C#动态调用系统DLL函数的类（不使用DllImport）

using System;
using System.Runtime.InteropServices;
using System.Reflection;
using System.Reflection.Emit;

public class DllCaller : IDisposable
{
    [DllImport("Kernel32.dll")]
    static extern IntPtr LoadLibrary(string lpFileName);

    [DllImport("Kernel32.dll")]
    static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName);

    [DllImport("Kernel32.dll")]
    static extern bool FreeLibrary(IntPtr hModule);

    private IntPtr libPtr;
    private MethodInfo method;
    /// <param name="dllFile">DLL文件的位置</param>
    /// <param name="functionName">函数名，注意带字符串的函数分W版跟A版</param>
    /// <param name="result">返回的数类型，如果返回void，使用typeof(void)</param>
    /// <param name="args">参数列表</param>
    /// <remarks>
    /// 注意:为了方便使用，返回类型跟参数类型都用一个实例表示，实例值没影响。
    /// 例如:用true,false表示bool; 0表示int; (byte)0表示byte, IntPtr.Zero表示指针等
    /// </remarks>
    /// <example>
    /// int MessageBox(
    /// HWND hWnd, 
    /// LPCTSTR lpText, 
    /// LPCTSTR lpCaption, 
    /// UINT uType
    /// ); 
    /// DllCaller MessageBox = new DllCaller(
    /// "user32.dll", "MessageBoxW", IntPtr.Zero, IntPtr.Zero, IntPtr.Zero, (uint)0
    /// );
    /// </example>
    public DllCaller(string dllFile, string functionName, object result, params object[] args)
    {
        if (dllFile == null) throw new ArgumentNullException();
        if (functionName == null) throw new ArgumentNullException();

        this.libPtr = LoadLibrary(dllFile);
        if (this.libPtr == IntPtr.Zero) throw new DllNotFoundException(dllFile);

        IntPtr procPtr = GetProcAddress(this.libPtr, functionName);
        if (procPtr == IntPtr.Zero) throw new EntryPointNotFoundException(functionName);

        AssemblyName asmName = new AssemblyName();
        asmName.Name = "DynamicAssembly";

        AssemblyBuilder asmBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(asmName, AssemblyBuilderAccess.Run);
        ModuleBuilder modBuilder = asmBuilder.DefineDynamicModule("DynamicModule");

        Type resultType = (result == typeof(void) ? typeof(void) : result.GetType());
        Type[] argTypes = new Type[args.Length];
        for (int i = 0; i < args.Length; i++)
            argTypes[i] = args[i].GetType();

        MethodBuilder funBuilder = modBuilder.DefineGlobalMethod(
            functionName,
            MethodAttributes.Public | MethodAttributes.Static,
            resultType,
            argTypes
            );

        ILGenerator ilGen = funBuilder.GetILGenerator();
        for (int i = 0; i < args.Length; i++)
            ilGen.Emit(OpCodes.Ldarg, i);

        if (IntPtr.Size == 4)
            ilGen.Emit(OpCodes.Ldc_I4, (int)procPtr);
        else if (IntPtr.Size == 8)
            ilGen.Emit(OpCodes.Ldc_I8, (long)procPtr);

        ilGen.EmitCalli(OpCodes.Calli, CallingConvention.StdCall, resultType, argTypes);
        ilGen.Emit(OpCodes.Ret);
        modBuilder.CreateGlobalFunctions();
        this.method = modBuilder.GetMethod(functionName);
    }

    public object Call(params object[] args)
    {
        return this.method.Invoke(null, args);
    }
    public void Dispose()
    {
        if (this.method != null)
        {
            if (this.libPtr != IntPtr.Zero)
                FreeLibrary(this.libPtr);
            this.libPtr = IntPtr.Zero;
            this.method = null;
            GC.SuppressFinalize(this);
        }
    }
    ~DllCaller()
    {
        this.Dispose();
    }
}

public class MarshalBuffer : IDisposable
{
    public static MarshalBuffer FromString(string Text)
    {
        MarshalBuffer buffer = new MarshalBuffer();
        buffer.ptr = Marshal.StringToCoTaskMemUni(Text);
        return buffer;
    }

    public IntPtr ptr;
    private MarshalBuffer() { }
    public MarshalBuffer(int cb)
    {
        this.ptr = Marshal.AllocHGlobal(cb);
    }
    public void Dispose()
    {
        if (ptr != IntPtr.Zero)
        {
            Marshal.FreeHGlobal(ptr);
            this.ptr = IntPtr.Zero;
        }
    }
    ~MarshalBuffer()
    {
        this.Dispose();
        GC.SuppressFinalize(this);
    }
    public override string ToString()
    {
        return Marshal.PtrToStringUni(this.ptr);
    }
    public int this[int Index]
    {
        get { return Marshal.ReadInt32(this.ptr, Index * 4); }
        set { Marshal.WriteInt32(this.ptr, Index * 4, value); }
    }
}

class Test
{
    public static void DrawScreenTest()
    {
        DllCaller getdc = new DllCaller("user32.dll", "GetDC", IntPtr.Zero, IntPtr.Zero);
        DllCaller textout = new DllCaller("gdi32.dll", "TextOutW", false, IntPtr.Zero, 0, 0, IntPtr.Zero, 0);
        IntPtr dc = (IntPtr)getdc.Call(IntPtr.Zero);
        MarshalBuffer text = MarshalBuffer.FromString("Hello World");
        Random rand = new Random();
        for (int i = 0; i < 100; i++)
            textout.Call(dc, rand.Next(1024), rand.Next(768), text.ptr, "Hello World".Length);
    }

    public static void GetComputerNameTest()
    {
        MarshalBuffer text = new MarshalBuffer(100);
        MarshalBuffer len = new MarshalBuffer(4);
        len[0] = 50;
        DllCaller getComName = new DllCaller("kernel32.dll", "GetComputerNameW", false, IntPtr.Zero, IntPtr.Zero);
        getComName.Call(text.ptr, len.ptr);
        Console.WriteLine(text.ToString());
        Console.WriteLine(len[0]);
    }

    static void Main()
    {
        DrawScreenTest();
        GetComputerNameTest();
    }
}