{           

if (char.IsDigit(e.KeyChar)== false && e.KeyChar != '.' && e.KeyChar!='\b')

//checking whether pressed key is number, decimal orbackspace.

//If false then we will set Handled variable ofKeyPressEventArgs to true.

//By doing this system will get a message that character of keypressed

//is executed and hence aplpa numeric character will not bedisplayed

//except decimal.

                e.Handled=true;

            elseif (e.KeyChar == '.')

                if(textBox1.Text.Contains('.'))

                    e.Handled=true;           

//checking whether decimal key is pressed or not, if truethen checking

//whether decimal is already present or not, as we can haveonly

//one decimal point in a text box.

        }