title: send HTTP request failed in C#
body: <p>I use following URL in my chrome and it works fine:
      <a href="https://westus.api.cognitive.microsoft.com/luis/v2.0/apps/62bea90c-9b0c-487b-8416-1a4d94772f99?subscription-key=29317e2237fb4b43a91959cadee6f143&amp;staging=true&amp;verbose=true&amp;timezoneOffset=480&amp;q=%E5%93%AA%E4%B8%AA%E5%9B%BD%E5%AE%B6%E8%8E%B7%E5%BE%972010%E5%B9%B4%E4%B8%96%E7%95%8C%E6%9D%AF%E7%AC%AC%E4%B8%80%E5%90%8D" rel="nofollow noreferrer">https://westus.api.cognitive.microsoft.com/luis/v2.0/apps/62bea90c-9b0c-487b-8416-1a4d94772f99?subscription-key=29317e2237fb4b43a91959cadee6f143&amp;staging=true&amp;verbose=true&amp;timezoneOffset=480&amp;q=哪个国家获得2010年世界杯第一名</a></p>
  
  <p>it returns json string like below:</p>
  
  <pre><code>{
    "query": "哪个国家获得2010年世界杯第一名",
    "topScoringIntent": {
      "intent": "Query",
      "score": 0.9818858
    },
    "intents": [
      {
        "intent": "Query",
        "score": 0.9818858
      },
      {
        "intent": "None",
        "score": 0.01755463
      }
    ],
    "entities": [
      {
        "entity": "2010年",
        "type": "builtin.datetimeV2.daterange",
        "startIndex": 6,
        "endIndex": 10,
        "resolution": {
          "values": [
            {
              "timex": "2010",
              "type": "daterange",
              "start": "2010-01-01",
              "end": "2011-01-01"
            }
          ]
        }
      },
      {
        "entity": "2010",
        "type": "builtin.number",
        "startIndex": 6,
        "endIndex": 9,
        "resolution": {
          "value": "2010"
        }
      },
      {
        "entity": "一",
        "type": "builtin.number",
        "startIndex": 15,
        "endIndex": 15,
        "resolution": {
          "value": "1"
        }
      }
    ]
  }
  </code></pre>
  
  <p>but when I send the URL in c#, see below:</p>
  
  <pre><code>    private void button1_Click(object sender, EventArgs e)
      {
          string uri = txtURL.Text;
          if (uri == null)
          {
              uri = @"https://westus.api.cognitive.microsoft.com/luis/v2.0/apps/62bea90c-9b0c-487b-8416-1a4d94772f99?subscription-key=29317e2237fb4b43a91959cadee6f143&amp;staging=true&amp;verbose=true&amp;timezoneOffset=480&amp;q=哪个国家获得2010年世界杯第一名";
          }
          var json_contents = new WebClient().DownloadString(uri); //exception thrown from this line
          txtJson.Text = json_contents;
      }
  </code></pre>
  
  <p>I got an error message:</p>
  
  <pre><code>'The path is not of a legal form.'
  </code></pre>
  
  <p>can anyone tell my the reason and how to fix this problem in my C# code</p>