+带有空操作数的字符串连接运算符

Question

A co-worker showed me a very strange behavior and I'd like to know if someone could explain me why.

A basic constructor with 2 string params:

    public MyClass(string str1, string str2)
    {
        this.s1 = str1;
        this.s2 = str2;
        this.s3 = Method(str2 + "._className", str1);
    }

Method is:

public string Method(string key, string defaultValue)
{
    List<string> list = _vars[key];
    if (list == null) return defaultValue;
    string res = "";
    foreach (string s in list)
    {
        if (res != "") res += ",";
        res += s;
    }
    return res;
}

When this ctor is called within an aspx page with str2 as null , all works fine because if an operand of string concatenation + is null , an empty string is substituted.

But when this ctor is called with str2 as null in a background thread, a NullReferenceException is fired.

The problem was solved by testing str2 != null before using it, but I'd really like to know why the same code sometimes fires an exception, sometimes not!

Here is the stack trace:

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()

Answer 1

There was an obscure bug in the .NET Framework's implementation of string concatenation, but it only affected concatenations of 4 objects, where one of the objects is non-null and provided an override of ToString that returned null. Clearly that situation isn't the case here.

This situation is most likely caused by one of the following:

• _vars is null when Method is called
• Due to a misuse of _vars in a multi-threaded application, the internal state of _vars has been corrupted, resulting in a NullReferenceException when operator [] is used.

Answer 2

The problem lies in the implementation of the Method Object. Since the + Operator implementation interprets a null value as an empty string . The actuall null value never enters the constructor when set in str2 . On the Opposite, str1 directly enters as null value and may depending on the implementation cause a null reference exception.