C# HttpClient, getting error Cannot add value because header 'content-type' does not support multiple values

Haven't got .NET 4.5 ready, but according to HttpContentHeaders.ContentType and MediaTypeHeaderValue, it should look something like this:

content.Headers.ContentType = new MediaTypeHeaderValue("text/html");

This error implies that you try to add an header that has already been added to the DefaultRequestHeaders (not only content-type header, but any other header that do not support multiple values).

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In my case, I was initiating the headers from two different places and accidentally added the same key header twice (for example content-type or Authentication).

Inside the startup.cs, IHttpClientFactory (documentation) like:

services.AddHttpClient("MyHttpClient", client =>
{
    client.BaseAddress = new Uri("https://www.google.co.il/");
    client.Timeout = new TimeSpan(0, 1, 0);
    client.DefaultRequestHeaders.Add("content-type", "application/json"));
    client.DefaultRequestHeaders.Add("Authorization", "some values"));
});

And got updated inside the client service:

HttpClient httpClient = this._httpClientFactory.CreateClient("MyHttpClient");    
httpClient.DefaultRequestHeaders.Add("content-type", "application/json")); //Throws Exception!
httpClient.DefaultRequestHeaders.Add("Authorization", "some values")); //Throws Exception!

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UPDATE: In cases you want to be sure that you can add header you can use carefully the DefaultRequestHeaders.Clear()

As soon as you assign a text value to the HttpContent by doing this-

HttpContent content = new StringContent(text);

the content type is automatically set for that content. This content type (in case of String Content) is - {text/plain; charset=utf-8}

So in the next step when you try to explicitly set the Content-Type header you get the error- Cannot add value because header 'Content-Type' does not support multiple values.

There are three ways by which you can set the content type and avoid this error:

Option 1. Specify the content type while setting the content

HttpContent content = new StringContent(text, System.Text.Encoding.UTF8, "text/html");

Option 2. Setting the ContentType property

HttpContent content = new StringContent(text);    
content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue("text/html");

Option 3. First remove the automatically assigned content-type header and then add that header again.

HttpContent content = new StringContent(text);  
content.Headers.Remove("content-type");  
content.Headers.Add("content-type", "text/html");