C#

class BaseFileClass
{
    protected void ProcessFile();
}

class XMLClass : BaseFileClass
{
    public override void ProcessFile()
    {
        //Process XML File
    }
}

class CSVCLass: BaseFileClass
{
    public override void ProcessFile()
    {
        //Process CSV File
    }
}


class ExcelClass: BaseFileClass
{
    public override void ProcessFile()
    {
        //Process Excel File
    }
}

Base b = null;
if( isXMl)
{
    b = new XMLCLass();
}
else if( isExcel)
{
    b = new ExcelCLass();
}
else if( isCSV)
{
    b = new CSVCLass();
}

b.ProcessFile();