c# - 从 SQL Server 表中选择 NULL 值

Question

我正在使用 C# 和 SQL Server 处理 ASP.NET MVC 4

我从下表中选取一行数据

CREATE TABLE [dbo].[Mem_Basic] (
[Id]          INT           IDENTITY (1, 1) NOT NULL,
[Mem_NA]      VARCHAR (100) NOT NULL,
[Mem_Occ]     VARCHAR (200) NOT NULL,
[Mem_Role]    VARCHAR (200) NOT NULL,
[Mem_Email]   VARCHAR (50)  NULL,
[Mem_MPh]     VARCHAR (15)  NULL,
[Mem_DOB]     DATE          NULL,
[Mem_BGr]     NCHAR (10)    NULL,
[Mem_WAnn]    DATE          NULL,
[Mem_Spouse]  VARCHAR (75)  NULL,
PRIMARY KEY CLUSTERED ([Id] ASC)
);

使用下面的代码

public MemberBasicData GetMemberProfile(int id)
{
        MemberBasicData mb = new MemberBasicData();
        using (SqlConnection con = new SqlConnection(Config.ConnectionString))
        {
            using (SqlCommand cmd = new SqlCommand("SELECT * FROM Mem_Basic WHERE Id="+id+"", con))
            {
                try
                {
                    con.Open();
                    SqlDataReader reader = cmd.ExecuteReader();
                   if(reader.Read()==true)
                    {
                        mb.Id = (int)reader["Id"];
                        mb.Mem_NA = (string)reader["Mem_NA"];
                        mb.Mem_Occ = (string)reader["Mem_Occ"];
                        mb.Mem_Role = (string)reader["Mem_Role"];
                        mb.Mem_Email = (string)reader["Mem_Email"];
                        mb.Mem_MPh = (string)reader["Mem_MPh"];
                        mb.Mem_DOB = (Convert.ToDateTime(reader["Mem_DOB"]));
                        mb.Mem_BGr = (string)reader["Mem_BGr"];
                        mb.Mem_WAnn = (Convert.ToDateTime(reader["Mem_WAnn"]));
                        mb.Mem_Spouse = (string)reader["Mem_Spouse"];
                   }
                }
                catch (Exception e) { throw e; }
                finally { if (con.State == System.Data.ConnectionState.Open) con.Close(); }
            } 
        }
        return mb;
    }

这显示了错误

Unable to cast object of type 'System.DBNull' to type 'System.String'.

(Mem_Email、MPh.. 等有时包含一个NULL 值.. 如果该值为 null 我想返回 null)。任何人都请帮助我。

Answer 1

如果你应该对所有其他变量做同样的事情，就做一些简短的:

 mb.Mem_Email = reader["Mem_Email"] == System.DBNull.Value ? null : (string) reader["Mem_Email"];