C# - Avoid Double Control Search In LINQ Query?

I have a Dictionary<string, bool> where key - control's ID and value - it's visible status to set:

var dic = new Dictionary<string, bool>
{
{ "rowFoo", true},
{ "rowBar", false },
...
};

Some of controls can be null, i.e. dic.ToDictionary(k => this.FindControl(k), v => v) will not work because key can't be null. I can do next:

dic
.Where(p => this.FindControl(p.Key) != null)
.ForEach(p => this.FindControl(p.Key).Visible = p.Value); // my own extension method

but this will call FindControl() twice for each key. How to avoid double search and select only those keys for which appropriate control exists? Something like:

var c= FindControl(p.Key);
if (c!= null)
return c;
but using LINQ.