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POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
 

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  1. POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
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    <p>I have a class that gathers 7-day forecast data from the Bureau or Meteorology and presents it on a web page. A script runs every 30 minutes to get updated data from the Bureau.</p> <p>The bureau provides data in a tab delimited format with a header row. After extracting all the fields, I put the values into a <code>Dictionary&lt;string,string&gt;</code> for parsing. For obvious reasons, the important field for organising the data is the "forecast_date". So before I begin parsing, I make sure that my dictionary actually contains this key.</p> <p>Here is a very simplified example of what I'm doing:</p> <pre><code>static object ForecastLockingObj= new object(); private void UpdateWeather() { if(isTimeForUpdate()) { lock(ForecastLockingObj) { if(isTimeForUpdate()) { Dictionary&lt;string, string&gt; ForecastData = Get7DayForecast(); int forecastDate = int.MinValue; if (ForecastData.ContainsKey("forecast_date") &amp;&amp; int.TryParse(ForecastData["forecast_date"], out forecastDate)) { //Parse the data SetNextUpdateTime(); } } } } } </code></pre> <p>This actually works for the most part. But very occasionally I'll get the following exception:</p> <pre><code>[KeyNotFoundException: The given key was not present in the dictionary.] System.ThrowHelper.ThrowKeyNotFoundException() +28 System.Collections.Generic.Dictionary`2.get_Item(TKey key) +7457036 CoA.WebUI.Controls.WeatherWidget.UpdateWeather() in C:\dev\WeatherWidget.cs:231 </code></pre> <p>Where line 231 is the if statement that checks the existence of "forecast_date" then attempts to parse it into an integer. Note; the service reliably renders dates as integers (e.g. 20130515) and so this is more of a sanity check.</p> <p><code>ContainsKey</code> should not be throwing this exception, so I feel it must be where I refer to <code>ForecastData["forecast_date"]</code> in my TryParse. </p> <p>My question is this; surely if ContainsKey returns false, then TryParse should not run. So why would it report the existence of a key in one statement and then deny its existence in the very next statement... whilst we are inside a lock and the Dictionary we're working on is non-static and local?</p> <p>As an aside; this typically happens in the afternoon, when the Bureau releases the next long range forecast. The exception happens for a few page-loads and then rights itself.</p> <hr> <p>Here is the full Get7DayForecast method</p> <pre><code>private Dictionary&lt;string, string&gt; Get7DayForecast() { int linenumber = 0; int locationNameKey = 0; List&lt;string&gt; keys = new List&lt;string&gt;(); Dictionary&lt;string, string&gt; myLocationData = new Dictionary&lt;string, string&gt;(); FtpWebRequest ftp = (FtpWebRequest)WebRequest.Create(ForecastURL); ftp.Method = WebRequestMethods.Ftp.DownloadFile; ftp.Credentials = new NetworkCredential(); FtpWebResponse ftp_response = (FtpWebResponse)ftp.GetResponse(); if (ftp_response.WelcomeMessage.StartsWith("230") &amp;&amp; ftp_response.StatusDescription.StartsWith("150")) { Stream ftp_responseStream = ftp_response.GetResponseStream(); StreamReader ftp_reader = new StreamReader(ftp_responseStream); while (ftp_reader.Peek() &gt;= 0) { linenumber++; string line = ftp_reader.ReadLine(); List&lt;string&gt; temp = (List&lt;string&gt;)line.Split(ForecastDelimiter).ToList&lt;string&gt;(); if (linenumber == 1) { //Break if the header line does not contain the fields we require if (!ForecastRequiredFields.All(line.Contains)) { break; } keys = temp; locationNameKey = keys.IndexOf(ForecastLocationFieldName); } else if (temp.Count == keys.Count &amp;&amp; temp[locationNameKey] == ForecastLocationName) { for (int i = 0; i &lt; keys.Count; i++) { myLocationData.Add(keys[i], temp[i]); } //Break if we've just parsed the data we were looking for break; } } ftp_reader.Close(); } ftp_response.Close(); return myLocationData; } </code></pre>
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    1. PTQuestion
    1. USIain Fraser
    1. USIain Fraser
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      1. POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
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      1. VTUpMod
    1. CODoes it matter that the dictionary is defined inside a lock? I figured anything defined within the lock is thread safe.
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      1. POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
      1. USIain Fraser
    2. COBy the `lock` statement, I assume that you are using multiples threads. There isn't other methods excluding itens from the same object returned by `Get7DayForecast`?
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      1. POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
      1. USfhelwanger
    3. COHmmm... no `Get7DayForecast` is only used by `UpdateWeather` inside the `lock` statement. It is also largely encapsulated; except for the FTP address of the service (yes... FTP) and the code for the City we're trying to get data on, all objects are defined in the method.
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      1. POThe given key was not present in the dictionary. Even when dictionary.ContainsKey("given_key") == true
      1. USIain Fraser
 

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