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// For complete examples and data files, please go to https://github.com/groupdocs-editor/GroupDocs.Editor-for-.NET

public static void GetHTMLExternalCSSContentsWithExternalResources()
        {
            // Obtain document stream
            Stream sourceStream = File.Open(Path.Combine(Common.sourcePath, Common.sourceFile), FileMode.Open, FileAccess.Read);
            using (InputHtmlDocument htmlDoc = EditorHandler.ToHtml(sourceStream))
            {
                string externalResourcePrefix = "GetResource?htmlDocumentFolderName=" + Common.sourceResourcesFolder + "&resourceFilename=Picture 3.png";
                //  Obtain CSS content
                string cssContent = htmlDoc.GetCssContent(externalResourcePrefix);

                Console.WriteLine(cssContent);
            }

            // close stream object to release file for other methods.
            sourceStream.Close();
        }