使用Skype4Com在C#中设置skype头像不起作用

我想用这个C#代码设置我的skype化身：

string path = "C:''a.jpg";
string cmdstr = "SET AVATAR" + " " + "1" + " " + path;
Command cmd = new Command(0, cmdstr);
skype.SendCommand(cmd);

这段代码不起作用，但如果我在VBA中使用这段代码，它会起作用：

Public skype As New Skype
Dim cmdStr, path As String
path = "C:'a.jpg"
cmdStr = "SET AVATAR" + " " + "1" + " " + path
skype.SendCommand skype.Command(0, cmdStr)

这与图像的延伸有关，还是我错过了什么？

            string path = "C:''b.jpg";
            var skype = new SKYPE4COMLib.Skype();
            var cmd = new SKYPE4COMLib.Command();
            cmd.Command = string.Format("SET AVATAR 1 {0}", path);              
            skype.SendCommand(cmd);