ASP.NET

//DropDownList inside the gridview
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        //identifying DropDownList
        DropDownList gridDropDownList = (DropDownList)sender;
        //selecting grid row
        GridViewRow gridRow = (GridViewRow)gridDropDownList.NamingContainer;
        //setting Index
        int selectedIndex = gridRow.DataItemIndex;
        //finding textbox in selected gridview row
        TextBox textBox = (TextBox)GridView1.Rows[selectedIndex].FindControl("yourTextBoxID");
        string selected = gridDropDownList.SelectedValue;
        if (textBox != null)
        {
            textBox.Text = selected;
        }
    }

function selectChoice(ddl, textboxID) {
    var chosenoption = ddl.options[ddl.selectedIndex];
    if (chosenoption.value != 'nothing') {
        document.getElementById(textboxID).value = chosenoption.value;
    }
}

protected void GridView1_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        DropDownList ddlChoice = e.Row.FindControl("ddlChoice") as DropDownList;
        TextBox txtSelectedValue = e.Row.FindControl("txtSelectedValue") as TextBox;

        string jScript = "selectChoice(this, '{0}');";
        ddlChoice.Attributes.Add("onchange", string.Format(jScript, txtSelectedValue.ClientID));
    }
}

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