Reads data frm config file

Question

Hi

I have code which is set 25 string lenght

[StringLength(25, ErrorMessageResourceType = typeof(CampaignResource.VendorCampaign), ErrorMessageResourceName = "InvalidLengthDescription1")]

 

Now i dnt want to set that length constant i need to change according to config file value,what value i am giving in config that value should take .How can do by using custom Attribute,plz gve sample code