How to create a C/C++ union by using attributes in C#
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.
In this code segment, all of the fields of TestUnion
start at the same location in memory.
[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 code is another example where fields start at different explicitly set locations.
[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 integer fields, i1
and i2
combined, share the same memory locations as lg
. Either lg
uses the first 8 bytes, or i1
uses the first 4 bytes and i2
uses the next 4 bytes. This sort of control over struct layout is useful when using platform invocation.
See also
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