字符串和？：运算符

我必须从像这样的字符串值构建查询字符串：

connString += "INSERT INTO (...) VALUES ( "+

_cd.userName "," +

//and there i'd like to use ?: operators:

_cd.lastLogin == "Null" ? "null" : _cd.lastLogin "," 

所以我的查询看起来像 INSERT INTO (...) VALUES ('name', null, (...))

但是当我使用它时，它会剪断我的弦，所以看起来

",null,1,2,'name', (...)";

好吧，我知道我可以使用var a，b，c，d然后检查是否(_cd.lastLogin == "Null) a = null将其放入字符串中，但是有很多变量。

什么是正确的使用方式？

@EDIT：代码：

string query =  "INSERT INTO PersonLogin(...) " + Environment.NewLine +
 "VALUES (" + _cD.userID + ","
 + "'" + _cD.number + "',"
 + "'" + _cD.dateCreate + "','"
 + _cD.lastLogin == "Null" ? ",null," : _cD.lastLogin + "',"
 + _cD.taken + ","
 + _cD.canLogin + ""+ Environment.NewLine;