由于其保护级别，“ImageUtils.ImageTransparency”无法访问

Question

我正面临这个错误

  

'ImageUtils.ImageTransparency'由于其保护而无法访问   水平

这是我的代码：

namespace ImageUtils
{
    class ImageTransparency
    {
        public static Bitmap ChangeOpacity(Image img, float opacityvalue)
        {

            Bitmap bmp = new Bitmap(img.Width,img.Height);
            Graphics graphics = Graphics.FromImage(bmp);
            ColorMatrix colormatrix = new ColorMatrix();
            colormatrix.Matrix33 = opacityvalue;
            ImageAttributes imgAttribute = new ImageAttributes();
            imgAttribute.SetColorMatrix(colormatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
            graphics.DrawImage(img, new Rectangle(0, 0, bmp.Width, bmp.Height), 0, 0, img.Width, img.Height, GraphicsUnit.Pixel, imgAttribute);
            graphics.Dispose();
            return bmp;
        }
    }
}

我想用它来制作透明图像。我在这段代码中使用它：

protected void bntChangeOpacity_Click(object sender, EventArgs e)
    {
        Image watermarkImage = Image.FromFile(myImage);

        using (TextureBrush watermarkBrush = new TextureBrush(watermarkImage))
        {
            Graphics gr = Graphics.FromImage(watermarkImage);

            gr.FillRectangle(watermarkBrush, new Rectangle(new Point(0, 0), new Size(watermarkImage.Width, watermarkImage.Height)));


            float opacityvalue = 50 / 100;

            ImageUtils.ImageTransparency.ChangeOpacity(watermarkImage, opacityvalue);


        }

    } 

请帮帮我..