void fftshift(ref FILE file1,ref FILE  file2,int Row,int Column,int flag)
{............}

------解决方案--------------------
struct _iobuf {
       char *_ptr;
       int   _cnt;
       char *_base;
       int   _flag;
       int   _file;
       int   _charbuf;
       int   _bufsiz;
       char *_tmpfname;
       };
typedef struct _iobuf FILE;
这是C++对FILE的定义，我想跟C应该差不多吧，C#需要自己定义结构相同的FILE。

[System.Runtime.InteropServices.StructLayoutAttribute(System.Runtime.InteropServices.LayoutKind.Sequential)]
public struct FILE {
  
   /// char*
   [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)]
   public string _ptr;
  
   /// int
   public int _cnt;
  
   /// char*
   [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)]
   public string _base;
  
   /// int
   public int _flag;
  
   /// int
   public int _file;
  
   /// int
   public int _charbuf;
  
   /// int
   public int _bufsiz;
  
   /// char*
   [System.Runtime.InteropServices.MarshalAsAttribute(System.Runtime.InteropServices.UnmanagedType.LPStr)]
   public string _tmpfname;
}
试试吧，不一定能解决问题。