEncode.Text = "Encode";
			this.btnEncode.UseVisualStyleBackColor = true;
			this.btnEncode.Click += new System.EventHandler(this.btnEncode_Click);
			// 
			// btnDecode
			// 
			this.btnDecode.Location = new System.Drawing.Point(288, 415);
			this.btnDecode.Name = "btnDecode";
			this.btnDecode.Size = new System.Drawing.Size(75, 23);
			this.btnDecode.TabIndex = 2;
			this.btnDecode.Text = "Decode";
			this.btnDecode.UseVisualStyleBackColor = true;
			this.btnDecode.Click += new System.EventHandler(this.btnDecode_Click);
			// 
			// txtEncoded
			// 
			this.txtEncoded.Location = new System.Drawing.Point(28, 155);
			this.txtEncoded.Multiline = true;
			this.txtEncoded.Name = "txtEncoded";
			this.txtEncoded.Size = new System.Drawing.Size(381, 228);
			this.txtEncoded.TabIndex = 3;
			// 
			// txtInFile
			// 
			this.txtInFile.Location = new System.Drawing.Point(28, 87);
			this.txtInFile.Name = "txtInFile";
			this.txtInFile.Size = new System.Drawing.Size(161, 20);
			this.txtInFile.TabIndex = 5;
			// 
			// openFileDialog1
			// 
			this.openFileDialog1.FileName = "openFileDialog1";
			// 
			// button1
			// 
			this.button1.Location = new System.Drawing.Point(212, 85);
			this.button1.Name = "button1";
			this.button1.Size = new System.Drawing.Size(43, 23);
			this.button1.TabIndex = 6;
			this.button1.Text = "...";
			this.button1.UseVisualStyleBackColor = true;
			this.button1.Click += new System.EventHandler(this.button1_Click);
			// 
			// lblBrowse
			// 
			this.lblBrowse.AutoSize = true;
			this.lblBrowse.Location = new System.Drawing.Point(35, 57);
			this.lblBrowse.Name = "lblBrowse";
			this.lblBrowse.Size = new System.Drawing.Size(113, 13);
			this.lblBrowse.TabIndex = 7;
			this.lblBrowse.Text = "Select a file to encode";
			// 
			// lblEncoded
			// 
			this.lblEncoded.AutoSize = true;
			this.lblEncoded.Location = new System.Drawing.Point(35, 126);
			this.lblEncoded.Name = "lblEncoded";
			this.lblEncoded.Size = new System.Drawing.Size(105, 13);
			this.lblEncoded.TabIndex = 8;
			this.lblEncoded.Text = "Encoded file content";
			// 
			// Form1
			// 
			this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
			this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
			this.ClientSize = new System.Drawing.Size(517, 684);
			this.Controls.Add(this.lblEncoded);
			this.Controls.Add(this.lblBrowse);
			this.Controls.Add(this.button1);
			this.Controls.Add(this.txtInFile);
			this.Controls.Add(this.txtEncoded);
			this.Controls.Add(this.btnDecode);
			this.Controls.Add(this.btnEncode);
			this.Name = "Form1";
			this.Text = "Form1";
			this.ResumeLayout(false);
			this.PerformLayout();

		}

		#endregion

		private System.Windows.Forms.Button btnEncode;
		private System.Windows.Forms.Button btnDecode;
		private System.Windows.Forms.TextBox txtEncoded;
		private System.Windows.Forms.TextBox txtInFile;
		private System.Windows.Forms.OpenFileDialog openFileDialog1;
		private System.Windows.Forms.Button button1;
		private System.Windows.Forms.Label lblBrowse;
		private System.Windows.Forms.Label lblEncoded;
		private System.Windows.Forms.SaveFileDialog saveFileDialog1;
	}
}
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