Problem of struct union in C#

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I Have this struct:

C#

public struct A
{
    public int first;
}
public struct B
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=32)]
    public char[] first;
}
[StructLayout(LayoutKind.Explicit)]
public struct CUnion
{
    [FieldOffset(0)]
    public A _a;

    [FieldOffset(0)]
    public B _b;
}
public struct C
{
    public int type;
    public CUnion u;
}

but when i instance new C struct, this error

C#

C c = new C();

// Exception
// An unhandled exception of type 'System.TypeLoadException' occurred in Microsoft.VisualStudio.HostingProcess.Utilities.dll
// Additional information: Could not load type 'CUnion' from assembly '...' because it contains an object field at offset 0 that is incorrectly aligned or overlapped by a non-object field.

Any ideia?

PS: fixed memory does not apply because the memory is not senquencial

Posted 3-Dec-14 2:14am

theafien

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Comments

Sinisa Hajnal 3-Dec-14 8:21am

It was long time ago, but isn't struct different from the class because it behaves like value type and has no constructor? Maybe I'm misremembering my C :)

theafien 3-Dec-14 8:33am

i'll use this struct in pinvoke function

1 solution

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Er. Dinesh Sharma · Answer 1 · 2014-12-03T02:33:00

Solution 1

Please Try this

C#

public struct A
   {
       public int first;
   }
   [StructLayout(LayoutKind.Sequential, Pack = 1)]
   public struct B
   {
       [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
       public char[] first;
   }
   [StructLayout(LayoutKind.Explicit)]
   public struct CUnion
   {
       [FieldOffset(4)]
       public A _a;

       [FieldOffset(0)]
       public B _b;
   }
   public struct C
   {
       public int type;
       public CUnion u;
   }

Posted 3-Dec-14 2:33am

Er. Dinesh Sharma

Comments

theafien 3-Dec-14 8:35am

why _a the offset is 4?

Er. Dinesh Sharma 3-Dec-14 8:54am

application memory is divided into blocks (in a 4-bytes base,) and every block has its own address.

theafien 3-Dec-14 9:01am

but i want share same memory block with A and B

Er. Dinesh Sharma 3-Dec-14 9:21am

You can not assign same memory block to same data types.