Question

I have assigned two string array:

string[] SelectColumns = {},WhereColumns={};

Both of them are full of data items. For example SelectColumns.length = 7,WhereColumns.Length=3;

When i went to implement them i got an exception: object reference not set to an instance of an object.
I am using them in below:

for (int i = 0; i < SelectColumns.Length; i++)
{
    DPS._SelectCol[i] = SelectColumns[i];
}

for (int i = 0; i < WhereColumns.Length; i++)
{
    DPS._WhereCol[i] = WhereColumns[i];
}

Here DPS is the object of a class, which is given below:

public class DefaultProfileSetting
{
    private string Server;

    public string _Server
    {
        get { return Server; }
        set { Server = value; }
    }

    private string Authentication;

    public string _Authentication
    {
        get { return Authentication; }
        set { Authentication = value; }
    }
    private string Login;

    public string _Login
    {
        get { return Login; }
        set { Login = value; }
    }
    private string Pass;

    public string _Pass
    {
        get { return Pass; }
        set { Pass = value; }
    }
    private string DB;

    public string _DB
    {
        get { return DB; }
        set { DB = value; }
    }
    private string Table;

    public string _Table
    {
        get { return Table; }
        set { Table = value; }
    }
    private string[] SelectCol;

    public string[] _SelectCol
    {
        get { return SelectCol; }
        set { SelectCol = value; }
    }
    private string[] WhereCol;

    public string[] _WhereCol
    {
        get { return WhereCol; }
        set { WhereCol = value; }
    }
}

Answer 1

You probably have just string array reference _SelectCol but not actual array and need to instantiate the _SelectCol string array to allocate memory to its elements.

DPS._SelectCol = new string [SelectColumns.Length];

for (int i = 0; i < SelectColumns.Length; i++)
{
    DPS._SelectCol[i] = SelectColumns[i];
}