I've never fully understood this property of the IHttpHandler. It is a property that you have to set when you implement the interface. I've assumed that setting it to true would be better for performance, but I am not sure what the negative side effects might be. Should I return true or false?
It is used to indicate if a single instance of the IHttpHandler will be used to process multiple concurrent requests. So if you set it to true it will improve performance but you must make sure that your code is thread safe because the ProcessRequest method might be invoked from multiple threads at the same time.
If your IHttpHandler implementation contains state (perhaps setup in the constructor and later used in ProcessRequest) then it can sometimes be useful to set IsReusable to false.
IsReusable keeps the handler in memory and able to handle multiple requests. When set to false, it has to create a new instance of the handler for each incoming request.
I had some issues with this property myself:
来源：https://stackoverflow.com/questions/2057394/should-i-set-isreusable-to-true-in-my-httphandlers