Why Are My Struct Parameters Ignored When Passing from C# to a C++ DLL?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I'm attempting to pass a struct from my C# application to a native C++ DLL function, but while the method is called correctly, it ignores my provided values and always uses default parameters.

In C++ the struct and function are declared as follows:

CSHARP
    [DllImport("cgm2vector_api.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Unicode)]
    private static extern CGM_STATUS cgm2vector(
    [MarshalAs(UnmanagedType.LPWStr)] string inputFile,
    [MarshalAs(UnmanagedType.LPWStr)] string outputFile,
    [MarshalAs(UnmanagedType.LPWStr)] string logDest,
    [MarshalAs(UnmanagedType.LPWStr)] string configFile,
    ref CGM2VECTOR_PARAMS @params);

public static CGM_STATUS Convert(string inputFile, string outputFile, int units, string logDest = null, string configFile = null)
{
    var u = (CGM2VECTOR_UNITS)units;

    var @params = new CGM2VECTOR_PARAMS
    {
        width = 0,
        height = 0,
        units = (int)u,
        demoflag = 0,
        lineScale = 1.0,
        logMode = 0,
        uncompressed = 0,
        colorMode = 0,
        reverseVideo = 0,
        mediaColor = 0,
        boundsCheckRatio = 0,
        outputType = 2,
        marginSize = [0, 0],
        marginUnits = (int)CGM2VECTOR_UNITS.CGM2VECTOR_PTS,
        lineWidthMin = -1,
        lineWidthMax = -1,
        extentCheckMargin = 0,
        CSSformat = 0,
        genRegions = 0,
        seqIdentifiers = 0,
        usePaths = 0,
        rotation = 0
    };

    return Cgm2VectorApi.cgm2vector(inputFile, outputFile, logDest, configFile, ref @params);
}

The C# struct is defined as:

CSHARP
[StructLayout(LayoutKind.Sequential)]
public struct CGM2VECTOR_PARAMS
{
    public double width;
    public double height;
    public int units;
    public long demoflag;
    public double lineScale;
    public long logMode;
    public int uncompressed;
    public long colorMode;
    public long reverseVideo;
    public long mediaColor;
    public float boundsCheckRatio;
    public int outputType;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
    public float[] marginSize;
    public long marginUnits;
    public double lineWidthMin;
    public double lineWidthMax;
    public float extentCheckMargin;
    public int CSSformat;
    public int genRegions;
    public int seqIdentifiers;
    public int usePaths;
    public double rotation;
}

I've also tried using LayoutKind.Explicit, but it did not alter the behavior. The function still runs with default parameters.

What could be causing this issue? Why are the parameters being ignored despite the function call succeeding?

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

The problem is that your C# struct layout does not match the native layout because you are using incorrect data types. In the C++ header, many fields are defined as long (which on Windows is a 32‑bit integer), but in C# you are declaring them as long (a 64‑bit integer). This misalignment causes the native function to ignore your passed values and use its defaults instead.

How to Fix It

Update your C# struct so that the data types exactly match the sizes expected by the C++ function. For example, change any field using C#’s long type (64‑bit) to int (32‑bit) for the corresponding C++ long fields.

Below is an updated C# struct that should match the native definition:

CSHARP
[StructLayout(LayoutKind.Sequential)]
public struct CGM2VECTOR_PARAMS
{
    public double width;
    public double height;
    public int units;
    public int demoflag;           // Changed from long to int
    public double lineScale;
    public int logMode;            // Changed from long to int
    public int uncompressed;
    public int colorMode;          // Changed from long to int
    public int reverseVideo;       // Changed from long to int
    public int mediaColor;         // Changed from long to int
    public float boundsCheckRatio;
    public int outputType;
    [MarshalAs(UnmanagedType.ByValArray, SizeConst = 2)]
    public float[] marginSize;
    public int marginUnits;        // Changed from long to int
    public double lineWidthMin;
    public double lineWidthMax;
    public float extentCheckMargin;
    public int CSSformat;
    public int genRegions;
    public int seqIdentifiers;
    public int usePaths;
    public double rotation;
}

Explanation

C++ long vs. C# long:
In Visual Studio (on Windows), C++ long is a 32‑bit integer but C#’s long is a 64‑bit integer. This mismatch shifts subsequent fields in memory.
Memory Layout:
Because the memory layout is off, when the unmanaged code reads the struct, it sees the wrong values (or the default values), which is why your configuration data is ignored.

Final Note

After updating your struct, rebuild your application and try again. The native function should now interpret your parameters correctly.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

No comments yet.

Answer by AstroCommander158 • 1 month ago

In C#, long is a keyword for .NET's System.Int64, which represents a 64-bit signed integer.

In C++, long's size can vary but it's often a 32-bit signed integer (check your compiler).

So if your C++ long's size is indeed 32-bit, you must use C#'s int (or .NET System.Int32) otherwise you'll get an offset mismatch in the structure and anything can and will happen.

PS: .NET Marshal.SizeOf, Marshal.OffsetOff and C++ sizeof, offsetof can be used also to compare overall structures offsets and sizes.

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Why Are My Struct Parameters Ignored When Passing from C# to a C++ DLL?

2 Answers

How to Fix It

Explanation

Final Note

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