c# - 我不断收到 An exception of type 'System.InvalidCastException' occurred in mscorlib.dll but was not handled in user code for RetailPrice

Question

protected void btnCalculateOrder_Click(object sender, EventArgs e)
{
    OrderSession();

    if (IsValid)
    {
        double RetailPrice = Convert.ToDouble(lblRetailPrice);
        double Quantity = Convert.ToDouble(txtQuantity);
        double Tax = (.9);

        double Subtotal = RetailPrice * Quantity;
        double TotalAmount = Subtotal + Tax;

        lblSubTotal.Text = Subtotal.ToString("c");
        lblTotalAmount.Text = TotalAmount.ToString("c");
    }

}

Answer 1

吹毛求疵，你不需要支架:

double Tax = (.9); //change to double Tax = .9; or double Tax = 0.9;

问题:

似乎您想Convert Controls 而不是Controls 的Text

double RetailPrice = Convert.ToDouble(lblRetailPrice); //I assume this is a Label Control
double Quantity = Convert.ToDouble(txtQuantity); //and this is a TextBox Control

尝试将它们更改为:

double RetailPrice = Convert.ToDouble(lblRetailPrice.Text);
double Quantity = Convert.ToDouble(txtQuantity.Text);