using System.Linq.Dynamic; 

    private DataTable GetFilteredDataSource()
    {

        DataTable DT = new DataTable();
        DataTable FilteredDT = new DataTable();
        string filterexpression = string.Empty;
        filterexpression = RadGrid1.MasterTableView.FilterExpression;
        DT = (DataTable)RadGrid1.DataSource;
        FilteredDT = DT.AsEnumerable()
        .AsQueryable()
        .Where(filterexpression)
        .CopyToDataTable();
        return FilteredDT;

    }

Severity    Code    Description    Project    File    Line
Error    CS1929    'IQueryable<DataRow>' does not contain a definition for 'Where' and the best extension method overload 'DbExpressionBuilder.Where(DbExpression, Func<DbExpression, DbExpression>)' requires a receiver of type 'DbExpression'    SSMS    D:\Projet\SSMS\SSMS\Windows\WinSchools.xaml.cs    528

Severity    Code    Description    Project    File    Line
Error    CS1503    Argument 2: cannot convert from 'string' to 'System.Func<System.Data.Entity.Core.Common.CommandTrees.DbExpression, System.Data.Entity.Core.Common.CommandTrees.DbExpression>'    SSMS    D:\Projet\SSMS\SSMS\Windows\WinSchools.xaml.cs    530