what is the difference between Boxing,Un-Boxing and typecasting

Typecasting is a conversion of an expression to a given type, provided that the types are derived from each other, or a suitable conversion method exists. For example, an integer value can be cast to a double, and vice versa:

int i = 999;
double d = 123.456;
i = (int) d;
d = i; // Implicit cast

Or

private void myButton_Click(object sender, EventArgs e)
    {
    if (sender is Button)
        {
        Button b = (Button) sender;
        }
    }

Or

foreach (Control c in Controls)
    {
    if (c is TextBox)
        {
        TextBox tb = (TextBox) c;
        }
    }

You cannot use casting to change between types where there is no defined conversion or relationship.

Boxing and unboxing are the process of "containing" a Value type (such as a integer, or struct) within a Reference type transparently so that it can be used as a reference. This is a bit complicated to explain, so have a look here: Using struct and class - what's that all about?[^] - there is a section on Boxing and what / why it is neccessary.