I want to customize list edit form(customedit.aspx) and want to design the list fields in Visual Studio - Visual WebPart and I will insert that WebPart in customized edit form(customedit.aspx). So, How can I pass the editing item value from edit form to (customedit.aspx) to Visual WebPart?
After creating the form you set it as default Edit form for that list. Then SharePoint will pass the ListID as well as the item Id as query string parameters.
You can also package all this into a deployable solutions as explained in below articles