Use the Append() method to append a character to StringBuilder : StringBuilder « String « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. String
3. StringBuilder

using System;
using System.Text;

class MainClass
{
  public static void Main()
  {
    StringBuilder myStringBuilder = new StringBuilder();
    
    myStringBuilder.Append("myString");
    
    myStringBuilder.Append('z', 5);


    Console.WriteLine(myStringBuilder);
  }
}

myStringzzzzz