'Find and Save' functionality not working

Question

First time poster, long time lurker. I am having some trouble with my ASP.NET page, and I hope someone can help me resolve my issue.

Basically, I have a bunch of checkboxes in a gridview, and two buttons: a 'find' button, and a 'save' button. The 'find' can set the value of the checkbox, but if a user unchecks it, I want to capture that change when the user hits 'save'. Currently, it does not work.

Relevant ASPX:

<%@ Page Language="C#" AutoEventWireup="true" EnableViewState="true" CodeBehind="FindTransactions.aspx.cs" Inherits="Basic.FindTransactions" MasterPageFile="~/Trans.Master" %>

Relevant Code Behind here:

Page:

public partial class FindTransactions : System.Web.UI.Page
{
    GridView _gridview = new GridView() { ID = "_gridView" };
    DataTable _datatable = new DataTable();
    Int32 _buyerID = new Int32();

    protected void Page_Load(object sender, EventArgs e)
    {
    }

"Find" button:

protected void Find_Click(object sender, EventArgs e)
{
    //truncated 
    _datatable.Rows.Add(
    //filled with other data from a custom object.
    );

    ViewState["_datatable"] = _datatable;
    ViewState["_buyerID"] = _buyerID;
    BuildGridView((DataTable)ViewState["_datatable"],(Int32)ViewState["buyerID"]);

}

BuildGridView function:

protected void BuildGridView(DataTable d, Int32 b)
{
    _gridview.DataKeyNames = new String[] {"Transaction ID"};
    _gridview.AutoGenerateColumns = false;
    _gridview.RowDataBound += new GridViewRowEventHandler(OnRowDataBound);

    for(Int32 i = 0; i < d.Columns.Count; i++)
    {
        Boundfield boundfield = new BoundField();
        boundfield.DataField = d.Columns[i].ColumnName.ToString();
        boundfield.HeaderText = d.Columns[i].ColumnName.ToString();
        _gridview.Columns.Add(boundfield);
    }

    _gridview.DataSource = d;
    _gridview.DataBind();
    //truncated
    Panel1.Controls.Add(_gridview);
}

Row Bound Event handler:

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        String controlID = "checkBox";
        CheckBox c = new CheckBox() { ID = controlID};
        c.Enabled = true;

        Boolean success;
        Boolean v;
        success = Boolean.TryParse(e.Row.Cells[8].Text, out v);
        e.Row.Cells[8].Controls.Add(c);

        if (success)
        {
            c.Checked = v;
            if (c.Checked)
            {
                //Will uncomment once other things work
                //e.Row.Visible = false;
            }
        }
        else
        {
            c.Checked = false;
        } 
    }
}

All of that works. Here is where it starts to break down:

"Save" button:

protected void Save_Click(object sender, EventArgs e)
{
    //Both for troubleshooting and both return 0. (Expected for datatable)
    Label1.Text = _gridview.Rows.Count.ToString();
    Label2.Text = _datatable.Rows.Count.ToString();

    /*truncated
    */
    if (grid.Rows.Count == 0)
    {
        BuildGridView((DataTable)ViewState["infoTable"], (Int32)ViewState["guestID"]);
    }
    foreach (GridViewRow r in grid.Rows)
    {
        if (r.RowType == DataControlRowType.DataRow)
        {
             CheckBox cb = (CheckBox)r.FindControl("checkBox");
             if (cb != null && cb.Checked)
             {
                 //This never seems to modify the label.
                 //Will put code to modify database here.
                 Label2.Text += "Hi " + r.RowIndex.ToString();
             }
         }
     }
 }

After I hit the save button, PostBack occurs and GridView is empty (Rows.Count is 0). ViewState appears to be lost before I get a chance to loop through the GridView rows to determine the checkbox values.

At the end of it all, I just want to capture the status of those checkboxes, changed by user interaction or not, by hitting the 'Save' button.

I found some other articles, but a lot of them haven't worked when I tried implementing the various fixes.

This one seems to be the closest that describes my issue, and the code is structured similarly, but I don't quite understand how to implement the fix: GridView doesn't remember state between postbacks

[New simplified code to illustrate problem:]

namespace GridViewIssue
{
    public partial class GridViewNoMaster : System.Web.UI.Page
    {
        GridView _gridView = new GridView() { ID = "_gridView" };
        DataTable _dataTable = new DataTable();

        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Find_Click(object sender, EventArgs e)
        {
            BuildDataTable();

            List<String> list = new List<String>();
            list.Add("1");
            list.Add("User");
            list.Add("10/12/2014");

            foreach (String s in list)
            {
                _dataTable.Rows.Add(
                    list[0],
                    list[1],
                    list[2]
                    );
            }

            BuildGridView();
            //Feedback.Text = _gridView.Rows.Count.ToString();
        }

        protected void Save_Click(object sender, EventArgs e)
        {
            Feedback.Text = "Save Clicked, PostBack: " + IsPostBack + ", GridView Row Count: " + _gridView.Rows.Count + ", GridView ViewState: " + _gridView.EnableViewState;
            foreach (GridViewRow r in _gridView.Rows)
            {
                if(r.RowType == DataControlRowType.DataRow)
                {
                     Feedback.Text = "In DataRow type" + _gridView.Rows.Count;
                }
            }
        }

        protected void BuildDataTable()
        {
            _dataTable.Columns.Add("Transaction ID", typeof(String));
            _dataTable.Columns.Add("Name", typeof(String));
            _dataTable.Columns.Add("Date", typeof(String));
        }

        protected void BuildGridView()
        {
            for (Int32 i = 0; i < _dataTable.Columns.Count; i++)
            {
                BoundField b = new BoundField();
                b.DataField = _dataTable.Columns[i].ColumnName.ToString();
                b.HeaderText = _dataTable.Columns[i].ColumnName.ToString();
                _gridView.Columns.Add(b);
            }

            _gridView.DataKeyNames = new String[] { "Transaction ID" };
            _gridView.AutoGenerateColumns = false;
            _gridView.DataSource = _dataTable;
            _gridView.DataBind();

            Panel1.Controls.Add(_gridView);
        }
    }
}