c# - 我如何将从数据库中检索到的 int 转换为字符串

Question

我有一个登录表单，我想从数据库中选择 userID(以 int 的形式)，并将其存储为 字符串.

string insertQuery = 
  "SELECT UserID FROM Customers WHERE Email = @Email AND Password = @Password";

SqlCommand com = new SqlCommand(insertQuery, conn);

com.Parameters.AddWithValue("@Email", tbEmail.Text);
com.Parameters.AddWithValue("@Password", tbPassword.Text);

string result = (string)com.ExecuteScalar();

但是在我登录之后，我得到了这个错误:

System.InvalidCastException: 'Unable to cast object of type 'System.Int32' to type 'System.String'.

Answer 1

如果记录不存在(即光标空)怎么办？让我们阅读并检查我们是否至少有一条记录:

// Keep Sql being readable
string insertQuery = 
  @"SELECT UserID 
      FROM Customers 
     WHERE Email = @Email 
       AND Password = @Password";

// Do not forget to dispose IDisposable
using (SqlCommand com = new SqlCommand(insertQuery, conn)) {
  com.Parameters.AddWithValue("@Email", tbEmail.Text);
  com.Parameters.AddWithValue("@Password", tbPassword.Text);

  using (var reader = com.ExecuteReader()) {
    string result = reader.Read()
      ? Convert.ToString(reader[0]) // record exists
      : null;                       // cursor is empty

    //TODO: put relevant code which works with result here
  }
}