Using External Javascript, CSS or Image File in a SharePoint WebParts

Button btn;
Image img;
string imgPath;

// Referring External Javascript
ClientScriptManager cs = Page.ClientScript;

// Include the required javascript file.
if (!cs.IsClientScriptIncludeRegistered("jsfile"))
cs.RegisterClientScriptInclude(this.GetType(), "jsfile", "/_wpresources/javascript/jsscript.js");
 
btn= new Button();
btn.Text = "Click here";
// specify function name here
btn.OnClientClick = "ButtonClick()";
this.Controls.Add(btn);

// Refering External CSS
Microsoft.SharePoint.WebControls.CssLink cssLink = new Microsoft.SharePoint.WebControls.CssLink();
cssLink.DefaultUrl = "/_wpresources/styles/styles.css";
this.Page.Header.Controls.Add(cssLink);

// Using External Image
imgPath = "/_wpresources/images/Imglogo.jpg";
img.ImageUrl = imgPath;
img.ID = "image1"; 

this.Controls.Add(img);