Unexpected Error ASP.net repeater FindControl

Using the following ItemDataBound to calculate values for my page fields but I am getting and Unexpected Error:

C#:

protected void rptLicenseHistory_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
    ((TextBox)e.Item.FindControl("txLicenseHistoryType")).Text = "Test";

}

ASP.NET:

<asp:Repeater ID="rptLicHistory" runat="server" onitemdatabound="rptLicenseHistory_ItemDataBound">
    <ItemTemplate>
        <div style="display:flex;width: 100%;align-items: center;">
            <div style="width:60px;">License:</div>
            <div style="width:170px;"><asp:TextBox ID="txLicenseHistoryType" runat="server"/></div>
        </div>
     </ItemTemplate>                    
     <SeparatorTemplate>
        <div style="height: 30px;"> </div>
     </SeparatorTemplate>
</asp:Repeater>

Textbox ID "txLicenseHistoryType" Should return "Test" but I get an Unexpected Error. Any ideas on what I am doing incorrectly?

You using the wrong control event. Most controls (even a plain text box) has a databind event.

You want to deal with EACH row, and thus you want to use the data "item" event.

In most cases for GridView, ListView and others, it usually called "row data bind" something or other.

In the case of a repeater, we want that even FOR EACH ITEM, and thus the markup should be this:

<asp:Repeater ID="rptLicHistory" runat="server"
    OnItemDataBound="rptLicHistory_ItemDataBound" >
    <ItemTemplate>
        <div style="display: flex; width: 100%; align-items: center;">
            <div style="width: 60px;">License:</div>
            <div style="width: 170px;">
                <asp:TextBox ID="txLicenseHistoryType" runat="server" /></div>
        </div>
    </ItemTemplate>
    <SeparatorTemplate>
        <div style="height: 30px;"> </div>
    </SeparatorTemplate>
</asp:Repeater>

Note the use of OnItemDataBound - for "each" item it means to help you remember.

So, now your code behind would be this:

protected void rptLicHistory_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
    if ((e.Item.ItemType == ListItemType.Item) ||
        (e.Item.ItemType == ListItemType.AlternatingItem))
    {
        Debug.Print($"Working on row = {e.Item.ItemIndex}");
        ((TextBox)e.Item.FindControl("txLicenseHistoryType")).Text = "Test";
    }
}

Note how you have to test if this is a "item" or a "alternate row" item. The reason is that you have heading rows, and other types of template rows, so you have to in code make sure you ONLY are grabbing + dealing with the row, or alternating row.

The above when run should look like this:

Edit: Get full data row for each row binding

So, say in place of "hard coded" assignment of "test"?

We could do this:

    protected void rptLicHistory_ItemDataBound(object sender, RepeaterItemEventArgs e)
    {
        if ((e.Item.ItemType == ListItemType.Item) ||
            (e.Item.ItemType == ListItemType.AlternatingItem))
        {
            DataRowView MyDataRow = (DataRowView)e.Item.DataItem;
            ((TextBox)e.Item.FindControl("txLicenseHistoryType")).Text = MyDataRow["HotelName"].ToString();
        }

Result: