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How to: Create a C/C++ Union Using Attributes (C# Programming Guide)

Visual Studio 2005

By using attributes you can customize how structs are laid out in memory. For example, you can create what is known as a union in C/C++ by using the StructLayout(LayoutKind.Explicit) and FieldOffset attributes.

Example

In this code segment, all of the fields of TestUnion start at the same location in memory.

C#
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[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)]
struct TestUnion
{
    [System.Runtime.InteropServices.FieldOffset(0)]
    public int i;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public double d;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public char c;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public byte b;
}

The following is another example where fields start at different explicitly set locations.

C#
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[System.Runtime.InteropServices.StructLayout(LayoutKind.Explicit)]
struct TestExplicit
{
    [System.Runtime.InteropServices.FieldOffset(0)]
    public long lg;

    [System.Runtime.InteropServices.FieldOffset(0)]
    public int i1;

    [System.Runtime.InteropServices.FieldOffset(4)]
    public int i2;

    [System.Runtime.InteropServices.FieldOffset(8)]
    public double d;

    [System.Runtime.InteropServices.FieldOffset(12)]
    public char c;

    [System.Runtime.InteropServices.FieldOffset(14)]
    public byte b;
}

The two int fields, i1 and i2, share the same memory locations as lg. This sort of control over struct layout is useful when using platform invocation.

See Also

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struct union sample

What is the C# equivalent of this code ;


typedef struct _CRYPT_KEY_SIGN_MESSAGE_PARA {
DWORD cbSize;
DWORD dwMsgAndCertEncodingType;
union {
HCRYPTPROV hCryptProv;
NCRYPT_KEY_HANDLE hNCryptKey;
} ;
DWORD dwKeySpec;
CRYPT_ALGORITHM_IDENTIFIER HashAlgorithm;
void * pvHashAuxInfo;
CRYPT_ALGORITHM_IDENTIFIER PubKeyAlgorithm;
}CRYPT_KEY_SIGN_MESSAGE_PARA, *PCRYPT_KEY_SIGN_MESSAGE_PARA;

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