Pass C# variable and use it as a Column Value in SQL Statement Query

Question

I need to use the value of a variable as one of my fields in my SQL insert query that I am generating. In PHP I do this:

if($currency = "USD")
{
 $columntoinsert = "USD_Currency";
} 
else
{
 $columntoinsert = "";
}

Then in my SQL statement I would execute the query like so:

$sqlinsertpayment = sqlsrv_query($connect,"INSERT INTO Payments_TBL(column1,column2,$columntoinsert,columnn)VALUES(????)",$params) or die("An error occurred trying to execute the statement");  

I want to do the same in C#. I have the following code to set the column I want to use

var amountfield = "";
  if (save_currency != "USD")
    {
      amountcolumn = "Amount";

     }
  else
   {
      amountcolumn = "USD_Amount";
   }

But I can't execute my sql query in since trying to use amountcolumn in the query string generates an Invalid column name error which is expected. How can I use the variable in this manner:

var sql = new sqlCommand("INSERT INTO Payments_TBL(column1,column2,myc#variablevaluehere,column3,....)Values(@value,@value,@value)",new sqlConnection(conn)); 

Open to other alternatives. All help will be appreciated.

Answer 1

Because the value for amountfield comes from your code logic, you can safely do this:

var sql = new sqlCommand(String.Format("INSERT INTO Payments_TBL(column1,column2,{0},column3,....)Values(@value,@value,@value)", amountcolumn),new sqlConnection(conn));

But never do things like this with strings coming from your users because that would make you open for SQL injection.

Answer 2

Its

var amountfield = save_currency != "USD" ? "Amount" : "USD_Amount";

var sql = new sqlCommand("INSERT INTO Payments_TBL(column1,column2," 
+  amountcolumn + 
",column3,....)Values(@value,@value,@value)",new sqlConnection(conn));

OR

var sql = new sqlCommand(
                   String.Format("INSERT INTO Payments_TBL(column1,column2,  
                   {0},column3,....)Values(@value,@value,@value)", amountfield), 
                   new sqlConnection(conn));