求助 Graphics 不能绘制透明效果。

#pragma once

namespace ClearRectangle {

    using namespace System;
    using namespace System::ComponentModel;
    using namespace System::Collections;
    using namespace System::Windows::Forms;
    using namespace System::Data;
    using namespace System::Drawing;

    /// <summary>
    /// Form1 摘要
    /// </summary>
    public ref class Form1 : public System::Windows::Forms::Form
    {
    public:
        Form1(void)
        {
            InitializeComponent();
            //
            //TODO:  在此处添加构造函数代码
            //
        }

    protected:
        /// <summary>
        /// 清理所有正在使用的资源。
        /// </summary>
        ~Form1()
        {
            if (components)
            {
                delete components;
            }
        }
    private: System::Windows::Forms::PictureBox^ pictureBox1;
    protected:
    private: System::Windows::Forms::Button^ button1;

    private:
        /// <summary>
        /// 必需的设计器变量。
        /// </summary>
        System::ComponentModel::Container ^components;

#pragma region Windows Form Designer generated code
        /// <summary>
        /// 设计器支持所需的方法 - 不要修改
        /// 使用代码编辑器修改此方法的内容。
        /// </summary>
        void InitializeComponent(void)
        {
            this->pictureBox1 = (gcnew System::Windows::Forms::PictureBox());
            this->button1 = (gcnew System::Windows::Forms::Button());
            (cli::safe_cast<System::ComponentModel::ISupportInitialize^>(this->pictureBox1))->BeginInit();
            this->SuspendLayout();
            //
            // pictureBox1
            //
            this->pictureBox1->BackColor = System::Drawing::Color::FromArgb(static_cast<System::Int32>(static_cast<System::Byte>(192)), static_cast<System::Int32>(static_cast<System::Byte>(255)),
                static_cast<System::Int32>(static_cast<System::Byte>(192)));
            this->pictureBox1->Location = System::Drawing::Point(0, 0);
            this->pictureBox1->Name = L"pictureBox1";
            this->pictureBox1->Size = System::Drawing::Size(200, 200);
            this->pictureBox1->TabIndex = 0;
            this->pictureBox1->TabStop = false;
            //
            // button1
            //
            this->button1->Location = System::Drawing::Point(51, 213);
            this->button1->Name = L"button1";
            this->button1->Size = System::Drawing::Size(97, 39);
            this->button1->TabIndex = 1;
            this->button1->Text = L"button1";
            this->button1->UseVisualStyleBackColor = true;
            this->button1->Click += gcnew System::EventHandler(this, &Form1::button1_Click);
            //
            // Form1
            //
            this->AutoScaleDimensions = System::Drawing::SizeF(6, 12);
            this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font;
            this->ClientSize = System::Drawing::Size(200, 266);
            this->Controls->Add(this->button1);
            this->Controls->Add(this->pictureBox1);
            this->Name = L"Form1";
            this->Text = L"Form1";
            (cli::safe_cast<System::ComponentModel::ISupportInitialize^>(this->pictureBox1))->EndInit();
            this->ResumeLayout(false);

        }
#pragma endregion
    private: System::Void button1_Click(System::Object^ sender, System::EventArgs^ e)
    {
        SolidBrush^ Brush1 = gcnew SolidBrush(Color::FromArgb(192, 255, 255)); //淡青色
        SolidBrush^ Brush2 = gcnew SolidBrush(Color::FromArgb(0x00000000));    //透明色
        Bitmap^ MyImage = gcnew Bitmap(200, 200);
        Graphics^ g = Graphics::FromImage(MyImage);

        //200x200全画面淡青色
        g->FillRectangle(Brush1, 0, 0, 200, 200);

        //100x100 左上部1/4正文形透明色
        g->FillRectangle(Brush2, 0, 0, 100, 100);

        //100x100 右下部1/4正方形 每一个像素设置为透明色
        for (int i = 100; i < 200; i++)
            for (int j = 100; j < 200; j++)
                MyImage->SetPixel(i, j, Color::FromArgb(0x00000000));

        this->pictureBox1->Image =(Image^) MyImage->Clone();

        delete MyImage;
    }
    };
}