in the report data source, i have one datatable, it contains the columns like
Invoice No, invoice Date, customer name, amount.

in this report user can select the report grouping by invoice date or  by customer name. when i am creating a formula like this,

if(?GroupSelection) = 'Customer' then
{Invoice.Customer}
else if(?GroupSelection) = 'Date' then
{Invoice.Date}

it shows exception like, "expecting string"... because {Invoice.Date} is a column of DateTime, but {Invoice.Customer} is a column of string type.

Please help me how to create a single report with dynamic grouping...

Thanks in advance.

this won't help you with the sorting on a string that was a date. Removes the option of grouping by different by date type options.

one way is to create 2 formulas

Group first on @group1