Invalidation

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A request that can change a resource removes the responses stored for it, so the next GET reads the new version from the server:

// The server answers with Cache-Control: max-age=3600
Assert.Equal("before", await httpClient.GetStringAsync("https://example.com/items/1"));

// A successful DELETE removes the stored responses of https://example.com/items/1
using var deleteResponse = await httpClient.DeleteAsync("https://example.com/items/1");
Assert.Equal(HttpStatusCode.NoContent, deleteResponse.StatusCode);

// The next GET is sent to the server
Assert.Equal("after", await httpClient.GetStringAsync("https://example.com/items/1"));

httpClient is an HttpClient with a caching handler.

Which requests invalidate

Every method other than GET, HEAD, OPTIONS and TRACE invalidates: POST, PUT, DELETE, PATCH, and the methods of your own.
Only when the server answers with a status code from 200 to 399. An error response leaves the stored responses in place.
These requests are never answered from the cache, and their responses are not stored.

What is removed

The stored GET and HEAD responses of the request URL.
The stored responses of the URLs in the Location and Content-Location headers of the response, when they are on the same host as the request.
The responses stored with No-Vary-Search for the same paths, whatever their query (see Vary and No-Vary-Search).

Every variant of these URLs is removed, whatever their Vary headers.

The cache only sees the requests sent through it. A resource changed by another client, or by another HttpClient, is only refreshed when its stored response becomes stale.