How to get value which has type "Double" and the value has 2 or 3 digits after decimal from a database?

What are your culture settings (Windows culture) when you execute the script? I assume your number format has something to do with that.

Did you mean Language for non unicode programs? It is "English(United States)" as of now

As our tools are unicode programs, the system culture. Is it en-us?

Forgot, can you post the code you used to convert to double?

yes it is en-us and below is my code

Dim rolerisk As Double = Connection.GetSingleProperty("AERole","RiskIndex",f.Comparison("Ident_AERole", rolename,ValType.String, CompareOperator.Equal)).Double

Ident_AErole is not a unique property.  So I would suspect that you are getting more than one result, and something in your cast is broken.  I'd try something more like:

public function TestDouble() As Double
    Dim dbRole as IEntity

    dbRole = Session.Source.Get("AERole","QER-AEROLE-CUSTOMADMIN-ADMIN")

    return dbRole.getvalue("RiskIndex").Double
End function

Or create a collection object and iterate through

Ident_AErole is not a unique property.  So I would suspect that you are getting more than one result, and something in your cast is broken.  I'd try something more like:

public function TestDouble() As Double
    Dim dbRole as IEntity

    dbRole = Session.Source.Get("AERole","QER-AEROLE-CUSTOMADMIN-ADMIN")

    return dbRole.getvalue("RiskIndex").Double
End function

Or create a collection object and iterate through

I am getting permission issue when I am trying to use the query which you posted. Error is 

Object of type One Identity Manager application roles does not exist in database or you do not have the relevant viewing permissions.

It was just a sample.  Replace "QER-AEROLE-CUSTOMADMIN-ADMIN" with the UID of an AERole of your choice.