How can I compare public keys in .NET?

I've got an X509Certificate2 containing a public key. I've got an RSACryptoServiceProvider (which came from calling SignedXml.CheckSignatureReturningKey), also containing a public key.

I want to find out if one came from the other. How can I compare the two?

You can compare the PublicKey property of signing certificates in the SignedXml.KeyIfo with signing key output from SignedXml.CheckSignatureReturningKey. This C# extension method does the job for me:

public static bool CheckSignatureReturningCertificate(this SignedXml signedXml, out X509Certificate2 signingCertificate)
{
    signingCertificate = null;
    AsymmetricAlgorithm signingKey;
    bool isValid = signedXml.CheckSignatureReturningKey(out signingKey);
    if (isValid)
    {
        IEnumerable<X509Certificate2> keyInfoCertificates =
            signedXml.KeyInfo.OfType<KeyInfoX509Data>()
                .SelectMany(x => x.Certificates.Cast<X509Certificate2>());

        signingCertificate = keyInfoCertificates.FirstOrDefault(x => x.PublicKey.Key == signingKey);
        if (signingCertificate == null)
        {
            throw new Exception("Signing certificate not found in KeyInfo.");
        }
    }

    return isValid;
}

Use it like this:

X509Certificate2 signingCertificate = null;
bool isValid = signedXml.CheckSignatureReturningCertificate(out signingCertificate);
if(isValid)
{
    // signingCertificate now contains the certificate used to sign
}