HttpWebRequestTests.cs

[TestMethod]
public void TestMethod1()
{
    // Arrange
    var uriThatThrows400 = new Uri("http://www.asp.net/<script>", true);
    var request = (HttpWebRequest)WebRequest.CreateDefault(uriThatThrows400);

    try
    {
        // Act
        request.GetResponse();
                
        // Assert
        Assert.Fail("GetResponse didn't throw the expected exception.");
    }
    catch (WebException ex)
    {
        var response = ex.Response;

        var responseStream = response.GetResponseStream();
        if (responseStream == null) Assert.Fail("GetResponseStream unexpectedly returned null.");

        using (responseStream)
        {
            using (var responseStreamReader = new StreamReader(responseStream))
            {
                var responseBody = responseStreamReader.ReadToEnd();

                // Assert
                StringAssert.Contains(responseBody, "<p>HTTP Error 400. The request URL is invalid.</p>");
            }
        }
    }
}