SA1214 (Readonly fields should appear before non-readonly fields) should not apply to fields when [StructLayout(LayoutKind.Sequential)] is in use.

[Kevin-Seiden]

Consider the following struct:

[StructLayout(LayoutKind.Sequential)]
public struct MyStruct
{
  private int myInt;

  [MarshalAs(UnmanagedType.LPStr)]
  private readonly string myString; // Produces SA1214

  // Accessor properties here
}

Since the struct is adorned with LayoutKind.Sequential, this struct is likely being used in marshaling to unmanaged memory and the order of the fields is imperative to the functionality of the code. I would expect StyleCop Analysers to not recomend SA1214 (or any field ordering rules) in this case.