Member Avatar for Gody

Hi freinds,

I need your help regarding the following piece of code.

public unsafe struct GUID
    {
        public int Data1;
        public System.UInt16 Data2;
        public System.UInt16 Data3;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
        public byte[] Data4;
    }

When I build the code, I get the following error:-

Usafe code may only appear if compiling with /unsafe

Please reply with suggestions.

Edited by mike_2000_17 because: Fixed formatting
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  • Latest Post Latest Post by SergioGP

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— that compiler message means the compiler saw an unsafe construct but your build isn't allowing unsafe code. is right that enabling unsafe compilation fixes it, but there are two common, often-better alternatives depending on what you actually need: (1) If you only use managed types (for example a normal byte[]), you can simply remove the unsafe keyword — no special compile flag is required. (2) If you need an inlined, fixed-size 8-byte buffer for exact unmanaged layout, use a fixed buffer (which does require unsafe).

A fixed-buffer example (requires /unsafe):

unsafe struct GuidFixed
{
    public int Data1;
    public ushort Data2;
    public ushort Data3;
    public fixed byte Data4[8];
}

To enable unsafe compilation if you really need it: set the MSBuild property, use the compiler switch, or enable it in Visual Studio for the active configuration. Example .csproj snippet:

<PropertyGroup>
  <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
</PropertyGroup>

Or on the command line: csc /unsafe MyFile.cs or dotnet build -p:AllowUnsafeBlocks=true. In Visual Studio: Project -> Properties -> Build -> check "Allow unsafe code" for the configuration you are building. Troubleshooting tips: make sure you change the right configuration (Debug/Release), then clean and rebuild if the error persists.

Recommendation: for most use cases you do not need a custom GUID struct — use System.Guid or a [StructLayout(LayoutKind.Sequential)] wrapper for P/Invoke and let the marshaller handle layout. Only enable unsafe when fixed buffers or pointers are required, and keep the scope minimal.

Member Avatar for SergioGP

Go to your Project properties and under Configuration Properties Click on build and then change Allow Unsafe Code blocks tu true.

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