[.NET 9] DllImport with Custom Marshaler Passes non-null address when null value for field is specified. #109033
Description
Description
In .NET 9 (Preview 2 and newer), passing null to a parameter which uses a CustomMarshaler results in a non-zero value passed to native code. This can lead to memory corruption or invalid memory reads, depending on what the native code does with the value.
Reproduction Steps
Attached reproduction: cimgui-stub.zip
To Reproduce
Run the csharp project (csharp.csproj) provided.
When ran with .NET 9 Preview 1 or older, the result is:
igDragInt called with:
label address: 2941681900960
v address: 750695999576
format address: 0
label value: label
v value: 0
v_speed: 0.1
v_min: 0
v_max: 100
format: null
flags: 0
When ran with .NET 9 Preview 2 or newer the result is:
igDragInt called with:
label address: 1824962101440
v address: 114923399832
format address: 14829735431805717965
label value: label
v value: 0
v_speed: 0.1
v_min: 0
v_max: 100
Fatal error. 0xC0000005
at ImGui.DragInt(System.String, Int32 ByRef, Single, Int32, Int32, System.String, Int32)
at Program.<Main>$(System.String[])
When null is passed to a string parameter with any CustomMarshaler specified, the generated DllImport stub passes a non-zero address. (Note: Marshaler is skipped, so implementation of marshaler used with CustomMarshaler is not relevant)
To Compile the Rust Library/ Native Code
[Note: I've included an x64 precompiled DLL in the csharp folder out of the box, this is only needed if you want to change the native DllImport target, e.g. reduce param count for debugging]
- Install
rustup: https://static.rust-lang.org/rustup/dist/x86_64-pc-windows-msvc/rustup-init.exe - cd into
cimgui_export cargo build --release- Result is in
target/release/cimgui_export.dll - Paste into
csharpfolder, replacing the existing file.
Expected behavior
format address should be 0 (null). Instead an unexpected address is used.
Actual behavior
format address is not 0 (null)
Regression?
.NET 9 (Preview 1) and older work fine.
Regression is introduced in Preview 2.
Known Workarounds
No response
Configuration
N/A
This reproduces in both x86 and x64 on Windows.
Other information
The reproduction is small enough that it can be included here for convenience:
var v = 0;
while (true)
{
ImGui.DragInt("label", ref v, 0.1f, 0, 100, null!, 0);
Thread.Sleep(1000);
}
static class ImGui
{
[DllImport("cimgui_export", CallingConvention = CallingConvention.Cdecl, EntryPoint = "igDragInt")]
[SuppressUnmanagedCodeSecurity]
[return: MarshalAs(UnmanagedType.I1)]
public static extern bool DragInt(
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalType = "csharp.UTF8Marshaller, csharp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null")]
string label,
ref int v,
float v_speed,
int v_min,
int v_max,
[MarshalAs(UnmanagedType.CustomMarshaler, MarshalType = "csharp.UTF8Marshaller, csharp, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null")]
string format,
int flags
);
}Rust part:
use std::ffi::CStr;
use std::os::raw::{c_char, c_int, c_float, c_uchar};
#[no_mangle]
pub extern "C" fn igDragInt(
label: *const c_char,
v: *mut c_int,
v_speed: c_float,
v_min: c_int,
v_max: c_int,
format: *const c_char,
flags: c_int,
) -> c_uchar {
// Print addresses of all pointers
println!("igDragInt called with:");
println!(" label address: {:?}", label as usize);
println!(" v address: {:?}", v as usize);
println!(" format address: {:?}", format as usize);
// Handle `label`
if label.is_null() {
println!(" label: null");
} else {
// Convert C string to Rust string
let c_label = unsafe { CStr::from_ptr(label) };
match c_label.to_str() {
Ok(s) => println!(" label value: {}", s),
Err(_) => println!(" label: Invalid UTF-8 string"),
}
}
// Handle `v`
if v.is_null() {
println!(" v: null");
} else {
// Safely read the integer value
let value = unsafe { *v };
println!(" v value: {}", value);
}
// Print other parameters
println!(" v_speed: {}", v_speed);
println!(" v_min: {}", v_min);
println!(" v_max: {}", v_max);
// Handle `format`
if format.is_null() {
println!(" format: null");
} else {
// Convert C string to Rust string
let c_format = unsafe { CStr::from_ptr(format) };
match c_format.to_str() {
Ok(s) => println!(" format value: {}", s),
Err(_) => println!(" format: Invalid UTF-8 string"),
}
}
println!(" flags: {}", flags);
// For demonstration, return true (1) to indicate success
1
}I can across this issue when trying older code with .NET 9 that used dear imgui bindings made with CppSharp. So in the repro I tried to model that.
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