Find SHA256 value of a File in CSharp (C#)

Use the below code to find the SHA256 value of a file. When the user clicks the btn_Generate_Click button the SHA256 value will be generated and displayed in a RichTextBox.
C# Code:

 private void btn_Generate_Click(object sender, EventArgs e)  
 {    
    hashOfFile("ENTER THE FILE PATH TO FIND SHA256");  
 }  
 

 public void hashOfFile(string fileToHash)  
 {  
    FileStream rdr = default(FileStream);  
    SHA256CryptoServiceProvider sha = new SHA256CryptoServiceProvider();  
    byte[] bytes = null;  
    string rtn = "";  

    if (File.Exists(fileToHash))  
    {  
        rdr = new FileStream(fileToHash, FileMode.Open, FileAccess.Read);  
        bytes = sha.ComputeHash(rdr);  
        rtn = ByteArrayToString(bytes);  
    }  

    richTextBox1.Text=rtn;  
 } 

 private string ByteArrayToString(byte[] arrInput)  
 {  
    System.Text.StringBuilder sb = new System.Text.StringBuilder(arrInput.Length * 2);  
    for (int i = 0; i <= arrInput.Length - 1; i++)  
    {  
        sb.Append(arrInput[i].ToString("X2"));  
    } 
 
    return sb.ToString().ToLower();  
 }