生成Avatar

在诸如MSDN等论坛中，每个用户默认会根据名字生成唯一不同的头像图片（如果没有上传自己的头像的话）。这个功能是怎么实现的呢？

1）需要到某个网站（Avatar）请求获取一个随机图片。

2）格式：www.gravatar.com/avatar/HASH?r=PG&s=256&default=identicon

数值（可以是MD5）

3）HASH的生成（本文主要讨论这个，其余参数可以打开去自行查看参数说明）：

   3.1）使用MD5CryptoServiceProvider的ComputeHash方法对已经转化成Byte数组的字符串加密。

   3.2）然后使用格式化把生成的Byte数组按照十六进制输出。

4）源代码如下：

Imports System.Net
Imports System.Security.Cryptography
Imports System.Text
Imports System.IO

Public Class Form1
    Private Async Function Generate(name As String) As Task(Of Stream)
        Dim s As Stream = Nothing

        Await Task.Factory.StartNew(Sub()
                                        Dim md5 As New MD5CryptoServiceProvider
                                        Dim bytes() As Byte = md5.ComputeHash(Encoding.UTF32.GetBytes(TextBox1.Text))
                                        Dim result As New StringBuilder

                                        For Each Str As String In bytes.Select(Function(b) b.ToString("x"))
                                            result.Append(Str)
                                        Next

                                        Dim req As WebRequest = WebRequest.Create("http://www.gravatar.com/avatar/" & result.ToString() & "?r=PG&s=256&default=identicon")

                                        Dim resp As WebResponse = req.GetResponse()
                                        s = resp.GetResponseStream()
                                    End Sub)
        Return s
    End Function
    Private Sub Form1_Click(sender As Object, e As EventArgs) Handles MyBase.Click

    End Sub

    Private Async Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
        Text = "请稍后……"
        Dim s As Stream = Await Generate(TextBox1.Text)
        PictureBox1.Image = Image.FromStream(s)
        Text = "生成完毕"
    End Sub
End Class

结果测试：