How can I ignore a field when marshalling a structure with P/Invoke

There's no way to make the CLR ignore a field. I would instead use two structures, and perhaps make one a member of the other.

struct MyNativeStructure 
{ 
    public int foo; 
    public int bar; 
} 

struct MyStructure 
{ 
    public MyNativeStruct native; 
    public int ignored; 
}