Marshal.SizeOf() cannot calculate size of object containing string

Marshal.SizeOf() will throw an exception when trying to calculate the lenght of an object of type MyClass.

Here is the class:

<StructLayout(LayoutKind.Sequential, Pack:=1)>
Public Class MyClass

    Public ReadOnly UniqueId As Long

    <MarshalAs(UnmanagedType.AnsiBStr, SizeConst:=60, SizeParamIndex:=0)>
    Public ReadOnly Name As String

End Class

This code will fail:

Dim MyObject = New MyClass()
Dim size  = Marshal.SizeOf(MyObject) 'will throw exception here. Why?

It will throw the exception "no meaningful size or offset can be computed"

How can I get the lenght of MyObject instead?

Solution

I believe that UnmanagedType.AnsiBStr can only be used on parameters (passed values) of a method signature.

A reference to a BSTR is a pointer to a length prefixed character array. As such it will be a .Net Intptr with a size of 4 or 8 bytes depending on process bitness (x32 or x64). If you need ANSI characters, you define that as part of the StructLayout declaration and tag the string as a UnmanagedType.BStr

<StructLayout(LayoutKind.Sequential, Pack:=1, CharSet:=CharSet.Ansi)>
Public Class [MyClass]
     Public ReadOnly UniqueId As Long
    <MarshalAs(UnmanagedType.BStr)>
     Public ReadOnly Name As String
End Class