c# dataGridView for loop to add data

your btn_click function. Every loop will initialize or create a new object inside the aaa(i) function

Every time aaa(i) is called in the for loop DataTable dt = new DataTable() will be called

public void aaa(int i)
{
    DataTable dt = new DataTable(); ///this will initialize every time, a new data table will be created every loop
    dt.Columns.Add("host");

    DataRow dr = dt.NewRow();
    for (int a = 1; a <= i; a++)
    {
        dr[a] = i;
    }
        dt.Rows.Add(dr);

    this.dataGridView1.DataSource = dt;
}

}

Might i suggest you pass the 254 int in your aaa(i) function and do the loop inside like

private void button1_Click(object sender, EventArgs e)
{
   aaa(254);
}


public void aaa(int i) //value of i = 254
{
        DataTable dt = new DataTable();
        DataRow dr = dt.NewRow();
        for (var s = 0; s <= i; s++ ) {
            for (int a = 1; a <= i; a++)
            {
                dr[a] = i;
            }
                dt.Rows.Add(dr);

            this.dataGridView1.DataSource = dt;
        }
    }
}

or if the reason for the loop in the button is just for the number limit in the loop inside void aaa then you can simplify it as

public void aaa(int i) //value of i = 254
{
    DataTable dt = new DataTable();
    DataRow dr = dt.NewRow();
    for (int a = 1; a <= i; a++)
    {
        dr[a] = i;
    }
    dt.Rows.Add(dr);

    this.dataGridView1.DataSource = dt;
}

}

I always do this like that

ListCollectionView collectionView;
collectionView = new ListCollectionView(*your list of items*);

datagridView1.ItemSource = collectionView;

Implementing like this give you in future open way for sorting, filtering etc. Here is link to ms documentation. where you will find more information about listCollectionView.