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PO+ string concat operator with a null operand
 

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  1. PO+ string concat operator with a null operand
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    <p>A co-worker showed me a very strange behavior and I'd like to know if someone could explain me why.</p> <p>A basic constructor with 2 string params:</p> <pre><code> public MyClass(string str1, string str2) { this.s1 = str1; this.s2 = str2; this.s3 = Method(str2 + "._className", str1); } </code></pre> <p>Method is:</p> <pre><code>public string Method(string key, string defaultValue) { List&lt;string&gt; list = _vars[key]; if (list == null) return defaultValue; string res = ""; foreach (string s in list) { if (res != "") res += ","; res += s; } return res; } </code></pre> <p>When this ctor is called within an aspx page with <code>str2</code> as <code>null</code>, all works fine because if an operand of string concatenation <code>+</code> is <code>null</code>, an empty string is substituted.</p> <p>But when this ctor is called with <code>str2</code> as <code>null</code> in a background thread, a <code>NullReferenceException</code> is fired.</p> <p>The problem was solved by testing <code>str2 != null</code> before using it, but I'd really like to know why the same code sometimes fires an exception, sometimes not!</p> <p>Here is the stack trace:</p> <pre><code>Exception: System.NullReferenceException Message: Object reference not set to an instance of an object. StackTrace: at MyClass..ctor(String str1, String str2) at AbandonedCartsNotificationJob.NotifyAbandonedCarts() in AbandonedCartsNotificationJobPartial.cs:line 39 at AbandonedCartsNotificationJob.work() in AbandonedCartsNotificationJob.cs:line 15 at MyRuntime.JobManager.run() at System.Threading.ThreadHelper.ThreadStart_Context(Object state) at System.Threading.ExecutionContext.runTryCode(Object userData) at System.Runtime.CompilerServices.RuntimeHelpers.ExecuteCodeWithGuaranteedCleanup(TryCode code, CleanupCode backoutCode, Object userData) at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading.ThreadHelper.ThreadStart() </code></pre>
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    1. USSylv21
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      1. VTUpMod
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    1. COI feel like something else is going on here. What is `this.s2` used for? What is the stack trace? Can you post reproducible code for us to test with? Where are you doing the null check? Is it before instantiating `MyClass` or before executing `Method` or at the beginning of the constructor?
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      1. USChris Sinclair
    2. COAre you sure you've tracked the issue down to the correct piece of code? What does `Method` do?
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      1. USDamien_The_Unbeliever
    3. COAre you sure? you tried same combination in Background thread?
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      1. PO+ string concat operator with a null operand
      1. USSriram Sakthivel
 

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