If strings are nullable by default, why do i get Unable to cast object of type 'System.DBNull' to type 'System.String'.?

Question

I have seen similar threads like this where the solution is a tertiary if.

My question is, why even get such an error if Strings are nullable?

I am reading a value of a text column in access using ado.net. Whenever there is a row with an empty text column i get that error.

Culprit:

while (dr.Read())
{
      UserList.Add(new UserInfo()
      {
        .
          DestributionGroup = (string)dr["Destribution Group"]
        .
      }
}
class UserInfo
{
.
    public string DestributionGroup;
.
}

Edit:

So in other words I have to convert all of my strings that I am reading from the DB into a line similar to this?

return (accountNumber == DBNull.Value) ? string.Empty : accountNumber.ToString ()

No other way around it?

Because `null != System.DBNull` – Heslacher Sep 12 '13 at 08:35 — Heslacher, Sep 12 '13 at 08:35

score 3 · Answer 1 · answered Sep 12 '13 at 08:38

3

Because null != DBNull.Value.

But you can check if it the value in the DataReader is null with the IsDbNull method:

DestributionGroup = dr.IsDbNull("Destribution Group") ? "" : dr.GetString("Destribution Group");

answered Sep 12 '13 at 08:38

Tim Schmelter

450,073
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score 0 · Answer 2 · answered Sep 19 '13 at 03:26

0

You can use this if you don't mind DestributionGroup being set to null when dr["Destribution Group"] is DBNull.Value :

DestributionGroup = dr["Destribution Group"] as string;

answered Sep 19 '13 at 03:26

Moe Sisko

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If strings are nullable by default, why do i get Unable to cast object of type 'System.DBNull' to type 'System.String'.?

2 Answers2