Is this call a method of unmanaged DLL call from C# is okay?

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C#

unmanaged

I have load an unmanaged dll from c# dot net. ITs signature is like below

This method returns the printer initialization string.
HRESULT WINAPI EndInit(
LPWSTR lpRbdFilename, LPSTR lpBuffer,
UINT uSize
) ;
Parameters
LpRbdFilename The file name of the RBD file. The string is Null-terminated. lpBuffer                
   A pointer to the buffer to receive the printer init string.
uSize               The maximum size of the buffer in TCHARs. The buffer length must include room 
for a terminating null character.

Return value
If the function succeeds, the return value is zero.
If the function fails, the return value is the error code.

What I have tried:

I have written the code like

[DllImport("tjp.dll", EntryPoint = "EndInit")]
public static extern string EndInit([MarshalAsAttribute(UnmanagedType.LPWStr)] string Rbd, char[] str, uint i);

and calling by sending by the following value by below code

var res = StartInit();
              
                if (res == 0)
            {

               
                    FileLogger.Info("Printer is connected.");
            }
               
                
                string path = "C:\\Users\\Iftekher Alam\\Desktop\\KBZ\\Emptly_Cheque_Printer\\noname.rbd";
               

            
                char[] buffer = new char[200000000];
            buffer[0] = 'H';
            buffer[1] = 'r';




            var loadEnd = EndInit(path, buffer, Convert.ToUInt32(buffer.Length));
                if (loadEnd == 0)
                {
                    FileLogger.Info("Document is printing...");
                }

EndInit

should return zero(0) int value if the method succeeded. But I got null value from the call.