“ System.InvalidCastException：指定的转换无效”-DateTime

Question

I am updating certain fields in my website, where I get the following error :

System.InvalidCastException: Specified cast is not valid.

And the code where the error is this(as shown in the error message) :

Line 77: productObj.QuantityInStock = dt.Rows[0]["QuantityInStock"].ToString();

Line 78:productObj.MinQuantity = dt.Rows[0]["MinQuantity"].ToString();

Line 79:productObj.DateUpdated =(DateTime)dt.Rows[0]["DateUpdated"]; //This is where error comes

Line 80:productObj.DateCreated = (DateTime)dt.Rows[0]["DateCreated"];

Line 81: }

My update query is like this :

public int Update(Product obj)
        {
            string query = "update tblProduct set ProdName=@pname,ProdDescription=@pdesc,ProdSize=@psize,ProdPrice=@pprice,QuantityInStock=@qis,MinQuantity=@mq WHERE ProductID=@pid";

            List<SqlParameter> lstParams = new List<SqlParameter>();
            lstParams.Add(new SqlParameter("@pid", obj.ProductID));
            lstParams.Add(new SqlParameter("@pname", obj.ProdName));
            lstParams.Add(new SqlParameter("@pprice", obj.ProdPrice));
            lstParams.Add(new SqlParameter("@pdesc", obj.ProdDescription));
            lstParams.Add(new SqlParameter("@psize", obj.ProdSize));
            lstParams.Add(new SqlParameter("@qis", obj.QuantityInStock));
            lstParams.Add(new SqlParameter("@mq", obj.MinQuantity));

            return DBUtility.ModifyData(query, lstParams);
        }

The Product class looks like this :

public class Product : IModel
    {
        public int ProductID;
        public string ProdName;
        public string ProdDescription;
        public string ProdSize;
        public string ProdPrice;
        public int CompanyID;
        public string ProdPhoto;
        public string QuantityInStock;
        public string MinQuantity;
        public int CategoryID;
        public DateTime DateCreated;
        public DateTime DateUpdated;
    }

Error seems to be in this method :

public Product SelectByID(int ID)
        {
            string query = "select * from tblProduct where ProductID=@pid";
            List<SqlParameter> lstParams = new List<SqlParameter>();
            lstParams.Add(new SqlParameter("@pid", ID));

            DataTable dt = DBUtility.SelectData(query, lstParams);

            Product productObj = new Product();
            if (dt.Rows.Count > 0)
            {
                productObj.ProdName = dt.Rows[0]["ProdName"].ToString();
                productObj.ProdDescription = dt.Rows[0]["ProdDescription"].ToString();
                productObj.CompanyID = Convert.ToInt32(dt.Rows[0]["CompanyID"]);
                productObj.ProductID = Convert.ToInt32(dt.Rows[0]["ProductID"]);
                productObj.CategoryID = Convert.ToInt32(dt.Rows[0]["CategoryID"]);
                productObj.ProdSize = dt.Rows[0]["ProdSize"].ToString();
                productObj.ProdPrice = dt.Rows[0]["ProdPrice"].ToString();
                productObj.QuantityInStock = dt.Rows[0]["QuantityInStock"].ToString();
                productObj.MinQuantity = dt.Rows[0]["MinQuantity"].ToString();
                productObj.DateUpdated = (DateTime)dt.Rows[0]["DateUpdated"]; //This where error comes
                productObj.DateCreated = (DateTime)dt.Rows[0]["DateCreated"];
            }

            return productObj;
        }

In my database too I have mentioned the data-type as DateTime , still why is this error coming ?

My method where I update looks like this :

protected void btnUpdate_Click(object sender, EventArgs e)
        {
            Product prodObj = new ProductLogic().SelectByID(Convert.ToInt32(Request.QueryString["ID"]));

            prodObj.ProdName = txtName.Text;
            prodObj.ProdPrice = txtPrice.Text;
            prodObj.ProdDescription = txtDesc.Text;
            prodObj.MinQuantity = txtMinQty.ToString();
            prodObj.QuantityInStock = txtInStock.ToString();
            prodObj.ProductID = Convert.ToInt32(Request.QueryString["ID"]); 

            int updateResult = new ProductLogic().Update(prodObj);

            if (updateResult > 0)
            {
                lblUpdateRes.Text = "Detail(s) updated successfully!";
                lblUpdateRes.ForeColor = System.Drawing.Color.Green;
            }
            else
            {
                lblUpdateRes.Text = "There was some error while updating your detail(s). Please try again later!";
                lblUpdateRes.ForeColor = System.Drawing.Color.Red;
            }

        }

Any ideas/suggestions ?

EDIT : 1 Here is an image of my table :

Answer 1

DateTime cannot be null in .NET. If your database can have null values then you can either change the type of DateUpdated to a nullable date/time ( DateTime? ):

productObj.DateUpdated = (DateTime?)(dt.Rows[0]["DateUpdated"] == DBNull.Value ? null : dt.Rows[0]["DateUpdated"]);

or use a "magic" value to represent null:

productObj.DateUpdated = (DateTime)(dt.Rows[0]["DateUpdated"] == DBNull.Value ? DateTime.MinValue : dt.Rows[0]["DateUpdated"]);

Answer 2

productObj.DateUpdated = Convert.IsDBNull(dt.Rows[0]["DateUpdated"])? 
  DateTime.MinValue : (DateTime)dt.Rows[0]["DateUpdated"];

or, if productObj.DateUpdated is nullable DateTime? ,

if (!Convert.IsDBNull(dt.Rows[0]["DateUpdated"]))
  productObj.DateUpdated.Value = (DateTime)dt.Rows[0]["DateUpdated"];