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

Question

I keep getting this error because the VendorID is null for that record. I've seen that this question has been asked a couple times and it seems like I should just do a check to see if null before it converts it and enters it in the dbtable, but I'm not sure how to do that.

public static PartOrder findPartOrder(string orderNo)
    {
        PartOrder aPartOrder = new PartOrder();
        OleDbDataAdapter myAdapter = new OleDbDataAdapter();
        if (aConnection.State == ConnectionState.Closed)
            aConnection.Open();
        OleDbCommand cmd = aConnection.CreateCommand();
        OleDbDataReader dbReader = null;
        cmd.CommandText = "SELECT * FROM PartOrder WHERE OrderNo = '" + orderNo + "'";
        dbReader = cmd.ExecuteReader();

        while (dbReader.Read())
        {
            aPartOrder.OrderNo = (string)dbReader.GetValue(0);
            aPartOrder.Length = (string)dbReader.GetValue(1);
            aPartOrder.Finish = (string)dbReader.GetValue(2);
            aPartOrder.Cost = (string)dbReader.GetValue(3);
            aPartOrder.PartDrawingNo = (string)dbReader.GetValue(4);
            aPartOrder.VendorId = (string)dbReader.GetValue(5);
            aPartOrder.ShopId = (string)dbReader.GetValue(6);
            aPartOrder.Completed = (string)dbReader.GetValue(7);
        }

        dbReader.Close();
        return aPartOrder;
    }

Again, the line "aPartOrder.VendorId = (string)dbReader.GetValue(5);" gets the error because VendorId is null.

score 3 · Accepted Answer · edited May 11 '12 at 06:20

3

for this particular line try this:

aPartOrder.VendorId = dbReader.GetValue(5)==DBNull?"":dbReader.GetValue(5).value;

better would be to write a helper function:

private static string MyToString(object o)
{
    if(o == DBNull.Value || o == null) 
       return "";

    return o.ToString();
}

and use it:

aPartOrder.VendorId = MyToString(dbReader.GetValue(5));

edited May 11 '12 at 06:20

Nikhil Agrawal

47,018
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answered May 11 '12 at 05:21

TheVillageIdiot

40,053
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score 1 · Answer 2 · answered May 11 '12 at 05:21

1

One Line Answer

aPartOrder.VendorId = dbReader.IsDBNull(5) ? "" :(string)dbReader.GetValue(5);

answered May 11 '12 at 05:21

Nikhil Agrawal

47,018
22
121
208

KV Prajapati · Answer 3 · 2012-05-11T05:31:39.123

Use the OleDbDataReader.IsDBNull(ordinal) to verify the field value.

if(!dbReader.IsDBNull(5))
  aPartOrder.VendorId = dbReader.GetString(5);

Suggestion: Always use parameters/precompiled sql statement to prevent SQL-Injection attack.

cmd.CommandText = "SELECT * FROM PartOrder WHERE OrderNo = @OrderNo";
cmd.Parameters.Add("@OrderNo",System.Data.OleDb.OleDbType.VarChar,20).Value=orderNo;
dbReader = cmd.ExecuteReader();

score 0 · Answer 4 · answered May 11 '12 at 05:24

0

dbReader.GetValue(5).HasValue ? dbReader.GetValue(5).ToString() : String.Empty

answered May 11 '12 at 05:24

rt2800

3,045
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Unable to cast object of type 'System.DBNull' to type 'System.String'. Error

4 Answers4