After some long research i found a way to do it . If someone else is looking for how to sign a message using PKCS7 format using a certificate in C# then here it is,
public byte[] SignMsg(
Byte[] msg,
X509Certificate2 signerCert)
{
// Place message in a ContentInfo object.
// This is required to build a SignedCms object.
ContentInfo contentInfo = new ContentInfo(msg);
// Instantiate SignedCms object with the ContentInfo above.
// Has default SubjectIdentifierType IssuerAndSerialNumber.
// Has default Detached property value false, so message is
// included in the encoded SignedCms.
SignedCms signedCms = new SignedCms(contentInfo);
// Formulate a CmsSigner object, which has all the needed
// characteristics of the signer.
CmsSigner cmsSigner = new CmsSigner(signerCert);
// Sign the PKCS #7 message.
Console.Write("Computing signature with signer subject " +
"name {0} ... ", signerCert.SubjectName.Name);
signedCms.ComputeSignature(cmsSigner);
Console.WriteLine("Done.");
// Encode the PKCS #7 message.
return signedCms.Encode();
}
Found the information from the link http://msdn.microsoft.com/en-us/library/ms180961%28v=vs.85%29.aspx .