The reason I published this post is because I didn't find any result when googling the error description and solved it myself at last.

If you're asp.net developers you probably know the ObjectDataSource object, which represents a business object that provides data to data-bound controls in multi-tier Web application architectures.

I like this object, most of the time this object can solve you all the annoying steps logic of calling the BL/DAL object in order to retrieve the data and populate the wanted presentation control.

On one of my working on website's pages (which is quite complex one that knows to list data from several sources and procedures), I am using such of object as a data source in the main GridView that renders a list of records. In order to interact with each different select method I had to set every time the SelectMethod property and it's specific parameters in the code-behind. Until now everything is just fine...

It seems that working this way affects the other postback events on the page, (because after event postback, the OnInit and OnLoad events are being called first and just after it the event handler itself is being called), here my page was crashed and gave this error message: "The Select operation is not supported by ObjectDataSource '<objectdatasource_id>' unless the SelectMethod is specified."

This error caused because the page expected the SelectMethod property to be initialized between the OnInit and OnLoad methods and just after it the rest of the events.

The resolution is quite easy in this case;

First, you need to remove the ObjectDataSourceID property definition from the control properties' definitions layout in the control source and set the DataSource for the control to the desired one in the OnInit method. After it, in the OnPreRender method call the control's DataBind method in order to bind the data source. This last action will allow to any event to happen and just after it to set up the Control (the GridView in my case) with data.

protected override void OnInit(EventArgs e)
{
   base.OnInit(e);
   MyGridView.DataSource = MyObjectDataSource;
}

protected override void OnPreRender(EventArgs e)
{
   base.OnPreRender(e);
   MyGridView.DataBind();
}

I hope it'll help anyone...