How to replace text with random text?

static void Main()
{
    Console.WriteLine(GetString(20));
}

static Random rand = new Random();

private static char GetRandomChar()
{
    return (char)rand.Next('a', 'z');
}

private static string GetString(int length)
{
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < length; i++)
    {
        sb.Append(GetRandomChar());
    }

    return sb.ToString();
}

Private Shared Sub Main()
    Console.WriteLine(GetString(20))
End Sub

Shared rand As New Random()

Private Shared Function GetRandomChar() As Char
    Return CChar(rand.[Next]("a"C, "z"C))
End Function

Private Shared Function GetString(length As Integer) As String
    Dim sb As New StringBuilder()
    For i As Integer = 0 To length - 1
        sb.Append(GetRandomChar())
    Next

    Return sb.ToString()
End Function

Friend Class Program
    ' Methods
    Private Shared Function GetRandomChar() As Char
        Return DirectCast(Program.rand.Next(&H61, &H7A), Char)
    End Function

    Private Shared Function GetString(ByVal length As Integer) As String
        Dim sb As New StringBuilder
        Dim i As Integer
        For i = 0 To length - 1
            sb.Append(Program.GetRandomChar)
        Next i
        Return sb.ToString
    End Function

    Private Shared Sub Main()
        Console.WriteLine(Program.GetString(20))
    End Sub


    ' Fields
    Private Shared rand As Random = New Random
End Class