Index was outside the bounds of the array, but i can't see the error

Question

int n =Count("SELECT COUNT(*) FROM information_schema.SCHEMATA");; //return 6

TreeNode[] db_name = new TreeNode[n];
MySqlCommand cmd = new MySqlCommand("show databases", connection);
MySqlDataReader dataReader = cmd.ExecuteReader();

for(i=0;i<n;i++)
{
    dataReader.Read();
    db_name[i] =  new TreeNode(dataReader[i].ToString());
}

Why do I get IndexOutOfRangeException was unhandled,Index was outside the bounds of the array? If the Count() funciont return 6 that means there are 6 rows so 6 fields [0][1][2][3][4][5] I put a breakpoint in the for loop and I get the error at the second loop, when i=1. How can I fix that? I can't see the error. Thanks in advance.

Your code is based on a wrong assumption that `show databases` command would return a single column for each row in `information_schema.SCHEMATA`. This assumption is incorrect - the `information_schema.SCHEMATA` has a row for each database schema, not for each column returned by `show databases`. — Sergey Kalinichenko, Feb 11 '14 at 13:30

score 2 · Accepted Answer · answered Feb 11 '14 at 13:25

The call to dataReader.Read() advances to the next available record.

When you call dataReader[i], you're probably trying to get a column that doesn't exist in your returned data. On the first iteration, dataReader[i] is trying to get the first column, but then on the second iteration it's just trying to get the second column, then third, etc. You're probably not returning 6 columns, so that's going to throw an exception.

You could try something like this instead, assuming that each record is just a string:

for(i=0; i<n; i++)
{
    dataReader.Read();
    db_name[i] =  new TreeNode(dataReader.GetString(0));

          // or use dataReader[0].ToString(), but don't change the index of 0
}

Christos · Answer 2 · 2014-02-11T13:30:08.850

1

Please try the following.

int i=0;
if (dataReader.HasRows)
{
    while (dataReader.Read())
    {
        db_name[i] =  new TreeNode(dataReader.GetString(0));          
    }
    i++;
}

edited Feb 11 '14 at 13:30

answered Feb 11 '14 at 13:24

Christos

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Just because it isn't an array doesn't mean you can't access the data reader like one. If it didn't have the indexer the program wouldn't compile. MySqlDataReader probably implements [IDataReader](http://msdn.microsoft.com/en-us/library/system.data.idatareader%28v=vs.100%29.aspx) which has array-like access to the columns. – Dirk Feb 11 '14 at 13:28
@Drik Oh, I didn't know it. Thanks, Dirk – Christos Feb 11 '14 at 13:29

Index was outside the bounds of the array, but i can't see the error

2 Answers2