对网格视图标题中的图像进行排序

我有一个名为SearchGenericReport的Gridview。我想在网格视图列的标题中对图像进行排序。但是GetSortColumnIndex(string strrCol)总是返回- value.所以我无法添加图像。我在这里错过了什么？

public int GetSortColumnIndex(String strCol)
{
    DataTable result= Session["TaskTable"] as DataTable;
    foreach (DataControlField field in result.Columns)
    {
        if (field.SortExpression == strCol)
        {
            return SearchGenericReport.Columns.IndexOf(field);
        }
    }
    return -1;
}
void AddSortImage(GridViewRow headerRow)
{
    int selCol = GetSortColumnIndex(m_strSortExp);
    //if (-1 == selCol)
    //{
    //    return;
    //}
    // Create the sorting image based on the sort direction
    Image sortImage = new Image();
    if (SortDirection.Ascending == m_SortDirection)
    {
        sortImage.ImageUrl = "img/uparrow.png";
        sortImage.AlternateText = "Ascending";
    }
    else
    {
        sortImage.ImageUrl = "img/downarrow.png";
        sortImage.AlternateText = "Descending";
    }
    // Add the image to the appropriate header cell
    headerRow.Cells[selCol].Controls.Add(sortImage);
}

但是当我在网格视图上快速观看时。列 - 它在"枚举不产生任何结果"之类的结果中。并且它不会只进入每个循环。我已经修改了这样的代码 -int GetSortColumnIndex（） {

        // Iterate through the Columns collection to determine the index
        // of the column being sorted.
        foreach (DataControlField field in this.SearchGenericReport.Columns)
        {
            if (field.SortExpression == SearchGenericReport.SortExpression)
            {
                return SearchGenericReport.Columns.IndexOf(field);
            }
        }
        return -1;
    }
    void AddSortImage(int columnIndex,GridViewRow headerRow)
    {
        // Create the sorting image based on the sort direction
        Image sortImage = new Image();
        SortDirection direction = SearchGenericReport.SortDirection;
        if (direction == SortDirection.Ascending)
        {
            sortImage.ImageUrl = "img/uparrow.png";
            sortImage.AlternateText = "Ascending";
        }
        else
        {
            sortImage.ImageUrl = "img/downarrow.png";
            sortImage.AlternateText = "Descending";
        }
        // Add the image to the appropriate header cell
        headerRow.Cells[columnIndex].Controls.Add(sortImage);
    }