Operator '-' cannot be applied to operands of type string and string

Question

I want to create a new variable using two string variables. But I keep getting the error message

"operator '-' cannot be applied to operands of type string and string c#".

string query3 = "SELECT Quantity FROM Supplier WHERE [Supplier ID]='"+supplierid+"'";
string query4 = "SELECT Quantity FROM Supplier WHERE [Book ID] = '" + bookid + "'";
SqlCommand cmd3 = new SqlCommand(query3, con);
SqlCommand cmd4 = new SqlCommand(query4, con);
con.Open();
string temporaryquantity = cmd3.ExecuteScalar().ToString();
string temporaryquantitystocks = cmd4.ExecuteScalar().ToString();
string totalcostforstocks = (temporaryquantitystocks-temporaryquantity + quantity) * buyingpriceperbook;

"quantity" is in int type
"buyingpriceperbook" is in double type

Can anyone help me with this?

Seems pretty clear - you can't "subtract" two strings ,even iof those strings represent numeric values. You have to convert them to numeric types first. — D Stanley
– D Stanley, Commented Feb 27, 2019 at 18:58
You can't add two strings together... You get a scalar out of your execute operation then immediately ToString it... You are trying to sum strings. — Charleh
– Charleh, Commented Feb 27, 2019 at 18:59
@Charleh You can "add" two strings (meaning you can use the + operator), but that concatenates them. — D Stanley
– D Stanley, Commented Feb 27, 2019 at 18:59
Yeah true, you knew what I meant though (mathematically)! Using operator+ here is not going to give you the results you want :( — Charleh
– Charleh, Commented Feb 27, 2019 at 19:01

41686d6564 · Accepted Answer · 2019-02-27 19:08:39Z

0

You can not apply arithmetic operators to strings. Change to double or integer:

string query3 = "SELECT Quantity FROM Supplier WHERE [Supplier ID]='" + supplierid + "'";
string query4 = "SELECT Quantity FROM Supplier WHERE [Book ID] = '" + bookid + "'";

SqlCommand cmd3 = new SqlCommand(query3, con);
SqlCommand cmd4 = new SqlCommand(query4, con);
con.Open();
double temporaryquantity = Convert.ToDouble( cmd3.ExecuteScalar());
double temporaryquantitystocks = Convert.ToDouble(cmd4.ExecuteScalar());
double totalcostforstocks = (temporaryquantitystocks - temporaryquantity + quantity) * buyingpriceperbook;

edited Feb 27, 2019 at 19:08

41686d6564

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answered Feb 27, 2019 at 19:05

Sérgio Sereno

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Comments

D Stanley · Accepted Answer · 2019-02-27 19:05:10Z

0

Convert to a numeric type rather than strings:

int temporaryquantity = (int)cmd3.ExecuteScalar();

Other things to note:

ExecuteScalar will give you only the first value from your query - are your queries guaranteed to only return one value? Or are you expecting something to sum up multiple values?
ExecuteScalar returns an object, meaning it can be a string, a double, an integer, null, etc. If you cast to int you are assuming that the query is returning a non-null integer value, otherwise it will fail at runtime.

edited Feb 27, 2019 at 19:05

answered Feb 27, 2019 at 19:01

D Stanley

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2 Comments

15ee8f99-57ff-4f92-890c-b56153 Over a year ago

Also, he may need to check for null.

sachintha imindhu Over a year ago

Thanks a lot. Now it works perfectly fine. Thanks for explaining this to me.

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