How to convert LParam to Struct in c#

Question

I have the following c++ struct

typedef struct {
    char    szAccountNo[11];        
    char    szAccountName[40];      
    char    act_pdt_cdz3[3];        
    char    amn_tab_cdz4[4];        
    char    expr_datez8[8];         
    char    granted;                
    char    filler[189];            
}ACCOUNTINFO;

typedef struct {
    char    szDate          [14];   
    char    szServerName    [15];   
    char    szUserID        [8];    
    char    szAccountCount  [3];    
    ACCOUNTINFO accountlist [999];  
}LOGININFO;

typedef struct{
    int       TrIndex;
    LOGININFO *pLoginInfo;
}LOGINBLOCK;

and converted into C# class

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public class ACCOUNTINFO
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 11)]
            public string szAccountNo;      
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 40)]
            public string szAccountName;        
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 3)]
            public string act_pdt_cdz3;     
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 4)]
            public string amn_tab_cdz4;     
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 8)]
            public string expr_datez8;          
            public char granted;                
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 189)]
            public string filler;           
        };
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public class LOGININFO
        {
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 14)]
            public string szDate;   
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 15)]
            public string szServerName; 
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 8)]
            public string szUserID; 
            [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 3)]
            public string szAccountCount;   
            [MarshalAs(UnmanagedType.Struct, SizeConst = 99)]
            public ACCOUNTINFO[] accountlist;   
        };
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
        public class LOGINBLOCK
        {
            [MarshalAs(UnmanagedType.I4)]
            public int TrIndex;
            [MarshalAs(UnmanagedType.Struct, SizeConst = 1)]
            public LOGININFO pLoginInfo;
        };

I used the following two methods to convert the LParam within the WndProc.

1. TrStruct.LOGINBLOCK lb = new TrStruct.LOGINBLOCK();
                        lb = (TrStruct.LOGINBLOCK)m.GetLParam(typeof(TrStruct.LOGINBLOCK));

2. TrStruct.LOGINBLOCK lb = (TrStruct.LOGINBLOCK) Marshal.PtrToStructure(m.LParam, typeof(TrStruct.LOGINBLOCK));

However, it doesn't get success nor emits an regular error message, instead I get the message "A first chance exception of type 'System.TypeLoadException' occurred in mscorlib.dll" in Output window. Could anyone tell me what's wrong with my conversion?

For starters, your charset is ascii, not unicode. Also use the `fixed` keyword for value arrays. — leppie, Feb 18 '13 at 07:21
@leppie I don't understand using fixed keyword for value arrays. Could you be more specific or give an example? — icewall, Feb 18 '13 at 08:52
See http://msdn.microsoft.com/en-us/library/zycewsya(v=vs.80).aspx — leppie, Feb 18 '13 at 08:57
@leppie I still don't get where I should use 'fixed'. It seems that you refering to ACCOUNTINFO[] array, but the array should refer to the ACCOUNTINFO type. And I counldn't use 'fixed' for the non-primitive type ACCOUNTINFO — icewall, Feb 18 '13 at 12:47
Sorry, I did not realize they wont work on classes with layout. Use a struct then. — leppie, Feb 18 '13 at 13:17
@leppie It won't work with struct either. "Fixed size buffer type must be one of the following: bool, byte, short, int, long, char, sbyte, ushort, uint, ulong, float or double" — icewall, Feb 18 '13 at 23:46

score 0 · Answer 1 · answered Feb 18 '13 at 07:31

The first apparent problem with your code lies in CharSet.Unicode. Fields of you structure in unmanaged code are all defined as char which is a single-byte character. You should use CharSet.Ansi for all your structures.

The second problem is in last field of LOGININFO. It is defined as

[MarshalAs(UnmanagedType.Struct, SizeConst = 99)]
public ACCOUNTINFO[] accountlist;

but in your unmanaged code, it is

ACCOUNTINFO accountlist [999];

The SizeConst parameter should be changed to 999.

I tried all solutions you provided, but it doesn't work as before. Any other suggestions? — icewall, Feb 18 '13 at 08:51

How to convert LParam to Struct in c#

1 Answers1