Calling GetString on a SqlDataReader is giving me an InvalidCastException

Question

I am looking to move data in a arraylist for a Listbox, but it is giving error. Error :

An unhandled exception of type 'System.InvalidCastException' occurred in System.Data.dll Additional information: You can not convert an object of type 'System.Int32' on type 'System.String'.

This happens on line: fill_arry[i] = objReader.GetString(1);

Code:

public void Fill_List(ArrayList fill_arry)
{
    SqlDataReader objReader;
    SqlCommand objcmd =null;

    int i = 0;

    vsql = "SELECT [NOME] As Identificador,[RGP],[NOME],[ENDERECO],[CIDADE],[ESTADO],[TELEFONE],[CELULAR] FROM pescador";

    if (this.Conectar())
    {
        try
        {
            objcmd = new SqlCommand(vsql, objCon);
            objReader = objcmd.ExecuteReader();

            while (objReader.Read())
            {
                fill_arry[i] = objReader.GetString(1);
            }
        }
        catch (SqlException erro)
        {
            throw erro;
        }
        finally
        {
            this.Desconectar();
        }
    }
}

I wouldn't recommend using an ArrayList, go with a normal List instead. You don't have to cast from object either: http://stackoverflow.com/a/2309699/1661209 ArrayList has been deprecated in C#. — Max, Jul 20 '15 at 17:22
What is the type of `RGP` column? Looks like it is numeric type. — Soner Gönül, Jul 20 '15 at 17:23
RGP is a Int Type. I need get Name types to list on a listbox. — Matheus Saviczki, Jul 20 '15 at 17:25
So you only want to store strings in the ArrayList? This would give you a `List strList = new List();` Use `strList.Add("STRING");` to add values to this list. — Max, Jul 20 '15 at 17:26
Also, I recommend just using ```throw;``` instead of ```throw erro;``` - See http://stackoverflow.com/questions/881473/why-catch-and-rethrow-an-exception-in-c — Aaron D, Jul 20 '15 at 17:48
@AaronD Or just remove the catch altogether as it's not doing anything useful. — juharr, Jul 20 '15 at 18:30

score 3 · Answer 1 · edited May 23 '17 at 12:29

Edit:

No, I need to get the NOME

so [NOME] is the desired field, change:

fill_arry.Add(objReader.GetString(1));

To:

fill_arry.Add(objReader.GetString(0));

Note: As sstan wrote down in his answer, you are retrieving the [RGP] column value, is this what you want? Since it is an int you would have to convert. If you want to get [NOME] than change 1 to 0, since the index starts at zero.

Using a normal List is preferred over the old ArrayList. Casting isn't necessary using a List instead of an ArrayList. The ArrayList stores the object references. You would end up with a function like:

public void Fill_List(List<string> fill_arry)
{
    SqlDataReader objReader;
    SqlCommand objcmd =null;

    int i = 0;

    vsql = "SELECT [NOME] As Identificador,[RGP],[NOME],[ENDERECO],[CIDADE],[ESTADO],[TELEFONE],[CELULAR] FROM pescador";

    if (this.Conectar())
    {
        try
        {
            objcmd = new SqlCommand(vsql, objCon);
            objReader = objcmd.ExecuteReader();

            while (objReader.Read())
            {
                //The value is added to List<string> fill_arry here.
                fill_arry.Add(objReader.GetString(1));
            }
        }
        catch (SqlException erro)
        {
            throw erro;
        }
        finally
        {
            this.Desconectar();
        }
    }
}

Method should be called something like this:

List<string> strList = new List<string>();
Fill_List(strList);

Refer to below answer, to know why you shouldn't use an ArrayList, instead of a List.

A short quote from the answer:

ArrayList belongs to the days that C# didn't have generics. It's deprecated in favor of List. You shouldn't use ArrayList in new code that targets .NET >= 2.0 unless you have to interface with an old API that uses it.

SO answer

At the following line? `fill_arry.Add(objReader.GetString(1));`? The index starts at 0, so `GetString(1)` would give you the value from column [RGP] is this desired? — Max, Jul 20 '15 at 17:35
I think this answer is geared more towards the suggestion of getting rid of ArrayList instead of solving his problem. ArrayList takes any type so the cast error wouldn't be affected nor solved by changing ArrayList to something else. The other answer I think solves this problem. Delete this answer? — Thraka, Jul 20 '15 at 17:37
@Thraka I'm giving him the suggestion to change from ArrayList to List and why he should. Besides that is the note claryfing what is happening. — Max, Jul 20 '15 at 17:38
@Max I hear ya buddy :) But that is simply done in the comment to his answer, as you did. :) — Thraka, Jul 20 '15 at 17:39
@user3056183 I modified my answer, explaining why it is failing. Int to string conversion, since you are retrieving the wrong / second value. — Max, Jul 20 '15 at 17:41

sstan · Answer 2 · 2015-07-20T17:42:23.957

Using a List<T> instead of an ArrayList is, as others are saying, better practice. But it isn't the reason for your error.

The cause of your error is this: objReader.GetString(1).

This is trying to fetch the value of the column RPG as if it was a string, when in fact, according to the error, it is an int.

To get rid of the error, but still get the value as a string, just change it to objReader.GetInt32(1).ToString().

EDIT: Well, apparently, you want [NOME], not [RGP] :)

It would have been nice to specify that. The indexes on SqlDataReader start at 0. so to get NOME, you need to fetch it as:

objReader.GetString(0);

Or, better yet, if you want to avoid getting the indexes wrong, why not do it this way:

(string)objReader["Identificador"]; // named parameter instead of an index, yay!

Calling GetString on a SqlDataReader is giving me an InvalidCastException

2 Answers2