How Can I Fix 'Could Not Establish Trust Relationship' Errors When Making HTTPS Calls with HttpClient?

Use HttpClient to make secure HTTPS calls

Question:

When using HttpClient to make an HTTPS request, an error occurs: "The underlying connection was closed: Unable to establish trust for the SSL/TLS secure channel."

Solution:

To resolve this issue, please follow these steps:

1. Set security protocol:

   Specify the TLS protocol used by default. Add the following lines to your code:

   <code class="language-csharp">System.Net.ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;</code>

2. Modify HttpClient code:

   Modified HttpClient code to support HTTPS calls:

   <code class="language-csharp">HttpClient httpClient = new HttpClient();
   System.Net.ServicePointManager.SecurityProtocol |= SecurityProtocolType.Tls12 | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls;
   httpClient.BaseAddress = new Uri("https://foobar.com/");
   httpClient.DefaultRequestHeaders.Accept.Clear();
   httpClient.DefaultRequestHeaders.Accept.Add(
       new MediaTypeWithQualityHeaderValue("application/xml"));
   
   var task = httpClient.PostAsXmlAsync<devicerequest>("api/SaveData", request);</code>

Additional notes:

• This solution assumes that the server only supports higher TLS versions (for example, TLS 1.2).
• If the server requires a custom certificate or key, additional configuration is required.

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