Question

我有一个登录表单，想要从数据库中选择userID（形式为int），并将其存储为string。

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();

但是登录后，出现此错误：

System.InvalidCastException：'无法转换类型的对象 “ System.Int32”键入“ System.String”。

Answer 1

如果记录不存在（即光标为 empty ）怎么办？让我们阅读并检查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
  }
}

Answer 2

您可以尝试如下使用

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

Answer 3

最简单的解决方案可能是（假设查询始终返回结果）：

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

您还可以在查询中将其转换为nvarchar：

string insertQuery = "SELECT cast(UserID as nvarchar) FROM Customers WHERE Email = @Email AND Password = @Password";

Answer 4

ExecuteScalar返回对象类型，您可以将其转换为所需的任何类型

public override object ExecuteScalar ();

您可以调用它的ToString（）方法，并返回其值的字符串形式。

Answer 5

尝试一下

string result = string.Empty;

SqlCommand com = new SqlCommand(..);
..
object executedResult = com.ExecuteScalar();

if(executedResult != null) {
   result = executedResult.ToString();
}

希望有帮助，

Answer 6

下面是我的发现

    string userID;

using(SqlConnection conn = new SqlConnection(connectionString))
{
            string insertQuery = "SELECT UserID FROM Customers WHERE Email = @Email AND 
            Password = @Password";
            SqlCommand com = new SqlCommand(insertQuery, conn);
            com.CommandType = CommandType.StoredProcedure;
            com.Parameters.AddWithValue("@Email", tbEmail.Text.ToString().Trim());
            com.Parameters.AddWithValue("@Password", tbPassword.Text.ToString().Trim());
            SqlDataReader reader = com.ExecuteReader();
            while(reader.Read())
            {
              userID = reader["UserID"].ToString();
            }
}

如何将从数据库检索到的int转换为字符串

6 个答案: