Question

In the codebehind of a page.

How do i get hold of a webpart that exist on the page,
then add properties to that webpart using c#.

The webpart exists withing a webpartzone.

Do i need to do anything with SPWebPartManager?

Answer 1

Use SPWeb.GetLimitedWebPartManager. The following example demonstrates updating a property in a list view web part:

using (SPLimitedWebPartManager webPartManager =
    SPContext.Current.Web.GetLimitedWebPartManager("default.aspx",
        PersonalizationScope.Shared))
{
    try
    {
        foreach (WebPart webPart in webPartManager.WebParts)
        {
            if (webPart.Title == "Web Part To Update")
            {
                ListViewWebPart listViewWebPart = (ListViewWebPart)webPart;
                // TODO: Set property on web part
                webPartManager.SaveChanges(listViewWebPart);
                break;
            }
        }
    }
    finally
    {
        webPartManager.Web.Dispose();
    }
}

Instead of default.aspx you need to use the name of the current page relative to the SPWeb.