How to validate price range value in c#

Question

I'm working on my Add Materials for my Sales and Inventory. So far I need a validation where when adding a material, Selling Price must be greater than Purchase Price.

Here is my code and I'm getting an error in the if/else statement where the values of my txtPurchasePrice and txtSellingPrice are decimal(18, 2).

---

protected void btnAdd_Click(object sender, EventArgs e)
{
    if (txtSellingPrice.Text >= txtPurchasePrice.Text)
    {
        con.Open();
        SqlCommand cmd = new SqlCommand();
        cmd.Connection = con;
        cmd.CommandText = "INSERT INTO Materials VALUES (@UnitID, @Name, @SellingPrice, @PurchasePrice, " +
            "@Description, @Available, @CriticalLevel, @Maximum, @Status, @DateAdded, @DateModified)";                
        cmd.Parameters.AddWithValue("@UnitID", txtUnitID.Text);
        cmd.Parameters.AddWithValue("@Name", txtName.Text);
        cmd.Parameters.AddWithValue("@SellingPrice", txtSellingPrice.Text);
        cmd.Parameters.AddWithValue("@PurchasePrice", txtPurchasePrice.Text);
        cmd.Parameters.AddWithValue("@Description", txtDesc.Text);
        cmd.Parameters.AddWithValue("@Available", "0");
        cmd.Parameters.AddWithValue("@CriticalLevel", txtCritical.Text);
        cmd.Parameters.AddWithValue("@Maximum", txtMax.Text);
        cmd.Parameters.AddWithValue("@Status", "Available");
        cmd.Parameters.AddWithValue("@DateAdded", DateTime.Now);
        cmd.Parameters.AddWithValue("@DateModified", DBNull.Value);
        cmd.ExecuteNonQuery();
        con.Close();
        Helper.AddLog("1", "Add", "Added a new Material");
        Response.Redirect("~/Materials/Default.aspx");
    }
    else
    {
        error.Visible = true;
    }
}

---

My Error

CS0019: Operator '>=' cannot be applied to operands of type 'string' and 'string'

Why is that >= or <= or || or < or > is cannot be used? in this case?

Answer 1

I'm assuming txtSellingPrice and txtPurchasePrice are TextBox controls, hence their Text property is of type string . You can't use >= since that has no semantical meaning. You'll need to parse the text first into decimal :

var sellingPrice = decimal.Parse(txtSellingPrice.Text);
var purchasePrice = decimal.Parse(txtPurchasePrice.Text);

if (sellingPrice >= purchasePrice)
{
   // stuff
}

If you're not sure the text is a valid decimal value, use decimal.TryParse :

decimal sellingPrice;
if (!decimal.TryParse(txtSellingPrice.Text, out sellingPrice))
{
   // Not a valid decimal, do something.
}

decimal purchasePrice;
if (!decimal.TryParse(txtPurchasePrice.Text, out purchasePrice))
{
   // Not a valid decimal, do something.
}

if (sellingPrice >= purchasePrice)
{
   // stuff
}

Answer 2

Hwo can a string be greater than another string?

You need to convert the string to a numeric value before you do the comparison.

Since you are dealing with money, you should use Decimal :

if (Decimal.Parse(txtSellingPrice.Text) >= Decimal.Parse(txtPurchasePrice.Text))

Answer 3

要比较两个字符串的数值，您必须将它们解析为数值类型，例如double！

Answer 4

Use Convert.ToDecimal(txtSellingPrice.Text) >= Convert.ToDecimal(txtPurchasePrice.Text)

String value cannot be compared like that