C# AllowUnsafeBlocks (-unsafe): fix CS0227 "Unsafe code may only appear if compiling with /unsafe"

<AllowUnsafeBlocks>true</AllowUnsafeBlocks> in the project file, or dotnet build -p:AllowUnsafeBlocks=true, lets C# code use the unsafe keyword: pointers, fixed statements, pointer arithmetic, stackalloc into a pointer and function pointers. Without it, any unsafe block, method or type fails with error CS0227: Unsafe code may only appear if compiling with /unsafe. The property maps to the compiler option -unsafe (/unsafe).

Without and with the property

dotnet new console -n App -o . >/dev/null 2>&1
cat > Program.cs <<'EOF'
int[] data = [10, 20, 30];
unsafe
{
    fixed (int* p = data)
    {
        Console.WriteLine(*(p + 2));
    }
}
EOF
build() { dotnet build --no-incremental "$@" 2>&1 | grep -oE "(warning|error) CS[0-9]+[^[]*" | sort -u; echo "exit: ${PIPESTATUS[0]}"; }
echo "== default"
build
echo "== -p:AllowUnsafeBlocks=true"
build -p:AllowUnsafeBlocks=true
dotnet run --no-build

== default
error CS0227: Unsafe code may only appear if compiling with /unsafe
exit: 1
== -p:AllowUnsafeBlocks=true
exit: 0
30

The default build stops at CS0227. With the property set, the same code compiles and prints the third element, read through a pointer that fixed pins to the array.

What needs it

Setting it

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

Set it only in the projects that contain unsafe code, not repository-wide in Directory.Build.props, so a code review notices when unsafe code appears in a new project. For a quick experiment, dotnet build -p:AllowUnsafeBlocks=true works without editing the file (add --no-incremental if a previous build already succeeded with other settings).

Pitfalls

• Pointers into managed objects are only valid inside fixed (or with a pinned GCHandle); storing such a pointer and using it later reads memory the garbage collector may have moved.
• The runtime does not check pointer arithmetic: reading past the end of the array is undefined behaviour, not an IndexOutOfRangeException.
• CS0214 (Pointers and fixed size buffers may only be used in an unsafe context) is a different error: the project allows unsafe code, but the statement is outside an unsafe block or method.
• Language features also depend on the C# version: function pointers need C# 9 or later, see LangVersion.

FAQ