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POconcurrent .net web api service calls returning duplicate data
 

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  1. POconcurrent .net web api service calls returning duplicate data
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    <p><strong>The setup</strong></p> <p>We have a web app hosted on IIS 7.5 that uses a .net web api rest service to short-poll a SQL Server 2008 database. The service controller calls a stored procedure to read the data.</p> <p><strong>The problem</strong></p> <p>When concurrent requests are made to the same web service, we intermittently see the duplicate results, and sometime other processes results, returned to the browser.</p> <p><strong>C# service code</strong></p> <pre><code>namespace ImageApp_REST_Services.Repositories { public class ImageLinkRepository : IImageLinkRepository { private List&lt;ImageLink&gt; ImageLinks { get; set; } public IEnumerable&lt;ImageLink&gt; Get(String userId) { ImageLinks = new List&lt;ImageLink&gt;(); using (var cnnSQL = new SqlConnection(...)) { // opend connection to DB cnnSQL.Open(); try { SqlCommand cmmSQL = new SqlCommand("nVision_select_lcimagelinks_sp", cnnSQL); cmmSQL.CommandType = System.Data.CommandType.StoredProcedure; SqlParameter prmSQL = cmmSQL.Parameters.Add(new SqlParameter { ParameterName = "@LCIMGLINKUSERID", Value = userId }); SqlDataReader rdrSQL = cmmSQL.ExecuteReader(); if (rdrSQL.HasRows) { while (rdrSQL.Read()) { ImageLinks.Add(new ImageLink { // set new ImageLink object's properties imageTopicId = DBReader.SQLString(rdrSQL, "LCIMGLINKIMGTOPICID"), id = DBReader.SQLInt(rdrSQL, "LCIMGLINKPK"), recordId = DBReader.SQLString(rdrSQL, "LCIMGLINKRECORDID"), text = DBReader.SQLString(rdrSQL, "LCIMGLINKTEXT"), topicId = DBReader.SQLString(rdrSQL, "LCIMGLINKTOPICID"), topicItem = DBReader.SQLString(rdrSQL, "LCIMGLINKTOPICITEM"), url = DBReader.SQLString(rdrSQL, "LCIMGLINKURL"), user = DBReader.SQLString(rdrSQL, "LCIMGLINKUSERID") }); } } } catch (Exception) { } } return ImageLinks; } } </code></pre> <p>}</p> <p>Again, when multiple requests are hitting the service simultaneously, we occasionally see either duplicate records returned, or records belonging to another service call. </p> <p>We want the service to only return the data for the given request. Does anyone know what is wrong???</p>
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    1. USZengineer
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    1. CO`ImageLinks` should be a local variable
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      1. USJohn Saunders
    2. CO@JohnSaunders Why does defining `ImageLinks` inside the Get method seem to fix the problem, or, why does having it defined at the class level cause the problem?
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    3. COIt is only used within the Get method, so does not need to be at the class level. Each Get call modifies it, and there's no need for that.
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      1. USJohn Saunders
 

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