Symbolic derivative about functional and substitute function

Question

I have trouble with symbolic functional derivatives and substitute , for example,

result1 =[D[f[x, y], x]]

$result1 = \frac{\partial f[x,y]}{\partial x} $

next, i want take $f[x,y]=x+y$ substitute into result1. This is very useful in symbolic computation, especially when substituting functions into expressions after complex computations.

I have try this answer with Inactivate ,this is a very early version, and Inactivate will deactivate all partial derivatives. It's very inconvenient, for example, When I only care about the properties of f,

result1 = Inactivate[D[f[x, y] Cos[x], x], D]

the output is $\frac{\partial (\cos (x) f(x,y))}{\partial x}$ , is there any better method can able to substitute f into the expression while computing known functions as much as possible?

As the follows example,

 result1 =[D[f[x, y] Cos[x], x]]  
 result2 = result1/.{f[x,y]=x+y} 

I want output is $result1 = \frac{\partial ( f(x,y))}{\partial x}\cos (x)-\sin (x) f(x,y), result2 = \cos (x)-(x+y) \sin (x)$

Answer 1

Another way could be to simply define your replacement for $f(x,y)$ using Function notation as follows

ClearAll[f, x, y]
result1 = D[f[x, y] Cos[x], x]
myf = Function[{x, y}, x + y]
result2 = result1 /. f -> myf

This is the same trick used to verify solution of an ode by plugging in the solution in Function form back into the ode which has derivatives in it.

Answer 2

What about

result1 = D[f[x, y] Cos[x], x] (*-f[x, y] Sin[x] + Cos[x] Derivative[1, 0][f][x, y]*)
result1 /. f -> Function[{x, y}, x + y](*Cos[x] - (x + y) Sin[x]*)