Question

Is there a way to remove all items except first one from any type of collection (Control.Items, List ….) using LINQ only ?

Answer 1

How about something using reflection?

static void RemoveButFirst(object o){

    Type t = o.GetType();

    System.Reflection.MethodInfo rm = t.GetMethod("RemoveAt",
                                                 new Type[]{typeof(int)});
    System.Reflection.PropertyInfo count = t.GetProperty("Count");

    for (int n = (int)(count.GetValue(o,null)) ; n>1; n--)
        rm.Invoke(o, new object[]{n-1});

}   

This would work any time your collection exposed an int Count property and a RemoveAt(int) method, which I think those collections should.

And a more concise version, using dynamic, if you work with C# 4.0:

public static void RemoveBut(dynamic col, int k){
    for (int n = col.Count; n>k; n--)
        col.RemoveAt(n-1);
}