Develop Reference

Converting from string to char* only copies the first character

I have looked at most of the string to char* conversion SO answers, but it is not working for me. Here is my code:
public static void Main() {
string name = "ELEM";
unsafe{
fixed(char* name_ptr = name) {
Console.WriteLine(name_ptr->ToString());
}
}
}
// Output: E
I need to do this since I have to pass a char* to my C++ custom DLL. Why would it only copy the first character only, and how can I properly convert string to a char*?

You get the first character only, because name_ptr is nothing but a reference to a single character and when you call name_ptr->ToString() you actually call char.ToString().
You should use a StringBuilder instead to pass a string to a C/C++ DLL. See this question.

Robert Giesecke's Unmanaged Exports - OUT strings

I'm working from VBA into .NET.
I have a working version of an interface using CLI and stdcall
I'm trying to remove the resulting dependency on the C++ 2015 runtime, and it looks like I can do it using UnmanagedExports.
But I have a couple of questions.
Can I just use "ref string" as a parameter and have it work?
If so, can I replace it with "out string"?
In either cases, do I have to do any string/string length management?
I currently pass a couple of callbacks in as "int". From an example I saw elsewhere, it looks like on the C# side, using this, I should be able to replace those parameters with Func for a callback such as Function A(p1 as String, p2 as Long, p3 as String) as Long
Any advice would be much appreciated.

You need a combination of StrPtr, possibly StrConv on the Access side, and IntPtr on the .NET side:
'VBA7
Private Declare PtrSafe Function Command Lib "External.dll" (ByVal CommandName As String, ByVal Result As LongPtr, ByRef ResultLength As Long) As Long
'VBA pre7
Private Declare Function Command Lib "External.dll" (ByVal CommandName As String, ByVal Result As Long, ByRef ResultLength As Long) As Long
'Example to use.
'Result will be up to "i" characters - no new string involved
Dim i As Long, x As Long, strResult As String
i = 100
strResult = Space(i)
x = Command(CommandName, Arguments, StrPtr(strResult), i)
If you use StrConv, the string type is up to you. If you don't, the pointer will be pointing to a Unicode array.
C# side:
[DllExport("Command", CallingConvention.StdCall)]
public static int Command(string commandName, string arguments, IntPtr result, out /*or ref*/ int resultLength)
{
string inputStr = Marshal.PtrToStringUni(result); //Unicode
resultLength = inputStr.Length;
int x = MainFunc.Command(commandName, arguments, ref inputStr);
if(null == inputStr)
{
inputStr = "";
}
if(inputStr.Length > resultLength)
{
inputStr = inputStr.Substring(0, resultLength);
}
byte[] outputBytes = Encoding.Unicode.GetBytes(inputStr);
Marshal.Copy(outputBytes, 0, result, outputBytes.Length);
resultLength = inputStr.Length;
return x;
}

Optional function parameter with default value by reference

By automatic conversion of code written in VB.NET to C# I have such situation in function declaration of VB.NET:
Private Function dataArchiver(ByVal toPlace As String,
Optional ByVal aExtension As String = ".7z",
Optional ByRef createdName As String = "") As Integer
Tool for automatic conversion does this in C#:
private int dataArchiver(string toPlace,
string aExtension = ".7z",
ref string createdName = "")
And this of course don't work. Keyword "ref" before last argument is underlined with red.
Why is that so? Because string createdName may be (and don't have to be) generated in function and in that case have to be passed out from function.
It is important that this code can work with NET framework 3.5.
Any idea to get this working in C# without much reconcepting of VB.NET program?

You have to create overloaded methods for this (as you would have had to do before C# acquired the optional parameter feature):
private int dataArchiver(string toPlace, string aExtension)
{
string tempVar = "";
return dataArchiver(toPlace, aExtension, ref tempVar);
}
private int dataArchiver(string toPlace)
{
string tempVar = "";
return dataArchiver(toPlace, ".7z", ref tempVar);
}
private int dataArchiver(string toPlace, string aExtension, ref string createdName)
{
return 0;
}

You can use the out keyword instead in this case:
string createdName;
int retVal = dataArchiver("to place", out createdName, extension);
And your new method signature.
private int dataArchiver(string toPlace,
out string createdName,
string aExtension = ".7z")
{
createdName = "some value";
// rest of your code
}
The only change is that out can't have a method signature default value, so you will need to set it inside the method.

Using Delphi's stuct arrays and strings in C#

I've been trying to invoke a method that have been created in Delphi in the following way:
function _Func1(arrParams: array of TParams): Integer;stdcall;
type
TParams = record
Type: int;
Name: string;
Amount : Real;
end;
My code is:
[DllImport("some.dll", EntryPoint = "_Func1", CallingConvention = CallingConvention.StdCall)]
public static extern int Func(
[MarshalAs(UnmanagedType.LPArray, ArraySubType=UnmanagedType.Struct)] TParams[] arrParams)
And the struct is:
[StructLayoutAttribute(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct TParams
{
public int Type;
[MarshalAs(UnmanagedType.AnsiBStr)]
public string Name;
public double Amount;
}
When I am calling this method I'm getting the error:
Cannot marshal field 'Name' of type 'TParams': Invalid managed/unmanaged type combination (String fields must be paired with LPStr, LPWStr, BStr or ByValTStr).
However none of those combinations works, as Delphi's strings are prefixed with its length and it is Ansi for sure (I've tried it with other string parameters). Does anyone have a clue how to solve this?

