# V3061. Parameter 'A' is always rewritten in method body before being used.

The analyzer detected a possible error in a method's body. One of the method's parameters is rewritten before being used; therefore, the value passed to the method is simply lost.

This error can manifest itself in a number of ways. Consider the following example:

    void Foo1(Node A, Node B)
    {
      A = SkipParenthesize(A);
      B = SkipParenthesize(A);
      // do smt...
    }

There is a typo here that will result in the 'B' object being assigned an incorrect value. The fixed code should look like this:

    void Foo1(Node A, Node B)
    {
      A = SkipParenthesize(A);
      B = SkipParenthesize(B);
      // do smt...
    }

However, this bug can take trickier forms:

    void Foo2(List<Int32> list, Int32 count)
    {
      list = new List<Int32>(count);
      for (Int32 i = 0; i < count; ++i)
        list.Add(GetElem(i));
    }

This method was meant to initialize a list with some values. But what actually takes place is copying of the reference ('list'), which stores the address of the memory block in the heap where the list (or 'null' if memory wasn't allocated) is stored. Therefore, when we allocate memory for the list once again, the memory block's address is written into a local copy of the reference while the original reference (outside the method) remains unchanged. It results in additional work on memory allocation, list initialization, and subsequent garbage collection.

The error has to do with a missing 'out' modifier. This is the fixed version of the code:

    void Foo2(out List<Int32> list, Int32 count)
    {
      list = new List<Int32>(count);
      for (Int32 i = 0; i < count; ++i)
        list.Add(GetElem(i));
    }

You can look at examples of errors detected by the V3061 diagnostic.