运算符“-”不能应用于字符串和字符串类型的操作数

Question

我想使用两个字符串变量创建一个新变量。但是我不断收到错误消息

  

“运算符'-'不能应用于字符串和字符串c＃类型的操作数”。

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;

“数量”为int类型
“ buypriceperbook”为双字型

有人可以帮我吗？

Answer 1

转换为数字类型而不是字符串：

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

其他注意事项：

• ExecuteScalar仅会给您查询中的 first 值-您的查询是否保证仅返回一个值？还是您希望某些东西可以汇总多个值？
• ExecuteScalar返回一个object，表示它可以是字符串，双精度数，整数，空值等。如果强制转换为int，则表示 查询将返回非空整数值，否则它将在运行时失败。

Answer 2

不能将算术运算符应用于字符串。更改为双精度或整数：

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;