There are two main problems with this, use of open arrays and use of Delphi string.
Open arrays
Delphi open arrays are implemented by passing a pointer to the first element of the array and an extra parameter specifying the index of the last item, high in Delphi terminology. For more information see this answer.
Delphi strings
The C# marshaller cannot interop with a Delphi string. Delphi strings are private types, only to be used internally to a Delphi module. Instead you should use a null-terminated string, PAnsiChar.
Putting it all together you can write it like this:
Delphi
type
TParams = record
_Type: Integer;//Type is a reserved word in Delphi
Name: PAnsiChar;
Amount: Double;
end;
function Func(const arrParams: array of TParams): Integer; stdcall;
C#
[StructLayoutAttribute(LayoutKind.Sequential)]
public struct TParams
{
public int Type;
public string Name;
public double Amount;
}
[DllImport("some.dll")]
public static extern int Func(TParams[] arrParams, int high);
TParams[] params = new TParams[len];
...populate params
int retval = Func(params, params.Length-1);

To compliment David's answer, you can marshal to a Delphi string, but it's ugly. In C#, you have to replace all of your strings in the struct with IntPtr.
private static IntPtr AllocDelphiString(string str)
{
byte[] unicodeData = Encoding.Unicode.GetBytes(str);
int bufferSize = unicodeData.Length + 6;
IntPtr hMem = Marshal.AllocHGlobal(bufferSize);
Marshal.WriteInt32(hMem, 0, unicodeData.Length); // prepended length value
for (int i = 0; i < unicodeData.Length; i++)
Marshal.WriteByte(hMem, i + 4, unicodeData[i]);
Marshal.WriteInt16(hMem, bufferSize - 2, 0); // null-terminate
return new IntPtr(hMem.ToInt64() + 4);
}
This can directly be sent to Delphi, where it'll properly be read as a string.
Remember that you must free this string when you're done with it. However, GlobalFree() can't be called directly on the pointer to the string, because it doesn't point to the start of the allocation. You'll have to cast that pointer to a long, then subtract 4, then cast it back to a pointer. This compensates for the length prefix.

C# Dll Invoke String Encoding Problem

StringBuilder codeline = new StringBuilder(100);
protected virtual int OnCodeLine(int code, int docId, ref StringBuilder codeline)
{
////
}
This is what i get with *ref StringBuilder*
ĞĞÑĞÒĞÓĞÔĞÕĞÖĞ×ĞØĞÙĞÚĞÛĞÜĞİĞŞĞßĞàĞáĞâĞãĞäĞåĞæĞçĞèĞéĞêĞëĞìĞíĞîĞïĞğĞñĞòĞóĞôĞõĞöĞ÷ĞøĞùĞúĞûĞüĞıĞşĞÿĞÑÑÑÑÑÑÑ Ñ
ÑÑÑ
ÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑÑ
and only with StringBuilder i only get 3
This is what its suppose to return
300 078 9059431
By the way this the MICR Code from Cheques
[DllImport("mflib.dll")]
public static extern int mfScanFeeder(int mode, int font, int timeout);
[UnmanagedFunctionPointer(CallingConvention.StdCall)]
public delegate int MFS100_CodeLineDelegate(int code, int docId, ref StringBuilder codeline);
public event MFS100_CodeLineDelegate MFS100_CodeLine;
private static MFS100_CodeLineDelegate cache_CodeLine;
Update : Here is the Original Code that works in vb6
Public Function MFS100_OnCodeline(ByVal code As Long, ByVal docId As Long, ByVal codeline As String) As Long
Dim i As Integer
WriteEvent "OnCodeline:"
WriteEvent " code = " & code
WriteEvent " docId = " & docId
WriteEvent " codeline = " & codeline
MFS100_OnCodeline = -1 ' -1 means: sorting will be done by mfSort()
g_codeline = codeline
CScannerForm.TmrSort.enabled = True
End Function
Update 2
mfSetEvent((int)EventEnum.E_EVENT_CODELINE, cache_CodeLine);
[DllImport("mflib.dll")]
private static extern int mfSetEvent(int eventID, Delegate callsback);
When i use StringBuilder with ref i get a string that have
32361 length. Without ref i get
only the first value of the
string.
OnCodeLine is for the callback
from the scanner device. What is the
problem ?

You should not pass the StringBuilder by ref. When you do that you say you only get one character back. That's the hallmark of expecting ANSI encoding but actually receiving Unicode (UTF-16). Fix it by specifying the character set:
[UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet=CharSet.Unicode)]
public delegate int MFS100_CodeLineDelegate(int code, int docId, StringBuilder codeline);

I don't understand what are you trying to achieve, but if you want to collect a string generated inside OnCodeLine, you don't need to pass the StringBuilder by reference, as it is a reference type.
Just pass the StringBuilder without ref, populate it, and when you return you'll have the desired string in it.
Regarding what you get after calling the OnCodeLine, can you provide some info regarding the implementation?
Sorry, I didn't notice the PInvoke was involved!! :(