Getting Type of expression which stored in string

I have a Dictionary - stored names of variables with their types. Also, I have a string with expression. Is there any way to get Type of this expression?

Example:

• 
  

  dictionary

  
  
  
  
  • "x" : List<DateTime>
  
  
  • "a" : int
  
  
  
  


• string : "x[a+5]"


• expected result: DateTime

In addition, I also know all included namespaces.

This task is little part of global task: I trying to write web implementation for Workflow designer. The problem with "Assign" activity. I have strings with expression for left and right parts, but if left part not just variable name then I have a problem with detecting their type("Assign" activity requires types of arguments).