object 无法从 dbnull 转换为其他类型

Question

I have a error with my code, when i start test with my web api and it shows object cannot be cast from dbnull change to other types and i have ticked the null box in sql server database. i don't want to change anything in database. The Mobile and datetime columns are null in sql server. I used asp.net web api 2 to do this project.

My Questions: How to solve the error without doing anything in the database?

here is my code:

        public IHttpActionResult Get()
        {
            List<TestClass> draft = new List<TestClass>();
            string mainconn = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString;
            SqlConnection sqlconn = new SqlConnection(mainconn);
            string sqlquery = "Select * From tblTest";
            sqlconn.Open();
            SqlCommand sqlcomm = new SqlCommand(sqlquery, sqlconn);
            SqlDataReader sdr = sqlcomm.ExecuteReader();
            while (sdr.Read())
            {
                draft.Add(new TestClass()
                    {
                        UserId = Convert.ToInt32(sdr.GetValue(0)),
                        Name = sdr.GetValue(1).ToString(),
                        Mobile = sdr.GetValue(2).ToString(),
                        Access = Convert.ToInt32(sdr.GetValue(3)),
                        Date= Convert.ToDateTime(sdr.GetValue(4))
                    });
            }
            return Ok(draft);
        }

My database in below:

UserId Name     Mobile    Access  Date
11     John     NULL      2       2012-01-02 00:00:00.000
24     Fred     34786584  5       NULL
56     Emily    18375555  0       2014-03-04 00:00:00.000
76     Lydia    NULL      4       2015-09-08 00:00:00.000
87     Anna     12313147  5       2020-11-21 00:00:00.000
90     Mimi     27184641  1       NULL

Answer 1

you are trying to convert a null value into DateTime which causes an error. Make your date nullable by adding a? sign after the DateTime property.

Answer 2

Check by DateTime.TryParseExact

DateTime.TryParseExact("YOURVALUE","yyyyMMdd",CultureInfo.InvariantCulture,DateTimeStyles.None, out resultvalue)?resultvalue.toString():"";

Answer 3

You can check using IsDBNull :

 Date = sdr.IsDBNull(4) ? null : Convert.ToDateTime(sdr.GetValue(4))

if Date column has type 'datetime' or 'datetime2' you can use GetDateTime :

Date = sdr.IsDBNull(4) ? null : sdr.GetDateTime(4)

Answer 4

This code solved it

Date = (sdr.GetValue(4) != DBNull.Value) ? Convert.ToDateTime(sdr.GetValue(4)) : (DateTime?)null