I need help with my Code Its driving me CRAZY

[QuestionPlease]

well, I am to design a form(a calculator but without buttons when selecting an operator +-*/) that accepts an operand(such as a number) and operator(+-*/) and a second operand(another number) and then gives the result, I then
need to have it show an error message if an invalid operator is enter by the user. My problem is even if the right operator +,*,-,/ is entered it still shows an error
message stating its an invalid???

this has really been driving me crazy i can't seem to figure it out??


I really think its the isOperator method that is causing the issues??

private void btnCalculate_Click(object sender, EventArgs e)
        {
            try
            {
                if (IsValidData())
                {

                    decimal operand1 = Convert.ToDecimal(txtOperand1.Text);
                    decimal operand2 = Convert.ToDecimal(txtOperand2.Text);
                    string Operator1 = Convert.ToString(txtOperator.Text);

                    decimal result = 0m;


                    result = Calculate(operand1, Operator1, operand2);
                    txtResult.Text = result.ToString("n4");

                }
            }

            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, ex.GetType().ToString());
            }
        }

        private decimal Calculate(decimal operand1, string Operator1, decimal operand2)
        {
            if (Operator1 == "+")
            {
                operand1 += operand2;
            }
            else if (Operator1 == "-")
            {
                operand1 -= operand2;
            }

            else if (Operator1 == "*")
            {
                operand1 *= operand2;
            }
            else if (Operator1 == "/")
            {
                operand1 /= operand2;
            }

            {
                return operand1;
            }
        }
            public bool IsPresent(TextBox textbox, string name)
            {
                if (textbox.Text == "") 
                {
                    MessageBox.Show(name + " is a required field.", "Entry Error");
                        textbox.Focus();
                        return false;
                }

                return true;
            }

                
                public bool IsDecimal(TextBox textbox, string name)
            {
                try
            {
                Convert.ToDecimal(textbox.Text);
                return true;
            }
            catch (FormatException)
            {
                MessageBox.Show(name +  " Must be a decimal value.", "Entry Error");
                textbox.Focus();
                return false;
            }
        }

                public bool IsOperator(TextBox textbox, string name)
                {
                    string validOperator1 = "";
                    
                    try
                    {
                        validOperator1 = Convert.ToString(textbox.Text);

                        if (validOperator1 != "+" | validOperator1 != "-" | validOperator1 != "*" | validOperator1 != "/")
                        {
                            MessageBox.Show(name + " Must be a Valid Operator", "Entry Error");
                            textbox.Focus();
                        }

                        return false;

                    }

                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.Message, ex.GetType().ToString());
                    }
                    return true;
                } 




             public bool IsWithinRange(TextBox textbox, string name, decimal min, decimal max)
             {
                 decimal number = Convert.ToDecimal(textbox.Text);
                 if (number < min || number > max)
                 {
                     MessageBox.Show(name + " Must be between " + min + " and " + max + "  ", "Entry Error");
                     textbox.Focus();
                     return false;

                 }
                 return true;
             }

             public bool IsValidData()
             {
                 return

                      IsPresent(txtOperand1, "Operand 1") &&
                      IsDecimal(txtOperand1, "Operand 1") &&
                      IsWithinRange(txtOperand1, "Operand 1", 1, 999) &&


                      IsPresent(txtOperator, "Operator") &&
                      IsOperator(txtOperator, "Operator") &&




                      IsPresent(txtOperand2, "Operand 2") &&
                      IsDecimal(txtOperand2, "Operand 2") &&
                      IsWithinRange(txtOperand2, "Operand 2", 1, 999);


             }

        private void btnExit_Click(object sender, EventArgs e)
        {
            this.Close();
        }

        private void ClearResult(object sender, EventArgs e)
        {
            txtResult.Text = "";

        }
    }

}