How to evaluate string expression in SQL Server?

Question

For a string '100+200', how to evaluate it?

I want to get 300the output.

Answer 1

DECLARE @expression VARCHAR(MAX) = '100+200'
EXEC    (N'SELECT ' + @expression)

If it comes from the user, make sure that it clears

Answer 2

Seriously? How about this:

SELECT 100 + 200;