Sorting on String format Column not Appeared properly.

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C#

ASP.NET

sorting

Grid View having with various column between those Loan_number is string Field Column when sorting Loan_number column it is not sorted properly.
Ex:
On Page Load

Loan_number Column display as
9898398279
9898398277
9898398278
111
999
777

After Sorting Loan_number Column Appeared as

999
9898398277
9898398278
9898398279
111
777

Sorting Function as below:

protected DataTable SortDataTable(DataTable dataTable, bool isPageIndexChanging)
   {
       DataTable dataTable1 = new DataTable();
       if (dataTable != null)
       {
           DataView dataView = new DataView(dataTable);
           if (GridViewSortExpression != string.Empty)
           {
               if (isPageIndexChanging)
               {
                   dataView.Sort = string.Format("{0} {1}",
                   GridViewSortExpression, GridViewSortDirection);
               }
               else
               {

                   dataView.Sort = string.Format("{0} {1}",
                   GridViewSortExpression, GetSortDirection());
               }
           }
           dataView.ToTable("dataTable");
           dataTable1 = CreateTable(dataView);

           return dataTable1;
       }
       else
       {
           return new DataTable();
       }
   }