Httpclient This instance has already started one or more requests. Properties can only be modified before sending the first request

HttpClient.DefaultRequestHeaders (and BaseAddress) should only be set once, before you make any requests. HttpClient is only safe to use as a singleton if you don't modify it once it's in use.

Rather than setting DefaultRequestHeaders, set the headers on each HttpRequestMessage you are sending.

var request = new HttpRequestMessage(HttpMethod.Post, url);
request.Headers.Accept.Clear();
request.Headers.Accept.Add(new MediaTypeWithQualityHeaderValue("application/json"));
request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);
request.Content = new StringContent("{...}", Encoding.UTF8, "application/json");
var response = await _client.SendAsync(request, CancellationToken.None);

Replace "{...}" with your JSON.

Maybe my two cents will help someone.

I ran into this issue when refreshing the page when debugging the application.

I was using a singleton, but each refresh, it was trying to set the base address. So I just wrapped it in a check to see if the base address had already been set.

The issue for me was, it was trying to set the baseAddress, even though it was already set. You can't do this with a httpClient.

if (_httpClient.BaseAddress == null)
{
    _httpClient.BaseAddress = new Uri(baseAddress);
}