Using X509Certificate2 to get PrivateKey causes CryptographicException "Invalid provider type specified"

Hi, everyone,

I am developing a web application that uses X509Certificate2 to get a private key from a certification file. Code snippet looks like following:

        public static RSACryptoServiceProvider GetSignProviderFromPfx()
        {
            var strFileName = "c:\cer\mycerfile.pfx";
            var strPassword = "000000";            
            X509Certificate2 pc = new X509Certificate2(strFileName, strPassword, X509KeyStorageFlags.MachineKeySet);
            var ThePivateKey = pc.PrivateKey;

            return (RSACryptoServiceProvider)ThePivateKey;
        }

But the statement pc.Privatekey  causes a  System.Security.Cryptography.CryptographicException "Invalid provider type specified" . I'm sure the certification file has no problem, it really has a private key. And the property pc.HasPrivateKey is also return true. 

The test environment is VS2013,  window 7. 

I also tried following:

a. I debugged it in VS2013 with iis express, the problem occured.

b. I debugged it in another computer with same enviroment with mine, the problem occured too.

c. I published the application to a server with iis running on Windows Web Server 2008 R2,  it worked fine.

d. I published the application to widows azure website, it also worked fine.

Therefore, I guess the code snippet has no problem. The key reason raising the exception is that there may be some problem about runningenvironment.  I checked and compared the reading/writing right on the certification file in different environment, all of them are same.

Anybody can help?

Thanks.