分配字符串时，viewstate保持为空

Question

我正在为View状态分配文本并在另一个事件中使用它，但是viewstate仍为空，为什么？

if (txtSelectedID.Text != "")
{
    string ComplaintCode = Convert.ToString(row.Cells[4].Text);
    txtAssignComplaintCode.Text = ComplaintCode;
    ViewState["ComposeLetterSubject"] = txtSubject.Text + " " + "Complaint Code:" + " " + ComplaintCode;

}