Is there something like an (Object AS Class) Block (C#)

It is not a real issue, but i have some code. That consists of an abstract class Part with 2 extension classes to it called ContentPart and MultiPartPart.
 
now i have a lot of lines like :
 

Part part;
if (part is MultiPartPart){
    (part as MultiPartPart).obj1 = x;
    (part as MultiPartPart).obj2 = y;
    (part as MultiPartPart).obj3 = z;
}

 
Off course I could make a new MultiPartPart object and copy the pass object to it... But that is kind of out of the question, because copying the object would be just consuming more without any additional value.
 
but is there something like an "as" block in c#? Just like a using block?
 
it works like a charm how i got the code right now, but its just pagefilling all those as conversions..
 
Thanks in advance!