我的方法中出现空引用异常。如何解决？

Question

这是我的代码。我认为问题出在我的 declare @EmployeeId.

string strCommand = "insert into Employee (FirstName, LastName, Birthday, Inn)" +
                    " values (@FirstName, @LastName, @Birthday, @Inn)" +
                    " declare @EmployeeId int = @@identity";

SqlCommand command = new SqlCommand(strCommand, connection);

command.Parameters.AddWithValue("@FirstName", employee.FirstName);
command.Parameters.AddWithValue("@LastName", employee.LastName);
command.Parameters.AddWithValue("@Birthday", employee.Birthday);
command.Parameters.AddWithValue("@Inn", employee.INN);

SqlParameter parameter = new SqlParameter("@EmployeeId", System.Data.SqlDbType.Int);
parameter.Direction = System.Data.ParameterDirection.Output;
//command.Parameters.Add(parameter); //???

try
{
    connection.Open();
    command.ExecuteNonQuery();

    employee.EmployeeId = (int)parameter.Value; // Here I get the exception
}
catch (Exception e)
{
    Console.WriteLine(e.Message);
    Console.WriteLine(e.StackTrace);
    // new Exception("Employee Not Insertes");
}
finally 
{
    connection.Close();
}

return employee;

这只是一种根本不起作用的方法。对不起我的英语。

Answer 1

我会这样做：

string sql = "
    insert into Employee(FirstName,LastName,Birthday,Inn)
        output inserted.Id -- or inserted.employeeid, if that's the column name
        values(@FirstName,@LastName,@Birthday,@Inn);";

// You should create a new connection object for most queries. Really.
// Read this for more info why: https://softwareengineering.stackexchange.com/q/142065/8057
// You should also wrap that connection in a using block
using (var connection = new SqlConnection("connection string here"))
using (var command = new SqlCommand(sql, connection))
{
    //You should avoid AddWithValue(). In certain situation it can cause a severe performance penalty
    //Instead, you should use an explicit type and length
    command.Parameters.Add("@FirstName", SqlDbType.NVarChar, 15).Value =  employee.FirstName;
    command.Parameters.Add("@LastName", SqlDbType.NVarChar, 20).Value = employee.LastName;
    command.Parameters.Add("@Birthday", SqlDbType.DateTime).Value = employee.Birthday);
    command.Parameters.Add("@Inn", SqlDbType.Int).Value = employee.INN; //not sure what INN represents. Some Number?

    connection.Open();           
    employee.EmployeeId = (int)command.ExecuteScalar(); 
}
return employee;

请注意，这允许我完全删除带有异常的行。就此而言，它允许我在这里完全删除异常处理。相反，它可以移动到调用代码 - 它所属的位置 - 因为不再需要 finally 块来关闭连接。

我的方法中出现空引用异常。如何解决？

Null reference exception in my method. How to fix it?

c#

sql-server

ado.net