Example

main.cs

using System;
using System.Runtime.InteropServices;


public class MainClass
{
	[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
	private struct ModuleInputData
	{
		[MarshalAs(UnmanagedType.ByValArray, SizeConst = 260)]
		public unsafe fixed char name[260];
		public uint pid;
	}

	public unsafe static void Main(string[] args)
	{
		ModuleInputData data;
		var arg = args[0];
		fixed (char* ptr = arg)
		{
			for (int i = 0; i < 260; i++) {
				char chr = *(ptr + i);
				if (chr == '\0') {
					break;
				}
				data.name[i] = chr;
			}
			data.name[0] = ptr[0];
		}
	}
}

Makefile

main.exe: main.cs
	mcs /unsafe main.cs