CryptExportKey

[DllImport("Advapi32.dll", SetLastError = true)][return: MarshalAs(UnmanagedType.Bool)]
public static extern bool CryptExportKey(
   IntPtr hKey,
   IntPtr hExpKey,
   uint dwBlobType,
   uint dwFlags,
   IntPtr pbData,
   ref uint pdwDataLen
);

Microsoft documentation: Link