How Do I Set a Default User Agent on an Httpclient?

You can solve this easily using:

HttpClient _client = new HttpClient();
_client.DefaultRequestHeaders.Add("User-Agent", "C# App");

The following worked for me in a .NET Standard 2.0 library:

HttpClient client = new HttpClient();
ProductHeaderValue header = new ProductHeaderValue("MyAwesomeLibrary", Assembly.GetExecutingAssembly().GetName().Version.ToString());
ProductInfoHeaderValue userAgent = new ProductInfoHeaderValue(header);
client.DefaultRequestHeaders.UserAgent.Add(userAgent);
// User-Agent: MyAwesomeLibrary/1.0.0.0

Using JensG comment

Short addition: The UserAgent class also offers TryParse, which comes especially handy when there is no version number (for whatever reason). The RFC explicitly allows this case.

on this answer

using System.Net.Http;

using (var httpClient = new HttpClient())
{
    httpClient.DefaultRequestHeaders
      .UserAgent
      .TryParseAdd("Mike D's Agent");
    //User-Agent: Mike D's Agent
}

string agent="ClientDemo/1.0.0.1 test user agent DefaultRequestHeaders";
HttpClient client = new HttpClient();
client.DefaultRequestHeaders.Add("User-Agent", agent);

remark: use this structure to generate the agent name User-Agent: product / product-version comment

• product: Product identifier
• product-version: Product version number.
• comment: None or more of the infomation Comments containing product, for example.

references