C#-P/Invoke: How to call function that's return value is a struct

[inch]

Hello all..
I'm new to C# and I've not actually started serious coding. However I know I will need to call some legacy functions from a company DLL. I have tested this using P/Invoke and been quite successull. However, there are a few functions that return a struct as their return value. How do I go about calling these? A simple (fictional) example of the C source is below:

typedef struct _MxRec {
char Name[32];
double Mx[32];
} MxRec;
 
MxRec GetMatrix(/* some parameters */)
{
 
MxRec TheMatrix;
 
strcpy(TheMatrix.Name,"MyName");
 
TheMatrix.Mx[0]=1.0;
// More values
TheMatrix.Mx[11]=123.45;
 
return(TheMatrix);
}
 

[Plater]

I *think* you can create an identical instance of the struct in your code.
(i.e. re-declare it in C# code)
Then you can use that as the return type.

You may have to fiddle with Marshalling settings, but I am unfamiliar with those.

[r035198x]

Hello all..
I'm new to C# and I've not actually started serious coding. However I know I will need to call some legacy functions from a company DLL. I have tested this using P/Invoke and been quite successull. However, there are a few functions that return a struct as their return value. How do I go about calling these? A simple (fictional) example of the C source is below:

typedef struct _MxRec {
char Name[32];
double Mx[32];
} MxRec;
 
MxRec GetMatrix(/* some parameters */)
{
 
MxRec TheMatrix;
 
strcpy(TheMatrix.Name,"MyName");
 
TheMatrix.Mx[0]=1.0;
// More values
TheMatrix.Mx[11]=123.45;
 
return(TheMatrix);
}
 

You do realize of course that C# also has the struct datatype?

[inch]

You do realize of course that C# also has the struct datatype?

Yea, I got that. When I tried the code below I get the message:
Method's type signature is not PInvoke compatible.
This is the C# code I used:

[DllImport("CLibToCallFromCS.dll")]
public static extern MxRecArr ReturnStructArr(int Mode);
 
private void button7_Click(object sender, EventArgs e)
{
int var = 0;
MxRecArr MyRec = new MxRecArr();
MyRec.Initialize();
MyRec = ReturnStructArr(var);
}
 
public struct MxRecArr
{
[MarshalAs(UnmanagedType.ByValArray,SizeConst=12)]
public double[] Mx;
// Need to call this after declaration before it can be used
public void Initialize()
{
Mx = new double[12];
}
}

Using this same structure as a parameter in a PInvoke function call worked fine.

[inch]

Just a small update to this. If I try using a 'simple' struct that only has single variable members it works OK.
However using a struct with an array member fails.
There must be a way to do this, but I can't find it.

[Plater]

Ahhh, yes, I don't believe structs can have an array.
Something about not being able to assign enough space or something

[inch]

Ahhh, yes, I don't believe structs can have an array.
Something about not being able to assign enough space or something

But passing the same struct as a parameter works OK. It is only trying to set it's value when returned as the result of a call that fails.