Question

How can I change struct in external method ?

public void ChangeStruct (MyStruct myStruct) {
   myStruct.field1 = 10;
   return;
}

When I pass struct to ChangeStruct method after that method I would like myStruct to be changed.

Answer 1

You need to pass a reference to the struct instead of a copy using the ref keyword :

public void ChangeStruct (ref MyStruct myStruct)
{
   myStruct.field1 = 10;
}

ChangeStruct(ref someStruct);

Your current code create a full bit-for-bit copy of the struct before entering the method and it’s this copy that you are modifying, the ref keyword force the caller to pass a reference (managed pointer) to the structure instead of the copy.