Use the following code to setup RadMaskedEditBox. When you start typing you will notice that the ValueChanged event is fired but the ValueChanging is not: 

            this.radMaskedEditBox1.MaskType = MaskType.Standard;
            this.radMaskedEditBox1.Mask = "AAAA";

            this.radMaskedEditBox1.ValueChanged += radMaskedEditBox1_ValueChanged;
            this.radMaskedEditBox1.ValueChanging += radMaskedEditBox1_ValueChanging;

        private void radMaskedEditBox1_ValueChanging(object sender, CancelEventArgs e)
        {
        }

        private void radMaskedEditBox1_ValueChanged(object sender, EventArgs e)
        {
        }