public bool ValidateAppSignature(Android.Content.Context context)
{
    PackageInfo packageInfo = context.PackageManager.GetPackageInfo(context.PackageName, PackageInfoFlags.Signatures);
    //note sample just checks the first signature
    foreach (Signature signature in packageInfo.Signatures) {
    // SHA1 the signature
    String sha1 = GetSHA1(signature.ToByteArray());
    // check is matches hardcoded value
    return APP_SIGNATURE.Equals(sha1);
}

//computed the sha1 hash of the signature
public static String GetSHA1(byte[] sig)
{
    Java.Security.MessageDigest digest = Java.Security.MessageDigest.GetInstance("SHA1", "BC");
    digest.Update(sig);
    byte[] hashtext = digest.Digest();
    return BytesToHex(hashtext);
}

//util method to convert byte array to hex string
public static String BytesToHex(byte[] bytes)
{
    char[] hexArray = { '0', '1', '2', '3', '4', '5', '6', '7', '8',
        '9', 'A', 'B', 'C', 'D', 'E', 'F' };
    char[] hexChars = new char[bytes.Length * 2];
    int v;
    for (int j = 0; j < bytes.Length; j++)
    {
        v = bytes[j] & 0xFF;
        hexChars[j * 2] = hexArray[(int)((uint)v >> 4)];
        hexChars[j * 2 + 1] = hexArray[v & 0x0F];
    }
    return new String(hexChars);
}