文本框只能输入小数

if(!(((e.KeyChar >= '0') && (e.KeyChar <= '9')) || e.KeyChar <= 31))
            {
                if(e.KeyChar == '.')
                {
                    if ( ((TextBox)sender).Text.Trim().IndexOf('.') > -1)
                        e.Handled   = true;
                }
                else
                    e.Handled = true;
            }   
            else
            {
                if( e.KeyChar <= 31 )
                {
                    e.Handled = false ;
                }
                else if( ((TextBox)sender).Text.Trim().IndexOf('.') > -1 )
                {
                    if( ((TextBox)sender).Text.Trim().Substring(((TextBox)sender).Text.Trim().IndexOf('.') + 1 ).Length >= 4)
                        e.Handled = true ;
                }               
            }